Rotation in [CII]-emitting gas in two galaxies at a redshift of 6.8

TL;DR

This study presents the first ALMA spectroscopic confirmation and velocity structure analysis of two galaxies at z > 6.8 using [C II] emission, revealing rotation-dominated, turbulent disks early in cosmic history.

Contribution

It provides the first measurements of [C II] velocity structures in galaxies during Reionization, demonstrating the potential of ALMA for studying early galaxy dynamics.

Findings

Detected high [C II] luminosity in galaxies at z > 6.8.

Observed velocity gradients suggest rotation-dominated disks.

Galaxies have similar stellar-to-dynamical mass ratios as later epoch galaxies.

Abstract

The earliest galaxies are expected to emerge in the first billion years of the Universe during the Epoch of Reionization. However, both the spectroscopic confirmation of photometrically-selected galaxies at this epoch and the characterization of their early dynamical state has been hindered by the lack of bright, accessible lines to probe the velocity structure of their interstellar medium. We present the first ALMA spectroscopic confirmation of such sources at z > 6 using the far-infrared [C II]{\lambda}157.74{\mu}m emission line, and, for the first time, measurement of the velocity structure, for two galaxies at z = 6.8540+/-0.0003 and z = 6.8076+/-0.0002. Remarkably, the [C II] line luminosity from these galaxies is higher than previously found in `normal' star-forming galaxies at z > 6.5. This suggests that we are sampling a part of the galaxy population different from the galaxies found through detection of the Ly{\alpha} line. The luminous and extended [C II] detections reveal clear velocity gradients that, if interpreted as rotation, would suggest these galaxies have turbulent, yet rotation-dominated disks, with similar stellar-to-dynamical mass fractions as observed for H{\alpha} emitting galaxies 2 Gyr later at cosmic noon. Our novel approach for confirming galaxies during Reionization paves the way for larger studies of distant galaxies with spectroscopic redshifts from ALMA. Particularly important, this opens up opportunities for high angular-resolution [C II] dynamics in galaxies less than one billion years after the Big Bang.

Tables

Table 1. Table 1: Galaxy properties

ID	COS-3018555981	COS-2987030247
$z_{\mathrm{phot}}$	6.76$\pm$0.07	6.66$\pm$0.14
$z_{\left[\mathrm{CII}\right]}^b$	6.8540$\pm$0.0003	6.8076$\pm$0.0002
S/Na	8.2	5.1
$[$C 2] line flux ${\left(\mathrm{Jy}\mathrm{km}{s}^{-1}\right)}^b$	0.39$\pm$0.05	0.31$\pm$0.04
FWHM${}_{\left[\mathrm{CII}\right]}{}^{}\left(\mathrm{km}{s}^{-1}\right)_{}^b$	232$\pm$30	124$\pm$18
158$\mu$m continuum flux ($\mu$Jy)	87c	75c
$L_{\left[\mathrm{CII}\right]}$ $\left({10}^8{L}_{\odot }\right)$	4.7$\pm$0.5	3.6$\pm$0.5
$L_{\mathrm{UV}}$ $\left({10}^{11}{L}_{\odot }\right)$	1.1$\pm$0.1	1.3$\pm$0.1
$L_{\mathrm{IR}}$ $\left({10}^{11}{L}_{\odot }\right)$	1.3c	1.1c
SFRIR $\left(M_{\odot }{\mathrm{yr}}^{-1}\right)$	19c	16c
SFRUV $\left(M_{\odot }{\mathrm{yr}}^{-1}\right)$	19.2$\pm$1.6	22.7$\pm$2.0
$M_{\ast }$ $\left({10}^9{M}_{\odot }\right)$	${1.4}_{-0.2}^{+0.7}$	${1.7}_{-0.2}^{+0.5}$
$M_{\mathrm{dyn}}$ $\left({10}^9{M}_{\odot }\right)$	${10}_{-2}^{+3}$	$4_{-3}^{+9}$
$\Delta v_{\mathrm{obs}}/2{\sigma }_{\mathrm{tot}}$	$0.57\pm 0.16$	$0.52\pm 0.21$
$r_{1/2,\left[\mathrm{CII}\right]}$ (kpc)	$2.6\pm 0.8$	$3.1\pm 1.0$
${\beta }_{\mathrm{UV}}$	$-1.22\pm 0.51$	$-1.18\pm 0.53$
EW([O 3]+H$\beta$) ( Å)[Smit2015]	$1424\pm 143$	$1128\pm 166$

Full text

{affiliations}

Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK

Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA

Leiden Observatory, Leiden University, NL-2300 RA Leiden, Netherlands

Geneva Observatory, University of Geneva, Ch. des Maillettes 51, CH-1290 Versoix, Switzerland

UCO/Lick Observatory, University of California, Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA

Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218

Departmento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile

Centro de Astrofisica y Tecnologias Afines (CATA), Camino del Observatorio 1515, Las Condes, Santiago, Chile

Department of Physics and Astronomy, University of Louisville, Louisville KY 40292 USA

Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

We have obtained spectroscopy with the Atacama Large Millimetre Array (ALMA) at 241–245 GHz of two Lyman-break galaxies (LBGs) COS-3018555981 and COS-2987030247 at an estimated photometric redshift just below 7, roughly 800 million years after the Big Bang. These two sources are luminous in the rest-frame ultraviolet (UV; $L_{\rm UV}\sim 2\times L^{\ast}$[Stark2016]), but are still representative of ‘normal’ star-forming galaxies at $z\sim 7$ with a UV star-formation rate (SFR) of $19-23\,M_{\odot}\rm\,yr^{-1}$. They were selected based on their blue rest-frame optical colours measured in the 3.6 and 4.5$\micron$ Spitzer photometric bands[Smit2015], which strongly constrains the photometric redshift probability distribution to the redshift range $6.6<z<6.9$. These sources are among the most extreme [O 3]+H$\beta$ emitters known at $z\sim 7$[Smit2014, Smit2015]. We observed these sources in a 36 antennae configuration with ALMA (angular resolution of 1.1"$\times$0.7", equivalent to 5.8$\times$3.7 kpc at $z=6.8$) and 24 min on source integration time for each of the targets. We use this spectral scan to search for [C 2] in the redshift range $z_{\rm[CII]}=6.74-6.90$. Our results are summarized in Table 1.

We detect a line in each of the targets at $241.97\pm 0.01\,\rm GHz$ and $243.42\pm 0.01\,\rm GHz$, for COS-3018555981 and COS-2987030247 respectively, at $>5\sigma$ significance in both the one-dimensional spectra and the spectral line-averaged maps (Fig. 1). We derive spectroscopic redshifts of $z_{\rm[CII]}=6.8540\pm 0.0003$ and $z_{\rm[CII]}=6.8076\pm 0.0002$, in excellent agreement with the photometric redshift estimates of $6.76\pm 0.07$ and $6.66\pm 0.14$ for COS-3018555981 and COS-2987030247, and line-widths of $232\pm 30$ and $124\pm 18$ $\rm km\,s^{-1}$ respectively. While successful line-searches have confirmed far-infrared lines in submillimetre selected star-bursting galaxies at $z>6$[Riechers2013, Strandet2017], and a few tantalising "blind" (i.e. with no optical or near-infrared counterpart) [C 2] emitters have recently been detected at $\sim 4\sigma$[Aravena2016], this is the first time that normal star-forming galaxies in this early epoch, selected at optical or near-infrared wavelengths, have been confidently spectroscopically confirmed with ALMA.

We furthermore obtain upper limits on the far-infrared (FIR) dust continuum emission from the ALMA data. We find $\rm SFR_{\rm IR}<16-19\,M_{\odot}\rm\,yr^{-1}$, consistent with ‘normal’ star-forming galaxies in the local Universe[Leroy2012] and ruling out the presence of a dusty starburst in these sources. Fig. 2 shows that for the colour of the UV-continuum slope ($\beta_{\rm UV}\sim-1.2$) in these galaxies we would expect a higher dust content (IRX=$L_{\rm IR}/L_{\rm UV}$) in these galaxies if they were consistent with having the Meurer dust law, which is locally observed to apply for starburst galaxies[Meurer1999]. Scatter in the IRX-$\beta_{\rm UV}$ relation can be due to dust geometry effect, galaxy population age or the shape of the attenuation curve. However, for blue galaxies ($\beta_{\rm UV}\lesssim-0.5$) that scatter below the Meurer[Meurer1999] relation, such as seen in our galaxies, the most likely way to reproduce the low values of IRX is through a steeper attenuation curve, such as has been derived for the Small Magellanic Cloud[Prevot1984] (consistent with our measurements within 3$\sigma$), in combination with a potential increase in dust temperature at higher redshift.

The [C 2] $\lambda$158$\micron$ line is an important coolant for the neutral interstellar gas. For local galaxies the [C 2] line luminosity is well correlated with the star-formation rate of galaxies[deLooze2014]. In Fig. 2 we present the measured flux of the [C 2] lines as a function of SFR, which is consistent with the $z\sim 0$ SFR-$L_{\rm[CII]}$ relation[deLooze2014], and consistent with similarly bright galaxies observed at $z\sim 5-6$[Capak2015, Willott2015]. In contrast, [C 2] observations in the Epoch of Reionization to-date have shown these galaxies are significantly below the local relation[Ouchi2013, Ota2014, Maiolino2015, Pentericci2016, Knudsen2016]. This is likely due to the distinctly different selection of our targets compared to previous studies at $z>6.5$, where we select [O 3]$\lambda\lambda$5007,4959Å+H$\beta$ emitters as opposed to Ly$\alpha$ emitting galaxies.

Our sources have slightly higher star-formation rates and redder UV slopes ($\beta_{\rm UV}\sim-1.2$) than previously studied galaxies at this epoch, which could indicate more evolved and higher metallicity galaxies. Sources with extremely low oxygen abundance in the local Universe are typically found to be [C 2]-deficient [deLooze2014, Smith2017] due to their hard radiation field and therefore metallicity could be an important discriminator between [C 2]-bright and [C 2]-faint sources[Vallini2017]. Moreover, in local galaxies the star formation rate density drives a continuous trend of deepening [C 2] deficit as a function of increasing $\Sigma_{\rm SFR}$[Smith2017, DiazSantos2017], indicating local processes such as the radiation field intensity are important in driving [C 2] luminosity. If [C 2]-faint sources at $z>6$, currently unresolved in [C 2], have higher star-formation surface brightness than our galaxies, this could also explain the different SFR/$L_{\rm[CII]}$ ratios.

Furthermore, our sources have inferred high equivalent-width optical emission lines, which could suggest an ongoing starburst and potentially a high fraction of [C 2] emission emerging from H 2 regions. Starbursts and H 2 galaxies in the local Universe have slightly elevated [C 2] luminosities for a given SFR[deLooze2014] and therefore we could specifically be targeting the brightest [C 2] galaxies of the overall $z\sim 7$ galaxy population. Finally, while we do not have spectroscopy covering the Ly$\alpha$ line on COS-3018555981 and COS-2987030247, our sources could be weaker Ly$\alpha$ emitters than typically seen in spectroscopically confirmed sources at this redshift. Ly$\alpha$ emission is suggested to be inversely correlated with neutral gas column density[Stark2017] and can therefore affect the visibility of [C 2], which emerges both in the diffuse neutral and the warm ionized medium of a galaxy.

We determine the [C 2] half-light radii (deconvolved from the beamsize) that extend $2.6\pm 0.8$ and $3.1\pm 1.0$ kpc for COS-3018555981 and COS-2987030247, nearly twice the size of the UV in the brightest LBGs at this redshift[Bowler2017]. We use the spatial extent of the [C 2] detection to investigate the velocity structure of these sources, which reveals a projected velocity difference over the galaxy of $111\pm 28$ and $54\pm 20$ $\rm km\,s^{-1}$ for COS-3018555981 and COS-2987030247 respectively (Fig. 3), similar to the velocity gradients observed in two galaxies recently studied at $z\sim 5-6$[Riechers2014, Pavesi2016]. Given the low angular resolution of the observations, there are various ways to interpret these velocity gradients. A rotating galaxy disk would be one interpretation of these velocity fields, however, a merger of one or more [C 2] emitting galaxies, smoothed by the beamsize, could also appear as a regular rotational field. Furthermore, a bipolar outflow, or perhaps an inflow of gas could provide an additional velocity component to the [C 2] line that might give the impression of galaxy rotation.

We apply an observational criterion for the classification of rotation- and dispersion dominated systems based on the full observed velocity gradient $\Delta v_{\rm obs}$ and the integrated line width $\sigma_{\rm tot}$ of a galaxy, such that $\Delta v_{\rm obs}/2\sigma_{\rm tot}>0.4$ are likely to be rotation-dominated sources[ForsterSchreiber2009]. We compare this observed quantity in our galaxies with those measured from H$\alpha$ in galaxies at $z\sim 1-3$[ForsterSchreiber2009] in Fig. 4. Despite our sources being an order of magnitude smaller in stellar mass and at an epoch 2.5 billion years earlier in cosmic time we find $\Delta v_{\rm obs}/2\sigma_{\rm tot}$ values of $0.57\pm 0.16$ and $0.52\pm 0.21$, similar to the turbulent, yet rotationally supported galaxy disks at $z\sim 2$ [ForsterSchreiber2009]. Assuming a circularly symmetric galaxy disk model, we estimate a dynamical mass of $M_{\rm dyn}$ of $1.0_{-0.2}^{+0.3}$ and $0.4_{-0.3}^{+0.9}\times 10^{10}\,M_{\odot}$ for COS-3018555981 and COS-2987030247 respectively. Note, however, that the influence of turbulence in these sources could increase the dynamical mass estimates (by at most a factor of 2$\times$). These sources are a factor of $\sim$4–10$\times$ lower mass than the bright, UV-selected sources recently observed at $z\sim 5-6$, just $\sim$200–300 Myr later in cosmic time[Capak2015], which appear otherwise similar in their [C 2] and IR properties (Fig. 2). Furthermore, the stellar mass in our sources makes up $\sim$14 and 43% of the total dynamical mass that we measure (Fig. 4), in good agreement with the 33% estimated in the UV-selected sources at $z\sim 5-6$[Capak2015] and consistent with the wide range of values observed for star-forming galaxies at $z\sim 1-3$[ForsterSchreiber2009, Gnerucci2011]. These results indicate a significant gas fraction in the inner few kpc of our galaxies, consistent with hydrodynamical simulations of star-forming galaxies at this epoch[Fiacconi2017].

In conclusion, we present the first ALMA spectroscopic confirmations of normal star-forming galaxies in the Epoch of Reionization and a measurement of velocity structure using the [C 2] $\lambda$157.74$\micron$ emission line. These observations will serve at a pathfinder study that will enable larger samples of $z\sim 7$ galaxies to spectrosopically confirmed with ALMA, while the kinematic mapping of $z>6$ galaxies will adds a remarkable new dimension to the study of galaxies in their formative years.