A ${\bf 1.4}$ deg${\bf ^2}$ blind survey for CII], CIII] and CIV at ${\bf z\sim0.7-1.5}$. I: nature, morphologies and equivalent widths

TL;DR

This study presents the first large-scale blind survey of CII], CIII], and CIV emission line galaxies at redshifts 0.7 to 1.5, analyzing their properties, morphologies, and potential AGN activity using multi-wavelength data.

Contribution

It provides a uniform, unbiased sample of emission line galaxies at multiple redshifts and investigates their physical nature and AGN connection, which was previously limited by small samples.

Findings

CII] emitters are a mix of star-forming and AGN host galaxies.

CIII] emitters mainly trace star-forming galaxies with diverse morphologies.

CIV emitters are consistent with young, blue quasars with high accretion rates.

Abstract

While traditionally associated with active galactic nuclei (AGN), the properties of the CII] ($\lambda=2326$\,{\AA}), CIII] ($\lambda,\lambda=1907,1909$\,{\AA}) and CIV ($\lambda,\lambda=1549, 1551$\,{\AA}) emission lines are still uncertain as large, unbiased samples of sources are scarce. We present the first blind, statistical study of CII], CIII] and CIV emitters at $z\sim0.68,1.05,1.53$, respectively, uniformly selected down to a flux limit of $\sim4\times10^{-17}$ erg s$^{-1}$ cm$^{-1}$ through a narrow band survey covering an area of $\sim1.4$ deg$^2$ over COSMOS and UDS. We detect 16 CII], 35 CIII] and 17 CIV emitters, whose nature we investigate using optical colours as well as HST, X-ray, radio and far infra-red data. We find that $z\sim0.7$ CII] emitters are consistent with a mixture of blue (UV slope $\beta=-2.0\pm0.4$) star forming galaxies with disky HST structure and AGN with Seyfert-like morphologies. Bright CII] emitters have individual X-ray detections as well as high average black hole accretion rates (BHAR) of $\sim0.1$ $M_{\odot}$ yr$^{-1}$. CIII] emitters at $z\sim1.05$ trace a general population of SF galaxies, with $\beta=-0.8\pm1.1$, a variety of optical morphologies, including isolated and interacting galaxies and low BHAR ($<0.02$ $M_{\odot}$ yr$^{-1}$). Our CIV emitters at $z\sim1.5$ are consistent with young, blue quasars ($\beta\sim-1.9$) with point-like optical morphologies, bright X-ray counterparts and large BHAR ($0.8$ $M_{\odot}$ yr$^{-1}$). We also find some surprising CII], CIII] and CIV emitters with rest-frame equivalent widths which could be as large as $50-100$ {\AA}. AGN or spatial offsets between the UV continuum stellar disk and the line emitting regions may explain the large EW. These bright CII], CIII] and CIV emitters are ideal candidates for spectroscopic follow up to fully unveil their nature.

Figures

Tables

Table 1. Table 1 : Line emitters studied in this work, with rest-frame wavelength, ionisation energy χ 𝜒 \chi (Veilleux, 2002 ) , luminosity distance D L subscript 𝐷 L D_{\rm L} and redshift range traced by the NB392 filter, with central wavelength 3919 3919 3919  Å and FWHM of 52 52 52  Å. The sample is drawn from the emission line catalogue presented in Sobral et al. ( 2017 ) , which focuses on selection and properties of Ly α 𝛼 \alpha emitters at z ∼ 2.2 similar-to 𝑧 2.2 z\sim 2.2 .

Line	${\lambda }_{\mathrm{line}}$	$\chi$	$z_{\mathrm{line}}$	$D_L$	Comments
	(Å)	(eV)	at FWHM	(${10}^3$ Mpc)
Cii]	2326	11.3	$0.673-0.696$	4.14	likely traces shocks around AGN
Ciii]	1907, 1909	24.4	$1.039-1.066$	7.04	produced in SF and BLR of AGN
Civ	1549, 1551	47.9	$1.513-1.546$	11.47	likely produced in BLR of AGN or in gas around very massive stars

Table 2. Table 2 : Criteria for selecting a source as a C ii ], C iii ] or C iv emitter. The z spec subscript 𝑧 spec z_{\rm spec} ranges used correspond to the full transmission range covered by the NB filter. Note that we are using conservative z phot subscript 𝑧 phot z_{\rm phot} cuts to minimise contamination. Sources selected as Ly α 𝛼 \alpha by Sobral et al. ( 2017 ) using colour-colour selections were removed from the sample. The number of sources of each type, selected based on z spec subscript 𝑧 spec z_{\rm spec} and additional z phot subscript 𝑧 phot z_{\rm phot} are also listed.

Line	$z_{\mathrm{spec}}$ selection range	$z_{\mathrm{spec}}$ sources	$z_{\mathrm{phot}}$ selection range	$z_{\mathrm{phot}}$ sources	All
			(sources without $z_{\mathrm{spec}}$)
Cii]	$0.661-0.707$	03	$0.63-0.75$	13	16
Ciii]	$1.025-1.080$	04	$0.8-1.2$	30	34
Civ	$1.486-1.563$	14	$1.4-1.7$	03	17

Table 3. Table 3 : Rest-frame E ​ W rest 𝐸 subscript 𝑊 rest EW_{\rm rest} , observed ( U − B ) obs subscript 𝑈 𝐵 obs (U-B)_{\rm obs} and UV slope β 𝛽 \beta of the emitters which have photometric or spectroscopic redshifts. We also list the observed filters used for tracing the rest-frame UV. Note that the rest-frame wavelength traced for calculating the β 𝛽 \beta slopes vary slightly depending on the emitter type. See Section  4.3 for more details. The uncertainties reported represent the standard deviation of the sample.

Line	$z_{\mathrm{spec}}$	Mean $E{W}_{\mathrm{rest}}$	Median $E{W}_{\mathrm{rest}}$	Mean ${\left(U-B\right)}_{\mathrm{obs}}$	Median ${\left(U-B\right)}_{\mathrm{obs}}$	Mean $\beta$	Median $\beta$	Filters for
		(Å)	(Å)	(mag)	(mag)			$\beta$ slope
Cii]	$0.68$	$82\pm 56$	$74\pm 70$	$0.25\pm 0.20$	$0.26\pm 0.25$	$-2.0\pm 0.4$	$-1.9\pm 0.5$	$NUV$, $U$
Ciii]	$1.05$	$93\pm 59$	$87\pm 74$	$0.25\pm 0.37$	$0.21\pm 0.46$	$-0.8\pm 1.1$	$-0.6\pm 1.4$	$U$, $B$
Civ	$1.53$	$51\pm 46$	$34\pm 58$	$0.15\pm 0.37$	$0.21\pm 0.46$	$-1.9\pm 0.8$	$-1.6\pm 1.0$	$U$, $V$
Ly$\alpha$	$2.23$	$85\pm 57$	$77\pm 71$	$0.18\pm 0.25$	$0.23\pm 0.31$	$-1.6\pm 0.6$	$-1.7\pm 0.7$	$g$, $R$
H$\alpha$	$2.23$					$-1.0\pm 0.6$	$-1.0\pm 0.7$	$g$, $R$

Table 4. Table 4 : Summary of the optical, X-ray and radio properties of C ii ], C iii ] and C iv emitters. The full details on individual sources can be found in Tables  6 , 7 and 8 .

Line		HST			Chandra			Radio $1.4$ GHz
		coverage	morphology		coverage	counterpart		coverage	counterpart
Cii]		12/16	4 bright-core+disk, 7 disky, 1 interacting		5/16	2/5		13/16	0/13
Ciii]		19/34	2 bright-core+disk, 8 disturbed/interacting, 7 diffuse/spiral, 2 compact		9/34	1/9		22/34	1/22
Civ		10/17	10 point sources		4/17	4/4		12/17	3/14

Table 5. Table 5 : Average X-ray luminosity and BHAR, as resulting from a stacking analysis of the C ii ], C iii ] and C iv emitters (with z spec subscript 𝑧 spec z_{\rm spec} or z phot subscript 𝑧 phot z_{\rm phot} ), including direct detections. While C ii ] and C iv emitters are consistent with AGN activity, C iii ] values place in the SF galaxy regime. Note that removing the direct X-ray detections results in non-detections in the stacks because of the very low numbers of sources left. We also present SFR upper limits obtained from FIR and radio data. Note that individual detections are removed from the radio stack as a direct radio detection will be caused by an AGN rather than SF (which is the purpose of our radio stacks). We also report SFR measurements from the UV, assuming all UV radiation is coming from SF. We removed sources where ancillary data strongly indicated the presence of an AGN. Therefore, we report no average UV SFR for C iv emitters.

Line	$\log L_X$	$BHAR$	No sources	$SF{R}_{\mathrm{FIR}}$ $3\sigma$ upper limit	$SF{R}_{\mathrm{radio}}$ $3\sigma$ upper limit	Mean $SF{R}_{\mathrm{UV}}$	Median $SF{R}_{\mathrm{UV}}$
	(erg $s^{-1}$)	($M_{\odot }$ yr-1)		($M_{\odot }$ yr-1)	($M_{\odot }$ yr-1)	($M_{\odot }$ yr-1)	($M_{\odot }$ yr-1)
Cii]	$43.46\pm 0.56$	$0.10\pm 0.06$	5	0	0	$7\pm 10$	$2\pm 13$
Ciii]	$42.80\pm 0.37$	$0.02\pm 0.01$	9	0	0	$120\pm 345$	$24\pm 430$
Civ	$44.37\pm 0.08$	$0.83\pm 0.12$	4			–	–

Table 6. Table 6 : Properties of the z ∼ 0.7 similar-to 𝑧 0.7 z\sim 0.7 C ii ] emitters: coordinates, spectroscopic redshift information availability, HST optical morphologies and possible X-ray and radio counterparts. We also list the emitters’ line luminosity, rest-frame E ​ W rest 𝐸 subscript 𝑊 rest EW_{\rm rest} and observed ( U − B ) obs subscript 𝑈 𝐵 obs (U-B)_{\rm obs} colour. The error on the ( U − B ) obs subscript 𝑈 𝐵 obs (U-B)_{\rm obs} colour was calculated assuming the maximal magnitude errors of 0.05 0.05 0.05 . A fits version of this table can be found online.

CALYMHA ID	RA	DEC	$z_{\mathrm{spec}}$	HST	Chandra	Radio $1.4$ GHz	$\log L_{\mathrm{CII}]}$	$E{W}_{\mathrm{rest}}$		${\left(U-B\right)}_{\mathrm{obs}}$
	($hh:mm:ss$)	(${}_{}{}^{\circ }:_{}^{\prime }{:}^{\prime \prime }$)	confirmed	morphology	counterpart	counterpart	(erg s-1)	(Å)		(mag)
CALYMHA-S16-5	$10:02:37.01$	$02:40:28.9$	NO	face-on spiral	–†	NO	41.44	$56$$\pm$	$22$	0.27$\pm 0.07$
CALYMHA-S16-53	$10:01:54.84$	$02:16:20.3$	NO	face-on spiral	NO	NO	41.47	$239$$\pm$	$137$	0.55
CALYMHA-S16-61	$10:01:44.50$	$02:12:34.2$	NO	face-on spiral	NO	NO	41.42	$216$$\pm$	$130$	0.26
CALYMHA-S16-74	$10:01:31.30$	$02:54:42.8$	NO	–	–	NO	41.26	$277$$\pm$	$105$	0.30
CALYMHA-S16-100	$10:00:58.73$	$02:25:56.3$	YES	bright core & disk	YES	NO	41.88	$18$$\pm$	$1$	0.54
CALYMHA-S16-149	$10:00:13.03$	$01:51:54.0$	NO	disk galaxy	NO	NO	41.30	$74$$\pm$	$34$	0.55
CALYMHA-S16-178	$09:59:36.48$	$01:38:45.6$	NO	bright core & disk	–	NO	41.32	$43$$\pm$	$26$	0.38
CALYMHA-S16-181	$09:59:35.45$	$01:43:01.6$	NO	disk galaxy	–	NO	41.39	$79$$\pm$	$42$	0.33
CALYMHA-S16-256	$09:58:34.68$	$02:40:37.2$	NO	face-on spiral	–	NO	41.31	$155$$\pm$	$105$	0.08
CALYMHA-S16-258	$09:58:33.86$	$02:40:34.7$	NO	face-on spiral	–	NO	41.33	$99$$\pm$	$61$	0.14
CALYMHA-S16-265	$09:58:26.66$	$02:28:17.8$	YES	bright core & disk	–	NO	42.12	$128$$\pm$	$8$	0.13
CALYMHA-S16-275	$09:58:13.34$	$02:05:36.6$	YES	bright core & disk	YES	NO	42.04	$27$$\pm$	$7$	0.26
CALYMHA-S16-335	$09:57:25.01$	$02:31:19.9$	NO	–	–	–	41.18	$107$$\pm$	$46$	0.24
CALYMHA-S16-340	$09:57:24.26$	$02:38:21.1$	NO	–	–	NO	41.22	$94$$\pm$	$37$	0.51
CALYMHA-S16-358	$09:56:59.33$	$01:52:35.8$	NO	–	–	–	42.20	$16$$\pm$	$2$	0.29
CALYMHA-S16-388	$02:17:17.33$	$-05:14:24.7$	NO	interacting, three nuclei	–	–	41.64	$52$$\pm$	$5$	$-$0.26

Table 7. Table 7 : Same as Table  7 , but for C iii ] emitters at z ∼ 1.05 similar-to 𝑧 1.05 z\sim 1.05 . A fits version of this table is available online.

CALYMHA ID	RA	DEC	$z_{\mathrm{spec}}$	HST	Chandra	Radio $1.4$ GHz	$\log L_{\mathrm{CIII}]}$	$E{W}_{\mathrm{rest}}$		${\left(U-B\right)}_{\mathrm{obs}}$
	($hh:mm:ss$)	(${}_{}{}^{\circ }:_{}^{\prime }{:}^{\prime \prime }$)	confirmed	morphology	counterpart	counterpart	(erg s-1)	(Å)		(mag)
CALYMHA-S16-29	$10:02:14.28$	$02:32:57.8$	NO	merger	–†	NO	41.87	111$\pm$	51	0.30$\pm 0.07$
CALYMHA-S16-55	$10:01:53.71$	$02:18:58.3$	NO	disturbed	NO	NO	41.87	53$\pm$	27	0.31
CALYMHA-S16-73	$10:01:31.97$	$02:26:47.4$	NO	spiral	NO	NO	41.90	86$\pm$	50	0.17
CALYMHA-S16-114	$10:00:50.83$	$01:59:55.3$	NO	disturbed	NO	NO	41.65	61$\pm$	13	0.19
CALYMHA-S16-127	$10:00:38.50$	$02:00:04.3$	NO	interacting	NO	NO	41.69	90$\pm$	20	0.15
CALYMHA-S16-153	$10:00:09.19$	$01:54:50.0$	NO	disturbed	NO	NO	41.77	33$\pm$	15	0.29
CALYMHA-S16-162	$09:59:54.29$	$01:39:42.1$	NO	faint/diffuse	–	NO	41.80	179$\pm$	124	0.33
CALYMHA-S16-182	$09:59:35.33$	$01:48:18.4$	NO	spiral	NO	NO	41.76	194$\pm$	111	0.18
CALYMHA-S16-194	$09:59:27.79$	$02:53:13.6$	NO	–	–	NO	41.80	77$\pm$	45	0.48
CALYMHA-S16-205	$09:59:18.79$	$01:40:32.2$	NO	diffuse spiral	–	NO	41.78	37$\pm$	22	0.28
CALYMHA-S16-212	$09:59:16.06$	$01:50:48.1$	YES	bright core & disk	YES	YES	42.42	21$\pm$	3	0.23
CALYMHA-S16-218	$09:59:12.84$	$01:39:09.4$	NO	interacting galaxies	–	NO	41.78	96$\pm$	60	0.68
CALYMHA-S16-237	$09:59:03.00$	$01:38:47.4$	NO	compact galaxy	–	NO	42.36	73$\pm$	16	0.07
CALYMHA-S16-262	$09:58:29.69$	$02:48:04.3$	NO	diffuse galaxy	–	NO	41.80	92$\pm$	54	0.21
CALYMHA-S16-267	$09:58:22.82$	$01:54:55.8$	NO	disturbed galaxy	NO	NO	41.84	196$\pm$	114	0.62
CALYMHA-S16-268	$09:58:22.30$	$01:49:13.8$	NO	diffuse spiral	–	NO	41.85	144$\pm$	76	0.26
CALYMHA-S16-273	$09:58:19.85$	$02:05:05.6$	NO	interacting	NO	NO	42.02	96$\pm$	65	0.36
CALYMHA-S16-290	$09:57:51.22$	$02:35:30.1$	NO	faint, compact	–	NO	41.69	192$\pm$	99	0.04
CALYMHA-S16-291	$09:57:51.07$	$02:21:33.8$	NO	–	–	NO	41.68	141$\pm$	66	0.39
CALYMHA-S16-292	$09:57:50.74$	$02:21:32.4$	NO	–	–	NO	41.59	21$\pm$	9	0.21
CALYMHA-S16-293	$09:57:50.45$	$02:28:35.0$	NO	diffuse spiral	–	NO	41.59	87$\pm$	41	0.32
CALYMHA-S16-297	$09:57:48.46$	$02:24:01.4$	NO	–	–	NO	41.61	178$\pm$	103	1.79
CALYMHA-S16-314	$09:57:35.57$	$02:25:50.9$	NO	–	–	–	41.63	131$\pm$	65	0.29
CALYMHA-S16-317	$09:57:33.48$	$02:26:58.6$	NO	–	–	–	41.64	37$\pm$	15	0.30
CALYMHA-S16-326	$09:57:27.82$	$02:24:07.9$	NO	–	–	–	41.70	148$\pm$	68	0.05
CALYMHA-S16-328	$09:57:26.66$	$02:24:26.3$	NO	–	–	–	41.61	42$\pm$	18	0.17
CALYMHA-S16-349	$09:57:08.86$	$01:49:52.0$	NO	–	–	–	41.73	44$\pm$	33	0.23
CALYMHA-S16-352	$09:57:05.81$	$01:58:54.5$	NO	–	–	–	41.69	49$\pm$	38	0.07
CALYMHA-S16-363	$02:18:17.38$	$-04:51:12.2$	YES	–	–	–	42.96	14$\pm$	1	NaN‡
CALYMHA-S16-371	$02:17:49.92$	$-05:03:15.8$	NO	–	–	–	41.80	124$\pm$	53	$-$0.07
CALYMHA-S16-381	$02:17:36.53$	$-05:21:56.5$	YES	–	–	–	41.81	26$\pm$	3	0.12
CALYMHA-S16-385	$02:17:28.68$	$-05:18:49.3$	NO	–	–	–	41.72	68$\pm$	20	$-$0.30
CALYMHA-S16-394	$02:17:13.10$	$-05:02:14.6$	NO	–	–	–	41.78	69$\pm$	18	$-$0.22
CALYMHA-S16-412	$02:16:41.16$	$-05:13:19.2$	YES	bright core, faint arcs	–	–	41.87	39$\pm$	5	NaN

Table 8. Table 8 : Same as Table  6 , for C iv emitters at z ∼ 1.53 similar-to 𝑧 1.53 z\sim 1.53 . A fits version of this table can be found online.

CALYMHA ID	RA	DEC	$z_{\mathrm{spec}}$	HST	Chandra	Radio $1.4$ GHz	$\log L_{\mathrm{CIV}}$	$E{W}_{\mathrm{rest}}$		${\left(U-B\right)}_{\mathrm{obs}}$
	($hh:mm:ss$)	(${}_{}{}^{\circ }:_{}^{\prime }{:}^{\prime \prime }$)	confirmed	morphology	counterpart	counterpart	(erg s-1)	(Å)		(mag)
CALYMHA-S16-16	$10:02:29.16$	$02:09:31.7$	YES	p.s.†	–‡	NO	42.84	8$\pm$	1	0.33$\pm 0.07$
CALYMHA-S16-26	$10:02:17.98$	$01:58:36.8$	YES	p.s.	–	YES	43.02	66$\pm$	3	0.56
CALYMHA-S16-58	$10:01:47.28$	$02:47:29.4$	YES	p.s.	–	NO	42.71	41$\pm$	3	0.28
CALYMHA-S16-63	$10:01:41.26$	$02:23:08.2$	YES	p.s.	YES	NO	42.65	97$\pm$	33	$-$0.22
CALYMHA-S16-92	$10:01:11.98$	$02:30:25.2$	YES	p.s.	YES	NO	42.59	7$\pm$	1	0.41
CALYMHA-S16-118	$10:00:47.95$	$02:11:26.9$	YES	p.s.	YES	NO	42.77	12$\pm$	1	0.64
CALYMHA-S16-135	$10:00:28.08$	$01:55:47.6$	YES	p.s.	YES	NO	42.88	72$\pm$	4	0.35
CALYMHA-S16-226	$09:59:08.69$	$02:54:24.5$	YES	–	–	possible faint source	43.54	7$\pm$	1	0.62
CALYMHA-S16-236	$09:59:04.01$	$02:50:40.6$	NO	–	–	NO	42.23	138$\pm$	89	0.49
CALYMHA-S16-250	$09:58:48.86$	$02:34:41.2$	YES	p.s.	–	NO	43.36	24$\pm$	1	0.78
CALYMHA-S16-274	$09:58:15.50$	$01:49:22.8$	YES	p.s.	–	YES	43.38	16$\pm$	1	0.14
CALYMHA-S16-289	$09:57:52.22$	$02:35:24.0$	NO	faint p.s.	–	NO	41.99	156$\pm$	96	0.33
CALYMHA-S16-319	$09:57:31.70$	$02:25:52.3$	NO	–	–	–	41.99	90$\pm$	46	0.55
CALYMHA-S16-325	$09:57:28.32$	$02:25:41.9$	YES	–	–	–	43.76	31$\pm$	1	0.48
CALYMHA-S16-364	$02:18:16.80$	$-05:00:55.1$	YES	–	–	–	42.19	80$\pm$	28	$-$0.65
CALYMHA-S16-372	$02:17:47.16$	$-04:56:24.7$	YES	–	–	–	42.47	36$\pm$	2	0.08
CALYMHA-S16-375	$02:17:45.22$	$-05:01:52.0$	YES	–	–	–	42.47	19$\pm$	1	0.43

Equations

$$L_{\mathrm{line}}=4\pi D_L^2(line)F_{\mathrm{line}},$$

$$\beta =-\frac{m_1-m_2}{2.5{\log }_{10}\left(\frac{{\lambda }_1}{{\lambda }_2}\right)}-2,$$

$$(NUV-U)>1.0\mathrm{or}(NUV-B)>1.5$$

$$L_{\mathrm{X}}=4\pi D_{\mathrm{L}}^2{F}_{\mathrm{X}}(1+z{)}^{\Gamma -2}({\mathrm{erg}\mathrm{s}}^{-1}),$$

$$BHAR=\frac{(1-ϵ)L_{\mathrm{bol}}}{ϵc^2}({\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}),$$

$$SF{R}_{\mathrm{FIR}}=2.5\times 10^{-44}{L}_{\mathrm{FIR}}{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}.$$

$$SF{R}_{\mathrm{radio}}=3.18\times 10^{-22}{L}_{\mathrm{radio}}{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}.$$

$$A_{\mathrm{UV}}=2.31\beta +4.85,$$

$$SF{R}_{\mathrm{UV}}=8.8\times 10^{-29}{L}_{\mathrm{UV}}{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1},$$

$$L_{\mathrm{UV}}=4\pi 9.523\times 10^{38}\times 10^{-0.4(M_{\mathrm{UV},\mathrm{corr}}+48.6)}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{Hz},$$

Full text

††pubyear: 2017

1 Introduction

Rest-frame ultra-violet (UV) emission lines are of great importance for extragalactic astrophysics as they can be used to infer gas metallicities, temperatures and the strength of the ionising field (e.g. Shapley et al., 2003; Osterbrock & Ferland, 2006). Observing these lines in the local Universe is challenging. However, for galaxies at large cosmic distances the rest-frame UV emission lines are redshifted into the easily observable optical range. Historically rest-frame UV spectra were mainly used to spectroscopically confirm UV-bright galaxies at $z\gtrsim 3$ selected through the Lyman Break technique (Lyman Break galaxies, LBG, Steidel et al., 1996; Lowenthal et al., 1997). In the last 15 years, strong rest-frame UV lines have been discovered in a variety of galaxies at $z>1$, which could be used to constrain the physics in high-redshift counterparts of local galaxies (Best et al., 2000; Shapley et al., 2003; Erb et al., 2010; Rigby et al., 2015).

Intrinsically the brightest rest-frame UV line is Ly$\alpha$, which is produced in HII regions as well as in galaxies with an active galactic nucleus (AGN) (e.g. Ouchi et al., 2008; Nilsson et al., 2009; Cowie et al., 2010; Hayes, 2015; Matthee et al., 2016, 2017; Sobral et al., 2017). However, it is difficult to interpret observations of Ly$\alpha$, as the line is scattered by neutral hydrogen and easily absorbed by dust (e.g. Dijkstra, 2014; Hayes, 2015). The amount of Ly$\alpha$ that escapes a galaxy therefore depends on the properties of the interstellar medium.

Other UV lines, such as Cii] ($\lambda=2326$ Å), Ciii] ($\lambda,\lambda=1907,1909$ Å) and Civ ($\lambda,\lambda=1549,1551$ Å) (Table 1) have recently been explored as they can be relatively bright compared to Ly$\alpha$ and can be used either individually or in combination to constrain the physics of the host galaxy. Ciii] (ionisation potential of 24.4 eV, Veilleux, 2002), for example, was found to be the strongest UV line after Ly$\alpha$ in stacks of LBGs at $z\sim 3$ (Shapley et al., 2003) and about 10 per cent of the observed strength of Ly$\alpha$ in faint, lensed galaxies at $1.5<z<3$ (Stark et al., 2014). Civ (47.9 eV) is also a bright UV line. From photoionisation modelling, the Civ ratio to Ciii] ranges from 0.1 for low metallicity star-forming ($\lesssim 0.3Z_{\odot}$), up to 100 for high, supersolar metallicity ($\sim 2-2.5Z_{\odot}$). The typical Civ to Ciii] ratio is $\sim 1$ for star forming galaxies of solar-type metallicity (e.g. Gutkin et al., 2016; Feltre et al., 2016).

High ionisation rest-frame UV emission lines of Carbon were originally thought to originate from close to the AGN engine as they require a strong radiation field and high temperatures. Ciii] is a high-ionisation, intercombination doublet ($1907,1909$ Å) expected to be mostly produced in the outer parts of the broad line region (BLR) of the AGN (Osterbrock & Ferland, 2006). However, at $z\sim 2-3$, Ciii] emitters are also found in star forming (SF) galaxies and trace a slightly sub-solar metallicity, a high ionization parameter and a hard radiation field (Rigby et al., 2015; Bayliss et al., 2014). Photoionisation models presented in Jaskot & Ravindranath (2016) indicate that Ciii] can be produced in starburst galaxies and is the strongest line (with $\lambda<2700$ Å) after Ly$\alpha$.

UV collisionally excited lines such as Cii] and Civ are stronger in very high temperature regions ($2\times 10^{4}-10^{5}$ K) in the cooling region behind shocks around the AGN than in areas with lower temperature such as those that can be reached with photoionisation ($10^{4}$ K, Osterbrock & Ferland, 2006; Allen et al., 1998). Cii] and Civ are therefore expected to be more strongly produced in AGN hosting galaxies. Indeed, Baldwin (1977) found a correlation between the strength of the resonant Civ ($1548,1551$ Å) line and the continuum luminosity, indicating that the line emitting gas is located very close to the ionisation source. While traditionally associated with BLR emission, Civ was found to be correlated with gas temperature and an intense radiation field (Osterbrock & Ferland, 2006), such as the one caused by AGN or by massive stars after a recent SF episode (Stark et al., 2014; Mainali et al., 2017; Schmidt et al., 2017). In a study of radio galaxies, De Breuck et al. (2000) noted that Cii] is five times more sensitive to shock ionisation than high ionisation UV lines, such as Civ. Therefore, there is compelling evidence that the semi-forbidden Cii] (11.3 eV, Veilleux, 2002) line traces shocks and in combination with other lines is effective in determining the power source of ionisation (Best et al., 2000, 2002).

While independently Cii], Ciii] and Civ trace gas metallicity and electron density, in combination they can be used as estimators which are, to first order, independent of abundance, metallicity and dust-extinction (Best et al., 2002). Therefore, Cii], Ciii] and Civ line ratios are some of the best diagnostics to separate excitation by fast shocks and photoionisation in a hard photon spectrum (Allen et al., 1998).

Given their relative strength to Ly$\alpha$, Ciii] and Civ have been proposed as a good avenue for spectroscopically confirming high redshift galaxies (Stark et al., 2014, 2015a, 2015b), particularly within the epoch of reionisation, when Ly$\alpha$ scattering is expected to increase leading to a significant decrease of surface brightness. As a result, in recent years, targeted searches for Ciii] and Civ emitters at high redshift have emerged. For example, Rigby et al. (2015) detect Ciii] in 11 $z\sim 1.6-3$ lensed galaxies, and Bayliss et al. (2014) find strong Ciii] in a $z\sim 3.63$ lensed starburst. Stark et al. (2015a) present tentative detections of Ciii] in two galaxies at $z>6$, while Ding et al. (2016) detect Ciii] in one galaxy at $z\sim 5.7$. By contrast, Zitrin et al. (2015a) do not obtain a detection of Ciii] in a sample of 7 $z\sim 7-8$ photometric candidates. Very recently, Du et al. (2017) presented a spectroscopic study of a sample of continuum selected Ciii] emitters at $z\sim 1$. With a detection rate of $\sim 20$ per cent, their Ciii] emitters have much lower EW ($1.3$ Å) than higher redshift examples. Du et al. (2017) also found that the stronger EW sources appear in fainter, bluer and lower-mass galaxies. Schmidt et al. (2017) and Mainali et al. (2017) obtain a detection of Civ in a multiply-lensed $z=6.1$, Ly$\alpha$-emitting SF galaxy, but without a Ciii] detection. Despite the growing number of detections, the samples of Ciii] and Civ emitters suffer from selection biases (e.g. spectroscopically selected, lensed sources, redshift known from Ly$\alpha$).

Despite the potential importance of Cii], Ciii] and Civ for understanding AGN physics and the nature of stellar populations at high redshift, not much is known about these emitters in a statistical sense, as no blind studies have been performed. As such, the nature, number densities and EW distributions are largely unknown. We seek to improve our understanding of Cii], Ciii] and Civ emitters by performing the first blind survey of these lines, without any preselection in terms of Ly$\alpha$ or UV properties. Our sample is uniformly selected down to a flux of $\sim 4\times 10^{-17}$ erg s*-1* cm*-2*, in three redshift slices around $z\sim 0.7$, $\sim 1.0$, $\sim 1.5$ for Cii], Ciii] and Civ, respectively. The limiting observed EW is 16 Å and the limiting $U$ magnitude is $\sim 26.5$. The sources were discovered by exploring the $\sim 1.4$ deg2 CAlibrating LYMan-$\alpha$ with H$\alpha$ NB survey (CALYMHA, Matthee et al., 2016; Sobral et al., 2017) over the COSMOS and UDS fields.

Our results are presented in two parts. In this paper (Paper I), we use the emission line data in combination with multiwavelength observations in the optical, radio, X-ray and far infra red (FIR) to unveil the characteristics of individual Cii], Ciii] and Civ emitters selected with the CALYMHA survey. We study the likely physical origin of the emission lines and how their properties compare with AGN and SF galaxies at similar redshifts. In the companion paper (Paper II, Stroe et al., 2017b) we investigate the statistical properties of the Cii], Ciii] and Civ emitters through luminosity functions (LF) and obtain the volume-average line ratios relative to e.g. Ly$\alpha$ and H$\alpha$.

We organise the paper as follows: in Section 2 we present the CALYMHA parent sample, while in Section 3 we select the Cii], Ciii] and Civ emitters. We discuss the colour and EW properties of the emitters as well as their Hubble Space Telescope (HST), radio, far infra-red (FIR) and X-ray properties in Section 4. The interpretation of our results and the implication for the physics Cii], Ciii] and Civ production can be found in Section 5, with conclusions and outlook in Section 6.

Throughout the paper, we use a flat $\Lambda$CDM cosmology ($H_{0}=70$ km s*-1* Mpc*-1*, $\Omega_{M}=0.3$, $\Omega_{\Lambda}=0.7$), and perform calculations with the aid of the Wright (2006) cosmology calculator. All magnitudes are in the AB system and we use a Chabrier (2003) initial mass function (IMF).

2 Survey description

We use the CALYMHA sample of emission line galaxies to select Cii] ($z\sim 0.63$), Ciii] ($z\sim 1.05$) and Civ ($z\sim 1.53$) line emitters in the COSMOS and UDS fields. The CALYMHA survey design, observations and data reduction is presented in full in Sobral et al. (2017) and here we give a brief summary of the survey strategy and goals. The programme surveyed a combined area of $\sim 1.4$ deg2 in the COSMOS and UDS fields using a narrow band (NB) filter (NB392, central wavelength $\lambda_{C}=3918$ Å and width $\Delta\lambda=52$ Å) mounted on the Isaac Newton Telescope111http://www.ing.iac.es/Astronomy/telescopes/int/. In combination with ancillary broad band (BB) $U$ data ($\lambda_{C}=3750$ Å, $\Delta\lambda=720$ Å), the NB filter was designed to select line emitters with a particular focus on Ly$\alpha$ emitters at $z\sim 2.23$, and cross-match them with H$\alpha$ galaxies at the same redshift (Sobral et al., 2013). The main goal of the survey is to unveil the nature of Ly$\alpha$ emitter by studying the luminosity function (LFs) and determining Ly$\alpha$ escape fractions as function of galaxy properties both for H$\alpha$ and Ly$\alpha$ selected samples at $z\sim 2.2$ (Matthee et al., 2016; Sobral et al., 2017).

The CALYMHA COSMOS+UDS survey selected a total of 440 line emitters down to a $3\sigma$ line flux limit of $\sim 4\times 10^{-17}$ erg s*-1* cm*-2*, down to an observed EW limit of 16 Å. Based on spectroscopic and photometric redshifts, the emitter population contains a significant fraction of Cii], Ciii] and Civ emitters (Sobral et al., 2017), thus rendering CALYMHA an ideal sample to study these emitters in a statistical, unbiased way with a clear selection function. Given the width of the NB filter and their rest-frame wavelength, the line emitters are traced over a narrow redshift range (see Table 1).

2.1 Ancillary data

In addition to the CALYMHA NB and U band data, we use ancillary spectroscopic and photometric redshifts and photometry from the COSMOS and UDS surveys (Capak et al., 2007; Lawrence et al., 2007; Ilbert et al., 2009; Laigle et al., 2016). About $40$ per cent of our sources are faint in the $i$ and $K$ bands or are located in masked regions and are thus not included in the publicly available COSMOS and UDS catalogues. There are 80 emitters with spectroscopic redshifts, most of which also have a photometric redshift (Fig. 1, data from Yamada et al., 2005; Simpson et al., 2006; van Breukelen et al., 2007; Geach et al., 2008; Ouchi et al., 2008; Smail et al., 2008; Lilly et al., 2009; Ono et al., 2010). However, in 10 cases, only a spectroscopic redshift is available. We also include the redshifts derived by Sobral et al. (2017) from dual, triple and quadruple detection of emission lines in NB filters. These very precise photometric redshifts have accuracies close to a spectroscopic measurement. The total tally for sources with redshifts (spectroscopic or photometric) is 269, or 61 per cent of the total number of emitters.

We also explore the deep, publicly available HST data in the F814W filter (Koekemoer et al., 2007; Massey et al., 2010), Chandra space telescope X-ray observations (Elvis et al., 2009), FIR Herschel data (Oliver et al., 2012) and radio Very Large Array images at $1.4$ GHz (Schinnerer et al., 2004, 2010) in the COSMOS field to further investigate the nature of the line emitters. We employ direct detections as well as stacking techniques for this purpose. We note that the Chandra deep data is only available in a sub-area of the COSMOS field, hence only a fraction of the sources will have counterparts and/or coverage. The UDS field is partly covered with HST data as part of the CANDELS survey (Koekemoer et al., 2011).

3 Selecting Cii], Ciii] and Civ emitters

In order to select Cii], Ciii] and Civ emitters at the redshifts traced by the NB392 filter, we use a combination of spectroscopic and photometric redshifts.

3.1 Redshifts

For the COSMOS field, Ilbert et al. (2009) derived photometric redshifts using a range of templates, including star, galaxy and quasar templates for an $i$ band selected sample. Blindly using the galaxy templates results in large discrepancies between the chosen photometric redshift and the true redshift, when spectroscopy is available. Keeping in mind that a fraction of Cii], Ciii] and Civ emitters is possibly tracing AGN activity, we expect in many cases the quasar templates to perform better. Ilbert et al. (2009) also provide the $\chi^{2}$ for the best fit stellar, galaxy and quasar template. We found that simply choosing the template which provided the lowest $\chi^{2}$ fit worked well: for the sources with both $z_{\rm phot}$ and $z_{\rm spec}$, the two estimates matched (see Fig. 1). When choosing the best $z_{\rm phot}$ estimate based on $\chi^{2}$, $88$ per cent of the photometric redshifts are within 0.1 of the spectroscopic ones. In cases where neither template was a good fit (high $\chi^{2}>100$), all photometric redshift estimates were catastrophically off.

We also tested the new COSMOS $z_{\rm phot}$ catalogue presented in Laigle et al. (2016) using the same method of selecting the best template (minimising the $\chi^{2}$), but found that the Ilbert et al. (2009) photometric redshifts correlate better with the spectroscopic redshifts in our sample. Laigle et al. (2016) is selected in the near infrared and Ilbert et al. (2009) in the optical. Since our sources are optically selected (in the very blue), it is maybe unsurprising Ilbert et al. (2009) $z_{\rm phot}$ work better, given their weighting towards optical bands. In the case of UDS, a single photometric redshift estimate is available (Cirasuolo et al., 2010).

Overall, for the entire sample, 84 per cent of photometric redshifts are within 0.1 of the spectroscopic redshift (Fig. 1). The sample is however not spectroscopically complete, especially for fainter sources, so the photometric redshift accuracy derived here is not necessarily applicable for all the sources without a spectroscopic redshift (Fig. 2).

3.2 Final selection criteria

For a source to make the Cii], Ciii] or Civ emitter selection, we first remove all sources selected as Ly$\alpha$ by Sobral et al. (2017). It then has to fulfill at least one of the criteria listed below. We summarise the criteria in Table 2 and describe them below:

• •

A spectroscopic redshift within the range probed by the respective filter, within two FWHM. We choose this wider range since the filter transmission drops slowly towards its wavelength edges, effectively being sensitive to emitters at twice the FWHM. This also accounts for broad lines.

• •

If spectroscopy is not available, we select a source if is has a photometric redshift within $\sim 0.2$ of the redshift range the NB filter is sensitive to. Note our very conservative cuts are chosen to maximise the purity of the sample.

The redshift distribution of sources selected as Cii], Ciii] and Civ is shown Fig. 2. The narrow photometric redshift ranges chosen for selection ensures that we do not include bright, low redshift emitters such as [Oii] at $z\sim 0.05$, [Nev] at $z\sim 0.15$ and Mgi and Mgii at $z\sim 0.4$ in our sample. Our photometric redshift selection is conservative as there could be sources with $z_{\rm phot}$ in the $1.2-1.4$ range which could be either Ciii] or Civ (Fig. 2).

Sobral et al. (2017) used $BzK$ colour selections to further improve the completeness of their Ly$\alpha$ sample and thus include some sources with lower photometric redshifts. If we did not remove the Ly$\alpha$ selected sources, we would select an extra 9 Ciii] and 16 Civ sources. We remove sources selected as Ly$\alpha$ by Sobral et al. (2017) to ensure a high purity and obtain conservative, but secure samples. Note however, the highly unusual colours of Civ emitters (see Section 4.1), which means that some real Civ emitters might have been selected as Ly$\alpha$ and thus were removed from our sample. Spectroscopic follow up is required to further investigate this. See also discussion in Sobral et al. (2017) on the removal of the vast majority of Ciii] and Civ contaminants in CALYMHA, which is usually not done in other Ly$\alpha$ surveys.

4 Properties of the Cii], Ciii] and Civ samples

Table 2 lists the final samples of Cii], Ciii] and Civ emitters, which include spectroscopically confirmed sources and sources selected through their $z_{\rm phot}$. We have $3$ spectroscopically confirmed Cii] emitters in addition to $13$ $z_{\rm phot}$. In the case of Ciii] emitters, we have $4$ spectroscopically selected sources and $30$ with $z_{\rm phot}$. We obtain $14$ Civ sources with $z_{\rm spec}$ and $3$ with $z_{\rm phot}$. Note the particularly high spectroscopic completeness of the Civ sample, a likely result of the follow up of Chandra COSMOS sources.

In this section, we study the colour-colour properties as well as the colour and EW distributions with the aim of investigating the nature of the Cii], Ciii] and Civ emitters, as well as test the robustness of our sample. We also investigate the properties of the emitters using X-ray, radio and space telescope optical data. Tables 3 and 4 summarise the EW, UV, optical, X-ray and radio properties of the sample, while Tables 6, 7 and 8 describe individual Cii], Ciii] and Civ emitters. We list their sky coordinates, line luminosity, rest-frame $EW_{\rm rest}$, observed $(U-B)_{\rm obs}$ colours and describe their optical HST morphologies, and their X-ray and/or radio counterparts. Note that while most of the COSMOS part of the CALYMHA survey is covered by HST and VLA radio data, the deep Chandra data is only available for a sub-area. In the case of UDS, only HST data is available and for a small subarea of the field.

To describe the properties of our Cii], Ciii] and Civ sources, we will use the emission line luminosity, which is derived from the observed flux measured within $3^{\prime\prime}$ apertures (which corresponds to about 30 kpc at the redshift of our emitters, see also Sobral et al., 2017):

[TABLE]

where $\mathrm{line}$ is Cii], Ciii] or Civ and $D_{L}(line)$ is the luminosity distance at the redshift of each line (see Table 1).

4.1 Colour-colour properties

In SF galaxies, the $R-z$ versus the $J-Ks$ colour space probes the $4000$ Å break, which moves from between the $R$ and $z$ filters for sources at $z\sim 0.7-1.2$, to between $J$ and $Ks$ for sources at $z>2.1$. In Fig. 3, this is illustrated by the population of SF H$\alpha$ emitters from Sobral et al. (2013) which move from the lower-right side at $z\sim 0.8$ (large $R-z$, small $J-Ks$) of the plot towards the upper-right side (small $R-z$, larger $J-Ks$) at $z\sim 2.2$. For comparison, we also overplot the CALYMHA Ly$\alpha$ emitters at $z\sim 2.2$.

Some Cii] emitters at $z\sim 0.7$ are located in the colour space of SF galaxies at $z\sim 1.5$, so they have atypical colours for their redshift, indicating that while some may trace SF galaxies, some, as expected, probably result from ionisation in AGN through shocks.

Ciii] sources mostly lie in the region of $z\sim 0.8-1.0$ SF galaxies, possibly indicating a SF, rather than AGN nature of these emitters. Note that the most extreme Ciii] emitter, with a very low $R-z$ colour, is an AGN (see Section 4.6) and the most luminous in the emission line ($\sim 10^{42.4}$ erg s*-1*).

Given the large ionisation energy necessary to produce Civ, it is expected that Civ requires either an AGN or very hot stars. It is therefore perhaps not surprising that most Civ emitters at $z\sim 1.5$ do not have colours consistent with SF galaxies at that redshift, but lie in the region of $z\sim 2.2$ Ly$\alpha$ emitters. Note that all Civ sources with all the two colours required for Fig. 3 are spectroscopically confirmed, so the unusual colours cannot be attributed to a wrong selection. It is therefore crucial to consider the contamination by Civ emitters to samples of NB selected Ly$\alpha$ emitters: without redshifts, when using colours, many lower redshift Civ sources will be confused with higher redshift Ly$\alpha$ emitters, as noted by Sobral et al. (2017). Civ have unusual spectral shapes in other bands as well. For example, the criteria Konno et al. (2016) used for selecting Ly$\alpha$ emitters at $z\sim 2.2$ ($(U-NB)>0.5$ and $(B-NB)>0.2$), would select 7 spectroscopically confirmed Civ emitters as Ly$\alpha$ emitters. Note this corresponds to half the sample of confirmed Civ emitters. Even when using the criteria defined by Konno et al. (2016) to select ‘secure’ Ly$\alpha$ emitters ($(U-NB)>0.9$ and $(B-NB)>0.2$), we would still select 5 spectroscopically confirmed Civ into a Ly$\alpha$ sample. These Civ emitters are typically luminous, so they result in contamination of the bright end of the Ly$\alpha$ distribution (Sobral et al., 2017; Matthee et al., 2017). Because this contamination is mostly at bright fluxes, it is less important in deep but small area NB surveys (e.g. Trainor et al., 2015).

4.2 Observed and rest-frame $U-B$ colours

Fig. 4 displays the distribution of observed $(U-B)_{\rm obs}$ colours for our emitters, while the mean and median colours are listed in Table 3. For individual sources, the numbers are listed in Tables 6, 7 and 8. At $z\sim 0.7-1.5$, $(U-B)_{\rm obs}$ approximately traces the rest-frame UV. Note that all but 2 Ciii] emitters have both $U$ and $B$ measurements. Note however that $U$ is contaminated by the emission line, thus the colours should be interpreted with caution. The $(U-B)_{\rm obs}$ corresponds to approximately:

• •

Cii]: restframe 1380 Å and 2275 Å

• •

Ciii]: restframe 1865 Å and 2175 Å

• •

Civ: restframe 1510 Å and 2165 Å

All three types of emitters studied in this paper have relatively blue colours with mean $(U-B)_{\rm obs}$ colours in the $0.15-0.25$ range. Our emitters are consistent in colour to Ly$\alpha$ emitters selected at $z\sim 2.23$ in the CALYMHA survey, which stand at a mean of $0.18$. For the Ciii] and Cii] source with disky morphology as discussed in the previous section, the blue UV colours indicate a relatively dust-free environment.

For comparison, we also present a rest-frame $(U-B)_{\rm rest}$ colour, derived from the observed $i-z$ colour (Fig. 5). Our Ciii] emitters are on average slightly bluer ($(U-B)_{\rm rest}=0.38$) than the spectroscopic sample of Ciii] emitters at $z\sim 1$ from Du et al. (2017), which characterise their emitters as being blue and low-mass with little dust extinction. Note however that the two distributions are perfectly compatible within the full distribution of values.

4.3 UV slope $\beta$

The dust in a galaxy absorbs the UV radiation coming from an AGN or from massive, young stars. Despite it depending on many other properties (see for example Bouwens et al., 2009), the slope of the rest-frame UV continuum ($\beta$) is usually used as a simple tracer of the dust extinction in a galaxy. Here, we estimate $\beta$ for our Cii], Ciii] and Civ emitters, defined in the following way:

[TABLE]

where $m_{1}$ and $m_{2}$ are the magnitudes of the source in two observed filters which trace the rest-frame UV, preferentially around the $1500$ Å reference wavelength. $\lambda_{1}$ and $\lambda_{2}$ are the central wavelengths of the two filters.

Given that our emitters are at 3 different redshifts, it is not simple to have filters that trace exactly the same rest-frame wavelengths. The best choices of filters to trace the rest-frame UV included the $U$ filter, which can be contaminated by the emission lines. We chose filters to match the convention used in other studies and ease comparisons:

• •

Cii]: NUV, U

• •

Ciii]: U, B

• •

Civ: U, V

Note that for Ciii] emitters, the $U$ filter traces a slightly redder rest-frame wavelength compared to other studies and Cii] and Civ, which may bias $\beta$ to slightly redder values.

We list the averages of the $\beta$ slope in Table 3. The $\beta$ slopes of our emitters indicate a steep UV continuum with potential low dust attenuation, within the same ranges as Ly$\alpha$ emitters at $z\sim 2.23$, but with Ciii] being slightly redder.

We show the average $\beta$ slope compared to the average absolute UV magnitude $M_{\rm UV}$ in Fig. 6. The relation between the UV slope and absolute UV magnitude has been shown to not depend significantly on redshift for LBGs, thus making it a good probe for studying galaxies at all cosmic epochs (Bouwens et al., 2009; Smit et al., 2012).

Fig. 6 shows that the Cii] and and Civ emitters are consistent in UV properties with other Lyman break selected SF galaxies at higher redshift ($z\sim 2.5-4$, Bouwens et al., 2009) and with Ly$\alpha$ selected galaxies at $z\sim 2.2$. It is important to note that Ly$\alpha$ and LBG selected samples are generally biased towards blue, less massive, metal-poor SF galaxies (Oteo et al., 2015). Note that Ly$\alpha$ can also probe extremely dusty galaxies unlike the LBG technique (Oteo et al., 2015; Matthee et al., 2016). The average $\beta$ slope is consistent with the results obtained from the observed and intrinsic colours, indicating that Cii] and Civ emitters are relatively blue objects with little dust extinction. At first glance, this is quite surprising, because, as was discussed in previous sections, a large fraction of Cii] emitters and the bulk of Civ emitters have properties consistent with AGN. However, young, dust-free, quasar-like AGN will have steep UV continua, similar to those measured for Civ and Cii] emitters.

Ciii] emitters have relatively flat slopes, indicating a redder UV continuum. This means Ciii] emitters have $\beta$ slopes consistent with more general populations of SF galaxies such as those selected from H$\alpha$ (Oteo et al., 2015). The Ciii] emission line may therefore be a good, unbiased tracer of SF galaxies with a range of properties.

4.4 EWrest distribution

We also investigate the distribution of rest-frame $EW_{\mathrm{rest}}$ in the sample of emitters. We find that the average EWs are high. This could be caused by Ly$\alpha$ interlopers, which can have large observed EWs.

Therefore, as a further conservative step, we attempt to bring any contamination from Ly$\alpha$ emitters (which may have high EWs) to virtually zero. We do this by applying colour cuts targeting the Lyman break in $z\gtrsim 2$ galaxies to further remove any potential Ly$\alpha$ interlopers from the sample:

[TABLE]

The distributions of the resulting samples with very high purity are given in Fig. 7. Averages are listed in Table 3, while individual values are given in Tables 6, 7 and 8.

We find a significant population with large rest-frame EWs for all three emitter species, potentially extending up to $200$ Å. The distributions of the three populations drop in numbers towards high EWs. The small bump in the distribution of Ciii] emitters in the largest EW bin is not statistically significant. On average, Civ emitters have lower EWs compared to Cii] and Ciii], which correlates with the brighter average magnitudes of these Civ sources (see for example, Fig. 6). This is caused by the prevalence of quasars among Civ sources. Overall, there are relatively few sources with very low EWs close to our selection limit of $16$ Å. Because of the way the selection of emitters is performed (see for details Sobral et al., 2017), sources will be classified as emitters if they pass the EW limit cut of $16$ Å and a signal-to-noise cut. For low S/N sources, the EW needs to be higher for a source to pass the selection compared to a source which is bright in the BB. This explains the proportionally larger number of Civ emitters with low EW compared to Cii] and Ciii] sources. We would like to note that while EWs can be biased high because of a number of geometrical reasons (see discussion below, e.g. offset of emitting line regions with respect to underlying UV radiation), the line fluxes given our apertures will be reliable, as all the emission should be captured, more so when compared to, for example slit observations.

Another explanation for the larger EWs would be that EWs measured from the NB are not accurate with respect to EWs from spectroscopy. However, we do not believe this could be the case as NB EWs have been found to be reliable when compared to spectroscopic observations (e.g. for lines such as H$\alpha$, Ly$\alpha$ Sobral et al., 2015a, b, 2016), specifically for CALYMHA survey follow-up of Ly$\alpha$ emitters (Sobral et al. in prep). Some Cii] and Civ sources, including those with large EWs, are confirmed spectroscopically, however, none of the large EW Ciii] emitters has a $z_{\rm spec}$. The available spectroscopy is biased towards continuum bright sources, which do not have large EW. Despite potentially strong emission lines with large EW, many continuum-faint Cii], Ciii] and Civ emitters in our sample, with NB magnitudes below $22-23$, were never spectroscopically followed up precisely because they were not bright enough. While we removed sources of systematic errors and potential interlopers to the best of our abilities, without targeted spectroscopy it is not possible to fully confirm the large EW measurements.

A word of warning is that the errors on the rest-frame EW can be large, with an average of $\sim 60$ per cent of the EW value. Hence, some of the values can be $60$ per cent lower or higher than estimated here. Furthermore, as we will discuss in Section 4.5, our morphological results show that we may be tracing specific regions within galaxies with little to no UV continuum, which may bias the EW towards higher values.

Spectroscopic observations are necessary to pose tighter constraints on the EW values, reduce the error bars and further investigate the validity of NB observations for measuring the EW of Cii], Ciii] and Civ emitters. EW could also be overestimated due to variable sources, which we discuss in detail in Section A. While variability could explain part of the population, the entire population of large EW sources cannot be explained this way.

Another possibility to explain the large EW are offsets between the main line emission region and the galaxy stellar light (see Figs.  8, 9, 10 and 11). Such offsets could be caused by systematic astrometric errors, but can also be caused by a real physical separation in the peaks of the underlying continuum and the line emission. While investigating this avenue with the INT NB data alone is not possible because of the large point spread function (psf, $\sim 2$ arcsec), we discuss this in more detail using high resolution HST data in Section 4.5. As will be shown in Section 4.5, astrometric errors are likely not the cause of the large EW.

The distribution of rest-frame EW for the Cii] emitters extends up to $200$ Å (as shown in Fig. 7). The chances of all of these high EW sources being interlopers or variable sources is small as explained above. We have also been extremely conservative in our selection and one source with a large $EW_{\rm rest}$ of 84 Å as measured from our NB data has a spectroscopic redshift confirming it to be a Cii] emitter at $z\sim 0.68$ (although this specific source could still be variable).

The rest-frame EW distribution of Ciii] emitters extends up to large values, with an average of $\sim 100$ Å (Fig. 7). These values place our Ciii] sample in a different regime than other samples from the literature. Very recently, Du et al. (2017) published a spectroscopically-selected Ciii] sample at $z\sim 1$ with a median $EW_{\rm rest}$ of $1.3$ Å. They also compared with results from the literature at redshifts up to $\sim 6$ and found that, on average, the Ciii] emitters discovered till now have EW of at most $\sim 25$ Å and are hosted by young, low mass, SF galaxies. The average EW of our Ciii] emitters is also higher than what was found by Shapley et al. (2003) in stacks of LBGs, where their strongest Ly$\alpha$ emitters have mean rest-frame Ciii] EW of about $10$ per cent of Ly$\alpha$, or $\sim 5.4$ Å. The spectroscopically confirmed sources in our Ciii] sample have lower EWs than our average ($14\pm 1$, $21\pm 9$, $26\pm 3$ and $39\pm 5$ Å versus the average of $93\pm 59$ Å). Note that both our $z_{\rm spec}$ confirmed Ciii] sources and the literature samples were chosen for spectroscopic followed-up up because of their relatively bright nature. Since sources with spectra are continuum selected this will bias samples towards lower EWs, while our samples are line emission selected, thus finding higher EWs. Hence it is not surprising that the high EW Ciii] sources have not been confirmed with spectroscopy, in the absence of a dedicated, targeted spectroscopic follow-up programme. In addition, as mentioned earlier, any offset between the line emitting region and the stellar light could also explain the large EW we are measuring. Therefore, for continuum selected sources, placing the spectroscopic slit or fibre on the main continuum emitting region does not guarantee the line emission will be captured. Spectroscopy focusing on the line-emitting regions is however crucial to unveil the nature of these emitters and understand the source of the high EW emission.

The Civ emitters have average rest-frame EWs of $52$ Å. It is important to note that a significant fraction of these are also spectroscopically confirmed, including a source with a EW of $\sim 100$ Å measured using the NB and BB data (see Fig. 7). The values are consistent within the ranges measured from a large sample of $\sim 150$ Type II quasars at $z\sim 2-4.3$ by Alexandroff et al. (2013). They measure rest-frame EW ranging from 10 to 100  Å for their most secure Type II sample, with an average of about $40$ Å. Note that the average Civ line fluxes in Alexandroff et al. (2013) are about $(1-1.2)\times 10^{-16}$ erg s*-1*, which is smaller than our average of $7.7\times 10^{-16}$ erg s*-1*, but consistent within the spread of the values. Without spectroscopic information, many of our Civ sources would have been likely Ly$\alpha$ candidates at higher redshifts. While traditionally it has been assumed that large EW emitters are Ly$\alpha$, without clear, secure redshift information the emitter can be misidentified. It thus becomes apparent that Civ emitters can represent an important contaminating population, as was concluded in Sobral et al. (2017) and in Stroe et al. (2017b).

4.5 HST morphologies

Ancillary high-resolution space based optical data is crucial in unveiling the nature of our emitters, especially in understanding their colour properties from Sections 4.1, 4.2 and 4.3 and their high EWs (Section 4.4). For the sources with coverage in HST, we show their morphologies in Figs. 8, 9 and 10. The optical properties of the emitters are listed in Tables 6, 7 and 8 .

Twelve out of the 16 Cii] sources have HST coverage, revealing a mix of morphology types. Seven Cii] emitters have a disky or spiral morphology, while four have a very bright nucleus and a spiral structure, indicative of a Seyfert nature. These Seyfert-like sources are also the most luminous in the emission line and have the lower EW (see Section 4.4). The possible mixed nature of Cii] emitters, some being powered by SF and some by AGN is therefore not only supported by the optical and UV colours of the emitters (Sections 4.1, 4.2, 4.3), but also by the morphologies.

Out of our 24 Ciii] sources, there are 19 with HST coverage, presenting a wide variety of morphologies. Some Ciii] emitters have a spiral structure, others a disturbed morphology and multiple nuclei, others consisting of 2 or more interacting components. There are also two sources with a UV bright core, indicative of a Seyfert nature, which are also among the brightest in the Ciii] emission line.

Ten out of 17 Civ sources have space telescope coverage and they are all point sources even at HST resolution. The HST data support a scenario where Civ emitters are predominantly quasars: all four sources with Chandra coverage have a direct X-ray detection, with an additional three sources with a radio detection. We note that the line luminosities of these sources span the $L_{\rm CIV}\sim 10^{42.6}-10^{42.9}$ erg s*-1* range.

We find that many line emitters, especially Cii] and Ciii] are offset from the HST detection. This is illustrated in Fig. 11 where we plot the absolute offset between the emission line regions and the UV continuum against the rest-frame EW. As the distribution of offsets shows, larger offsets are more prevalent for Ciii] emitters, while few large offsets are detected in the Civ emitter sample. There is also a trend of increasing EW with spatial offset when looking at the population as a whole. It is important to note that sources where the galaxy disk is most offset from the emission line detection in CALYMHA are also among the sources with the largest EW (see discussion of the large EW in Section 4.4). While there are a few Cii] and Ciii] emitters with large EW at small offsets, the bulk of these emitters present offsets larger than 0.2 arcsec. At least for part of the Cii] and Ciii] sample, a reason for large $EW_{\rm rest}$ could be the offset between the main line emission region and the location of the bulk stellar light. Therefore, we have tentative evidence that some of the emission line regions can be offset by $1^{\prime\prime}$ from the brightest UV continuum component, which would result in large $EW$ (see Figs. 8 and 10). While the offset can be caused by a physical displacement, a caveat to note is the large PSF of the NB and BB INT observations used to calculate the EW, which can be prone to larger astrometric uncertainties and thus contribute towards the offsets we are measuring. We investigate the offset between CALYMHA sources and the HST counterparts and found that astrometric errors are in the $0.2-0.3$ arcsec range, which is the expected value for INT observations (Sobral et al., 2017). Therefore, the large offsets found here are most likely of a physical origin, either by the presence of an AGN, and in the case of SF sources, by an offset between the stellar light and the line emitting region. Another option, at least for some of the sources which are not spectroscopically confirmed, would be that they are in fact higher redshift, lensed Ly$\alpha$ emitters. This may explain the offsets as well as the large EW.

4.6 Chandra X-ray and VLA radio counterparts

While no emitters have direct FIR detections (and thus none can be extremely star-forming at the $1000$ M⊙ yr*-1* level), some sources do have an X-ray or radio counterpart, which we discuss here and summarise in Table 4. Five Cii] have Chandra coverage and 13 are covered by radio data. Nine Ciii] sources are covered with Chandra and 22 with radio coverage, while 4 Civ are covered with X-ray and 13 with radio.

In the case of Cii], the results from the optical morphologies and colours are supported by the X-ray data: the disk galaxies with Chandra coverage do not have an X-ray counterpart, while the Seyferts have counterparts. None of the Cii] sources is active in the radio (out of 13 with coverage). The HST data reveals that some of the Cii] emitters have a spiral-like morphology. In that canse any radio emission will be powered by SF, which is low compared to AGN-powered radio emission. Additionally, only 5 per cent of Seyferts are radio-loud. Therefore, the lack of radio detection for Cii] emitters is not surprising given the depth of the radio observations.

As mentioned in Section 4.5, the Ciii] sources have a range of morphologies. One of the two sources with a UV bright core, indicative of a Seyfert nature, has coverage in Chandra and VLA and possesses an X-ray and radio detection. As expected for normal SF galaxies, none of the other Ciii] sources have radio or X-ray counterparts. A detection would imply very large SFRs of $\sim 1000$ M*⊙* yr*-1*.

The radio and Chandra data for Civ, in agreement with HST, support a scenario where Civ emitters are predominantly quasars: all four sources with Chandra coverage have a direct X-ray detection, with an additional three sources with a radio detection.

In conclusion, the X-ray and radio data suggest that Ciii] is mostly powered by SF. Civ and some Cii] emitters are active in the X-ray, indicating they are young, actively accreting in the radiatively driven, quasar mode. Only a few Civ and no Cii] have radio detections, which would be indicative of a more evolved AGN in the mechanical, radio-loud phase.

4.7 Average SFRs and BHARs

4.7.1 Chandra stacking

To evaluate the level of activity of the central supermassive black hole in our emitters, we can estimate the black hole accretion rate (BHAR) which can be derived from X-ray data. We follow the method described in Calhau et al. (2017) to measure the average BHAR for the Cii], Ciii] and Civ emitters by stacking sources with Chandra coverage.

We make a $20^{\prime\prime}\times 20^{\prime\prime}$ cutout around the position of each source and take the mean to obtain an average image which is used to calculate the average flux of the emitters. We establish a stacking radius of 2 arcsec and convert the counts inside to flux by following the indicated procedure in Elvis et al. (2009), with a photon index $\Gamma$ of 1.4 and Galactic absorption of $2.7\times 10^{20}$ cm*-2*. We convert the flux $F_{\rm X}$ to an X-ray luminosity $L_{\rm X}$ using:

[TABLE]

We obtain the bolometric luminosity by multiplying by 22.4 (Lehmer et al., 2013). The BHAR is obtained by:

[TABLE]

where $\epsilon=0.1$ is the accretion efficiency (following Lehmer et al., 2013) and $c$ is the speed of light. Errors on both $L_{\mathrm{X}}$ and the BHAR are calculated by taking the standard deviation of the distribution obtained from a bootstrapping analysis of the data.

4.7.2 FIR and radio stacking

We use FIR and radio data to obtain an average SFR in our sources.

To obtain an estimation of the dust-obscured SFR from the FIR emission, we start by mean stacking our sources using the Herschel bands at 100, 160, 250, 350 and 500 $\mu$m (PACS and SPIRE instruments). Aperture corrections were applied for the PACS 100 and 160 $\mu$m bands as specified in the PACS PEP release notes. In the SPIRE 250, 350, 500 $\mu$m, the fluxes were taken from the peak value in each stack. Note that we do not have any detections and can only provide upper limits. We estimate the FIR luminosities by fitting modified black body templates (using the SWIRE template Library, Polletta et al., 2007) to the upper limits and integrating the best fit between 8 and 1000 $\mu$m. We use the total FIR luminosity $L_{\rm FIR}$ to compute the SFRs, assuming a Chabrier (2003) IMF by using:

[TABLE]

We find no detections in any of the stacks and thus can place upper limits on the SFRs. The results of the stacking do not change if we use all sources with coverage or only the sources with Chandra coverage.

Radio at $1.4$ GHz can be used as a dust-free SF indicator on timescales longer than FIR or emission lines. The radio stacking procedure is the same as the X-rays. To convert the radio luminosities $L_{\rm radio}$ to SFRs, we use the conversion determined by Yun et al. (2001), adapted to a Chabrier (2003) IMF:

[TABLE]

Note that in the case of radio, unlike FIR, a direct detection is caused by an AGN rather than SF. We find that if we remove all detections, our radio stacks all provide non-detections in the same range as the FIR ones.

4.7.3 Stacking results

The results of our X-ray, FIR and radio stacking analysis are listed in Table 5. The stacked X-ray image can be found in Fig. 12, which shows clear detections for Cii] and Civ and a very faint/non detection for Ciii]. We plot our $BHAR$ results and compare with similar measurements obtained for H$\alpha$ and FIR selected SF galaxies, Ly$\alpha$ emitters and AGN (Delvecchio et al., 2015; Stanley et al., 2015; Calhau et al., 2017, Calhau et al. in prep) in Fig. 13. Note however the low number statistics, driven by the small area of the publicly available Chandra images.

The stacking analysis indicates that Cii] emitters are bright in the X-rays and are consistent with AGN activity ($BHAR=0.10\pm 0.06$ $M_{\odot}$ yr*-1*), as revealed by Fig. 13. Cii] emitters accrete matter onto their BH at a much higher rate than SF galaxies. The Cii] emission, as suggested, for example, by Best et al. (2002) could then be in part powered by shocks near the BH.

Once the only source with an X-ray detection is removed, Ciii] emitters have an upper limit X-ray luminosity consistent with SF activity and a low BHAR ($0.02\pm 0.01$ $M_{\odot}$ yr*-1*), similar to what is measured from H$\alpha$ and FIR selected SF galaxies (Delvecchio et al., 2015; Calhau et al., 2017).

The average X-ray luminosity of the Civ sources is very high, $\sim 10^{44.4}$ erg s*-1*, supporting an active nucleus which accretes matter at a rate of $\sim 0.83$ $M_{\odot}$ yr*-1*. This value is well above the rate expected for main sequence SF galaxies, lying in the AGN region of Fig. 13. This is consistent with the optical properties of the Civ emitters discussed in previous sections.

There are no detections in the FIR bands, so we are only able to provide $3\sigma$ SFR upper limits of $15$, $75$, $141$ M⊙ yr*-1* for Cii], Ciii], Civ, respectively. Note that the different values are mostly driven by the different redshifts of the three types of emitters. The FIR non-detections are mostly caused by the relatively low number of sources in each stack compared to the detections from Calhau et al. (2017), who stacked about ten times more sources than included in this analysis. In order to further test this, we stack the H$\alpha$ emitters at $z\sim 0.84$ in the FIR, using the same number of sources as our Ciii] emitters and find no detection. We therefore conclude that Ciii] emitters are fully consistent with being similar to H$\alpha$ selected samples at a similar redshift, including the fact that, as seen before, they are slightly redder than LBGs.

Similarly, we do not have a detection in the radio stacks after removing direct detections (which are caused by AGN, not SF). These upper limits, as for the FIR, are not very stringent ($\sim 17,35,115$ $M_{\odot}$ yr*-1* for Cii], Ciii] and Civ respectively). The range of typical SFR from H$\alpha$ emission at $z\sim 0.8-1.5$ is in the range $8-16$ $M_{\odot}$ yr*-1* (Sobral et al., 2013), well below our FIR and radio depths.

4.7.4 UV SFRs

We also attempt to estimate the SFRs of Cii], Ciii] and Civ emission from the restframe UV emission. While UV emission is produced by young, massive stars, thus indicating recent SF episodes, this SFR estimator can be heavily affected by the dust content of the host galaxy. As well, in the cases of galaxies hosting an AGN, accretion can also produce significant UV, making it harder to disentangle the contribution from SF.

We estimate the dust extinction $A_{\rm UV}$ in the restframe UV by using the dust extinction law from Calzetti et al. (2000):

[TABLE]

where $\beta$ is the UV restframe slope. In principle this relation has been defined for $\beta$ measured between $1300$ and $2600$ Å. However, since these wavelengths are not available directly, we use the $\beta$ as measured from our data (see Section 4.3) with the warning that we are tracing slightly different rest-frame wavelengths which can introduce uncertainties in the dust attenuation estimates.

We obtain the $SFR_{\rm UV}$ from the UV data using the conversion presented in Kennicutt (1998), corrected for the Chabrier (2003) IMF:

[TABLE]

where:

[TABLE]

where $M_{\rm UV,corr}$ is the absolute UV magnitudes corrected for dust attenuation $A_{\rm UV}$.

We list the average and median SFRs for Cii] and Ciii] in Table 5. We would like to point out that these values depend on a number of assumptions and can be affected by uncertainties. The $\beta$ slope is not measured exactly at the wavelengths defined in Calzetti et al. (2000). A large unknown is the proportion of UV emission that can actually be attributed to SF, and how much to other support such as AGN activity. To alleviate this as much as possible, we remove sources which we were able to classify as AGN from our ancillary data. In the case of Civ emitters, which we classified predominantly as quasars, we do not report an UV based SFR. As discussed in the paper, the properties of Civ emitters are consistent with a population of AGN/quasars, especially for bright sources ($M_{\rm UV}\sim-22$). Therefore most of the UV for Civ sources comes from accretion precesses onto the central BH.

We note that for both Cii] and Ciii], the mean SFR is much larger than the median value, indicating a few outliers with very large SFR bias the measurements.

The $SFR_{\rm UV}$ values for Cii] emitters are typically lower than those of Ciii] emitters, with averages a factor of 25 lower and medians a factor of 10 lower. This supports the scenario discussed in previous sections of the paper, where Cii] emitters are most likely hosted in SF galaxies and/or AGN. The average SFR for Ciii] emitters is consistent with values obtained in stacks of typical, H$\alpha$-selected SF galaxies, using FIR as SF tracer. Calhau et al. (2017) find a values of $13$ M⊙ yr*-1* at $z\sim 0.85$ and $32$ M⊙ yr*-1* at $z\sim 1.5$. Nevertheless, this comparison should be made with caution given the difficulty in comparing different SFR estimators.

5 Discussion

We investigate the nature of Cii], Ciii] and Civ line emitters by jointly interpreting their emission line properties with broad band optical colours as well as ancillary space-based optical and X-ray data.

5.1 Cii] emitters $z\sim 0.68$

Cii] emitters at $z\sim 0.68$ have optical colours spanning across the colour space of SF galaxies at their redshift into a regime more typical of SF sources at higher cosmic distances. Such unusual optical colours may indicate that while part of the Cii] emitter population is powered by SF, at least some sources host an AGN, in agreement with luminosity function and cosmic ratio results from Stroe et al. (2017b). This is also supported by the HST morphology of Cii] sources: while some sources have spiral-like morphologies, part of the population, particularly the sources with atypical colours, have a bright nucleus accompanied by a disk. While the bright nucleus could be indicative of a nuclear starburst surrounded by a stellar disk, this scenario is not supported by the X-ray data which reveals that these Cii] emitters have BHAR rates consistent with an AGN at their core. Cii] emitters have relatively steep $\beta$ slopes, which indicate a relatively blue UV continuum. This is also supported by the lack of a detection in the FIR stacks of these sources. Cii] emitters also have high rest-frame EWs, which could be caused by either the offset we observe between the main stellar disk and the line emitting region, or by AGN activity.

Traditionally, it was thought that Cii] emission is mostly triggered in shocks around AGN (De Breuck et al., 2000; Best et al., 2000, 2002), however this resulted from studies of this line being done almost exclusively in bright, active galaxies. Overall, our results indicate that Cii] emitters may actually powered in part by blue, SF galaxies at lower luminosities and in part by Seyfert-like sources with young, actively-accreting AGN at brighter line luminosities.

5.2 Ciii] emitters at $z\sim 1.05$

Most Ciii] emitters at $z\sim 1.05$ have optical colours consistent with SF galaxies at their redshift. Furthermore, their low average BHAR is also consistent with SF galaxies. The $\beta$ UV slopes of Ciii] emitters are consistent with H$\alpha$ selected galaxies, indicating Ciii] emission might be suitable avenue for selecting a wide range of SF galaxies. Ciii] sources are also mostly X-ray and radio quiet, as expected for SF galaxies. Most emitters have disky, clumpy or disturbed optical morphologies, indicative of either single or interacting SF galaxies. The offset optical disk or the clumpy/disturbed morphologies might explain the large EW we find for some Ciii] emitters. The few direct X-ray and radio detections ($<10$ per cent) correlate with a bright nucleus plus a disk optical morphology, which indicates that the brightest Ciii] emitters host an AGN at their core.

Traditionally Ciii] emission was thought to originate from AGN. More recent results focusing on SF galaxies find that $\sim 20$ per cent of local galaxies have the Ciii] feature and the strongest Ciii] emission is fostered at low metallicities (Rigby et al., 2015). Part of their sample are Wolf-Rayet galaxies, indicating recent (few Myrs), short-lived ($<1$ Myr) strong starbursts. At higher redshifts ($1.5-3$), Stark et al. (2014) find that almost all of their Ciii] emitters also possess strong Ly$\alpha$. Stanway et al. (2016), in agreement with Jaskot & Ravindranath (2016), showed that the presence of binary stars can increase the ionising flux by up to $60$ per cent in low metallicity ($<0.3Z_{\odot}$) environments, which could also lead to a boost in Ciii] production.

While these works indicate that strong Ciii] is produced in relatively low-metallicity SF galaxies, our results strongly support a scenario where Ciii] emitters trace a more general population of SF galaxies at $z\sim 1$, with very few bright emitters being powered by AGN. This is similar to what is found in studies of Ly$\alpha$ and H$\alpha$ emitters at $z>2$ (Matthee et al., 2015; Konno et al., 2016; Sobral et al., 2017; Matthee et al., 2017). However, as shown in Stroe et al. (2017b), at fixed UV magnitude of line flux, the number density of Ciii] emitters is substantially lower than H$\alpha$ or LBG SF galaxy populations, indicating only a fraction of galaxies have strong Ciii]. The average line ratios imply that Ciii] is $\sim 20$ times fainter than H$\alpha$ and $\sim 4$ times fainter than observed Ly$\alpha$ (Stroe et al., 2017b).

An open question relates to the large $EW_{\rm rest}$ we measure for bulk of the Ciii] population. According to photoionisation modelling, the largest EWs that can be reached with star formation are of the order of $25$ Å (Jaskot & Ravindranath, 2016). As discussed earlier in the paper, the large EW cannot be solely attributed to systematic errors in the EW estimation or effects such as variability. Whether the discrepancies between our observations and the modelling can be attributed to spatial offsets between emission line regions we cannot ascertain for sure with the current data. Spectroscopic follow-up will reveal the detailed physics of the Ciii] emitters, as well as confirm or dismiss the large EW measured here.

5.3 Civ emitters $z\sim 1.53$

The direct detections in the X-ray, the high accretion rates characteristic for bright AGN, the universal optical quasar morphologies, the unusual optical colours and blue UV slopes and large rest-frame EW, all reveal that Civ is produced in intense radiation fields characteristic to young, virtually dust-free actively accreting quasars. The number distribution of Civ emitters as function of line luminosity also indicates they are primarily a quasar population, being well described by a power-law (Stroe et al., 2017b).

In a sample of lensed, low mass galaxies at $z\sim 1.5-3$, Stark et al. (2014) find ubiquitous joint detections of Civ and Ciii], which require young stellar populations, increased electron temperatures and an enhanced ionising output from metal poor gas and stars. While the modelling is still uncertain, their work therefore indicates that the Civ emission is mainly driven by young stars, without particular need for an AGN contribution.

Our results however, seem to support a scenario where the bulk of the Civ is produced in galaxies hosting powerful AGN at least at $z\sim 1.5$. One important aspect to note is that the rest-frame UV magnitudes of the sources from Stark et al. (2014) are similar to ours, so a comparison is appropriate from this point of view. However, the galaxies in the Stark et al. (2014) sample are at a higher redshift and also span on average lower EW values ($<15$ Å for the Stark sample versus $34\pm 58$ Å for our sample).

6 Conclusions

We characterise the properties Cii], Ciii] and Civ emitters at $z\sim 0.68,1.05,1.53$. These sources were selected from the first blind, statistical study of Carbon emitters, uniformly selected in the COSMOS and UDS field as part of the CALYMHA survey. In this paper, we focus on the UV, optical, X-ray, FIR and radio properties of these emitters. Our main results are:

• •

Cii] emitters at $z\sim 0.68$ have disky structure or bright nucleus with a disk morphology (Fig. 8) and blue UV colours (Figs. 4, 6). Some of the brightest line emitters are strong X-ray sources (Fig. 12), with black hole accretion rates (Fig. 13) characteristic of AGN. Our results therefore indicate that Cii] traces a mixed population of SF galaxies and AGN, especially at the brightest line luminosities. For the brighter emitters, our results are in accordance with the theoretical expectation that Cii] emission is triggered by shocks around AGN.

• •

Ciii] emitters at $z\sim 1.05$ in our study have typical optical colours for SF galaxies (Fig. 3) and morphologies indicative of either isolated or interacting galaxies (Fig. 9). Their relatively blue rest-frame UV and optical colours (Figs. 5, 6) and low black hole accretion rates (Fig. 13) are consistent with general populations of SF galaxies.

• •

The point-like HST morphologies (Fig. 10), the strong X-ray detections implying high accretion rates (Figs. 12,  13) and the blue optical and UV colours (Figs. 4, 6) indicate that Civ emitters are almost universally blue/non-dusty quasars.

• •

We discover a large, previously-undetected population of Cii], Ciii] and Civ emitters with large rest-frame EW, of above $50$ Å, potentially up to 200 Å (Fig. 7). In stark contrast, the typical EW of continuum selected Ciii] samples from the local Universe up to $z\sim 6$ is $\lesssim 25$ Å. These large EW might be caused by emission line regions offset from the stellar disk or because of the AGN nature of some of the sources.

While historically thought to originate in AGN, our study revealed some interesting properties of Cii], Ciii] and Civ emitters at $z\sim 0.7-1.5$. While Civ emitters are almost universally quasars, Ciii] emission is consistent with being produced in a wide variety of SF galaxies and Cii] emission may trace either an AGN or SF. Further spectroscopy is crucial however to unveil their powering source and the physics of the large EW we measure in a fraction of the population. In the companion paper, Stroe et al. (2017b), we further explore the samples and derive luminosity functions and cosmic average line ratios.

Acknowledgements

We would like to thank the anonymous referee for her/his valuable input that helped improve the clarity and interpretation of our results. DS acknowledges financial support from the Netherlands Organisation for Scientific research (NWO), through a Veni fellowship. IO acknowledges support from the European Research Council in the form of the Advanced Investigator Programme, 321302, cosmicism. CALYMHA data is based on observations made with the Isaac Newton Telescope (proposals 13AN002, I14AN002, 088-INT7/14A, I14BN006, 118-INT13/14B, I15AN008) operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Also based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under ESO programme IDs 098.A-0819 and 179.A-2005. We are grateful to E. L. Wright and J. Schombert for their cosmology calculator. We would like to thank the authors of NumPy (van der Walt et al., 2011), SciPy (Jones et al., 2001), Matplotlib (Hunter, 2007) and AstroPy (Astropy Collaboration et al., 2013) for making these packages publicly available. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA’s Astrophysics Data System. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in Ochsenbein et al. (2000). This research has made use of “Aladin sky atlas” developed at CDS, Strasbourg Observatory, France (Bonnarel et al., 2000; Boch & Fernique, 2014). The CALYMHA catalogue used for this study is publicly available from Sobral et al. (2017).

Appendix A Effect of variability on the measured EW

Since the NB and BB observations were taken in different years, it is possible that any variable source (such as an AGN) is falsely selected as a line-emitter or has an over- or under- estimated EW. This is because, on such timescales, the continuum UV magnitudes of AGN may change by $\sim 1$ magnitude (e.g. Ulrich et al., 1997; de Vries et al., 2003; Favre et al., 2005; Sarajedini et al., 2011; Micheva et al., 2017). Thus, there is the possibility that some of the large EW sources are not line-emitters, but variable sources caught in a high state in the NB, but in a low state in the BB, thus mimicking a colour excess. Another possibility is that some of the EW are overestimated because of variability.

We empirically estimate the number of variable sources that may contaminate our sample as follows. If such variable sources exist, we expect not only to find them in the sample of line-emitters, but also in samples of absorbers (with negative excess). Therefore, we select galaxies with statistically significant negative excess (from Sobral et al., 2017). Among these, we focus on X-ray AGN by matching negative excess sources with the catalogue of X-ray detected sources from Civano et al. (2012), resulting in 13 matches (all with available spectroscopic redshifts). We remove four sources for which the absorption feature corresponds to Ly$\alpha$ forest between $912-1216$ Å, as well as two sources for which the absorption feature can be explained by Civ absorption. The remaining 7 AGN are at redshifts $0.9<z<2.0$, and no obvious absorption feature is present at the wavelength traced by the NB. Thus, we assume that these 7 AGN are in fact variable sources that mimic an absorption feature. These AGN have randomly distributed excesses between -0.4 and -1.1. If we would assume that such variable sources with such excess exist, we would expect 7 line-emitters to be variable sources with $EW_{\rm obs}\approx 30-110$ Å. Therefore, while a few of the large EW sources might be variable AGN, variability cannot fully explain the distribution of EW. One possibility we cannot rule out is variability with a smaller magnitude ranges ($-0.2$ to $0.2$ mag), because of photometric errors. Therefore while some EW may be boosted, in some cases, the measured EWs will be lower than in reality. Spectroscopic follow-up is required to completely verify some of the highest EWs.

Appendix B Properties of individual Cii], Ciii] and Civ emitters

In Tables 6, 7 and 8 we list the individual properties of Cii], Ciii] and Civ emitters, respectively. We give the coordinates of the sources, their optical HST morphology, possible X-ray and radio counterparts, as well as line luminosity, rest-frame EW ($EW_{\rm rest}$) and the observed $(U-B)_{\rm obs}$ colour.