'Scraggy' dark halos around bulge-less spiral galaxies

TL;DR

This study investigates the dark matter halos of bulge-less spiral galaxies, finding they have significantly lower dark halo mass-to-light ratios compared to bulgy spirals, indicating less dark matter presence.

Contribution

It provides the first estimate of dark halo mass-to-light ratios for bulge-less spiral galaxies using satellite orbital motions.

Findings

Bulge-less spirals have low dark halo mass-to-light ratios (~20).

Dark halos around bulge-less galaxies are about half as massive as those around bulgy spirals.

Sample includes 220 galaxies with 43 dwarf companions and 7 in the Local Volume.

Abstract

We use a sample of 220 face-on bulge-less galaxies situated in the low density environment to estimate their total mass via orbital motions of supposed rare satellites. Our inspection reveals 43 dwarf companions having the mean projected separation of 130 kpc and the mean-square velocity difference of 96 km/s. For them, we obtain the mean orbital-mass-to-K-band luminosity ratio of $20\pm3$. Seven bulge-less spirals in the Local Volume are also characterized by the low mean ratio, $M_{orb}/L_K = 22\pm5$. We conclude that bulge-less Sc-Scd-Sd galaxies have poor dark halos, about two times lower than that of bulgy spiral galaxies of the same stellar mass.

Tables

Table 1. Table 1: Face-on bulge-less galaxies with orbital mass estimates.

Galaxy	$B_t$	$V_{LG}$	mod D	T	$\log L_K$	$\Delta V$	$R_p$	$\log M_{orb}$	$M_{orb}/L_K$
	mag	km/s	mag		$L_{\odot }$	km/s	kpc	$M_{\odot }$
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
IC1562	13.60	3771	33.56	c	10.69	122	60	12.02	21
NGC0255	12.41	1694	31.70	c	10.45	37	173	11.45	10
						-72	243	12.17	52
ESO542-004	15.00	5657	34.50	d	10.29	-66	199	12.01	52
UGC02043	14.96	5316	34.40	c	10.67	-18	128	10.69	1
ESO479-022	15.39	7266	35.07	c	10.61	45	168	11.60	10
UGC02323	15.66	8139	35.33	c	10.99	-281	41	12.58	39
UGC02692	14.07	6337	34.77	c	11.09	-42	92	11.28	2
						114	233	12.55	29
NGC1376	12.85	4137	33.82	c	11.14	-115	134	12.32	15
						-100	157	12.27	14
NGC1599	14.10	3947	33.73	c	10.60	100	155	12.26	46
UGC03703	15.35	7260	35.14	c	10.72	-96	189	12.31	39
UGC04380	15.05	7554	35.24	c	10.86	134	208	12.64	60
UGC05169	15.44	7699	35.29	d	10.45	-81	41	11.50	11
NGC2967	12.28	1679	32.16	c	10.80	101	191	12.36	36
UGC05474	14.89	5847	34.71	cd	10.54	42	80	11.22	5
						-117	154	12.39	71
						-67	161	11.93	24
UGC05483	15.13	6014	34.77	c	10.62	-21	46	10.38	1
NGC3506	13.16	6252	34.87	c	11.42	1	135	8.20	0
PGC034006	14.13	7543	35.26	c	11.25	168	190	12.80	35
NGC3596	11.79	1062	31.32	cd	10.42	-101	25	11.48	12
						-60	60	11.41	10
NGC3938	10.87	841	30.87	c	11.03	-179	156	12.77	55
						-82	166	12.12	12
						-191	168	12.86	68
						35	223	11.51	3
						27	251	11.33	2
IC3271	14.57	7083	35.13	c	10.97	63	103	11.68	5
NGC4653	12.77	2471	33.08	c	10.85	37	68	11.04	2
						13	96	10.45	0
NGC5434	13.94	4587	34.21	c	10.84	115	41	11.81	9
NGC5468	12.95	2734	33.40	c	10.90	68	55	11.48	4
PGC058201	15.69	8562	35.51	c	10.70	43	54	11.07	2
IC1221	14.59	5706	34.63	cd	10.64	75	96	11.80	14
NGC6821	13.62	1680	31.86	d	10.33	-28	127	11.07	6
NGC7137	13.05	1977	32.16	c	10.57	152	77	12.32	56
NGC7495	13.76	5133	34.28	c	11.06	-39	73	11.12	1
NGC7535	14.28	4884	34.17	cd	10.63	8	47	9.55	0
						-100	174	12.31	48
						-15	182	10.68	1
ESO605-016	13.23	7999	35.28	c	11.55	-68	142	11.89	2
(1) galaxy name; (2) apparent $B$-magnitude from HyperLEDA (Makarov et al. 2014);
(3) radial velocity with respect to the Local Group center;
(4,5) distance modulus and morphological type from HyperLEDA;
(6) logarithm of K-band luminosity expressed in the solar units;
(7) radial velocity difference of satellite relative to the central galaxy;
(8) a satellite projected separation;
(9) logarithm of orbital mass; (10) orbital mass-to-luminosity ratio.

Table 2. Table 2: Companions of face-on bulge-less galaxies in the Local Volume.

Galaxy	$B_t$	$R_p$	$\Delta V$
	mag	kpc	km/s
IC 342	9.1	0	0
KK35	15.7	15	-97
UGCA 86	13.5	89	36
NGC 1560	12.1	312	-67
NGC 1569	11.8	312	-126
Cam A	14.8	326	-77
Cam B	16.1	365	46
Cas 1	15.3	526	4
M 101	8.3	0	0
NGC 5477	14.2	44	73
PGC2448110	17.3	92	14
NGC 5474	11.5	96	46
UGC 8882	15.8	110	104
GBT1355+54	-	152	-33
Holm IV	13.8	163	-106
NGC 5585	11.2	429	79
dw1343+58	15.7	605	-13
DDO194	14.5	647	3
NGC6946	9.6	0	0
KK 251	16.5	77	86
UGC 11583	15.9	86	87
KK 252	16.7	105	99
KKR 55	17.0	178	-8
KKR 56	17.6	311	-83
Cepheus 1	15.4	525	18
KKR 59	15.7	631	-43
KKR 60	18.0	672	-54
NGC 628	9.8	0	0
UGC 1171	15.7	132	81
UGC 1176	14.4	150	-27
SDSS0138+14	18.0	153	86
KDG 10	16.3	296	132
AGC 112454	17.5	297	14
UGC 1056	14.8	374	-62
JKB 142	18.4	404	65
UGC 1104	14.5	481	29
SDSS0142+13	17.4	501	15
NGC 3184	10.3	0	0
KUG1013+414	15.4	90	-85
SDSS1028+42	17.5	445	-34
NGC 3104	13.6	543	8
SDSS1025+43	17.3	561	58
KUG1004+392	15.6	629	9

Table 3. Table 3: Suites of companions around the Local Volume bulge-less spirals.

Galaxy	D	n	$\Delta m_{12}$	$⟨R_p⟩$	${\sigma }_V$	$\log L_K$	$\log M_{orb}$	$M_{orb}/L_K$
	Mpc		mag	kpc	km/s
IC 342	3.28	8	3.0	319	76	10.60	12.28	48
M 101	6.95	9	3.2	204	64	10.79	11.93	14
NGC 6946	7.73	8	5.8	323	68	10.99	12.05	11
NGC 628	10.19	9	4.6	309	68	10.60	12.11	32
NGC 3184	11.12	5	3.3	454	49	10.52	11.87	22
NGC 253	3.94	7	2.0	500	60	11.04	12.18	14
NGC 5236	4.92	10	4.4	294	61	10.86	12.03	15
Mean	-	8	3.8	343	65	10.77	12.06	22

Equations

$$M_{orb}=(32/3\pi )(1-2e^2/3{)}^{-1}\times G^{-1}\times ⟨\Delta V^2\times R_p⟩.$$

Full text

††pagerange: ”Scraggy” dark halos around bulge-less spiral galaxies–LABEL:lastpage††pubyear: XXX

1 Introduction.

Among the galaxies with stellar masses of more than, $10^{10}M_{\odot}$ the flat disc-like objects without the evidence of the central bulge constitute about 10%. Almost all of them are classified as late-type spirals: Sc, Scd, and Sd according to de Vaucouleurs et al. (1976). Some of them demonstrate small pseudo-bulges formed during the process of the secular disc evolution. Unlike classical bulges, the pseudo-bulges show a nearly exponent stellar density profile and the evidences of current star formation (Kormendy & Kennicutt 2004). The observed abundance of the spiral discs without spheroidal stellar component imposes a challenge to models of the hierarchical clustering of galaxies via their consecutive merging (Kormendy et al. 2010).

Bulge-less galaxies are the most distinguishable when they are seen strictly edge-on. A Reference Flat Galaxy Catalog = RFGC (Karachentsev et al. 1999) lists 4236 ”flat” galaxies covering the all sky. Karachentseva et al. (2016) have selected from the RFGC catalog 817 ultra-flat galaxies (UFG) with an apparent axial ratio $a/b>10$. The environment of the UFGs is characterized by low spatial density, a deficit of massive satellites and almost total absence of dwarf spheroidal satellites with old stellar population.

Inner structure of the UFGs is practically unseen due to the disc inclination. To determine the bulge-less galaxy structural features, Karachentsev & Karachentseva (2019) collected 220 galaxies of the Sc, Scd, and Sd types, seen almost face-on. According to statistical analysis, about half of the galaxy sample have bar-like substructures, in most part of the discs the unresolved (star-like) nuclei are seen, and a considerable part (27–50)% of the face-on galaxies have peripheral distortion of their spiral pattern.

There is an idea that secular stability of very thin stellar discs requires the presence a very massive dark halo around them (Banerjee & Jog 2013). The shape of this halos is supposed to be nearly spherically symmetric. Unfortunately, the rotation curves obtained for the UFGs, extend not so far from the center (Uson & Matthews 2003, Makarov et al. 2001), which makes it impossible to evaluate the true dimension and mass of the dark halo. Determining the total dark halo mass by radial velocities and projected separations of the satellites is more promising method. However, the difficulties arise even with such approach. The search for satellites with measured radial velocities around the UFGs shows that $\sim$60% of ultra-flat galaxies have not any detected physical satellites inside the virial radius $R_{vir}\simeq 250$ kpc, about 30% of the UFGs enter together with a rather bright neighbors in scattered associations (filaments, walls), and only $\sim$10% of ultra-flat galaxies are the dominant objects in physical multiple systems (Karachentsev et al. 2016). In the following we present the results of searches for satellites around 220 late type spiral galaxies seen face-on as well as the estimates of the total mass of the bulge-less galaxies.

2 Satellites around face-on bulge-less galaxies.

To reveal the physical satellites, we have used the option ”To Search for Nearby Objects” proposed in the NASA Extracalactic Database (=NED, http://ned.ipac.caltech.edu/). Around each bright enough galaxy from the list by Karachentsev & Karachentseva (2019) with absolute magnitude $M_{B}<-19\aas@@fstack{m}0$ we have searched for companions with radial velocity difference $\mid\Delta V\mid<300$ km/s ranging in the projected separation $R_{p}=250$ kpc. In the next stage, we left only those conditionally isolated cases where other neighbors brighter than the central object were absent within $R_{p}=750$ kpc around the face-on galaxy under study. In the process, we excluded a lot of neighboring objects that turned out to be stars or parts of a normal galaxy. Altogether, we detected 43 satellites around 30 face-on galaxies. The remaining galaxies from our list do not have satellites in the specified limits of $\mid\Delta V\mid$ and $R_{p}$, or they have as neighbors other bright galaxies violating the isolation condition.

In Table 1 data on the face-on bulge-less galaxies and their satellites are given. Before presenting the data we have to do some explanations. The total $B$-magnitude is corrected for Galactic extinction; the inner extinction for face-on galaxies we prove to be zero. The $K$- band magnitude is calculated via the corrected $B$- magnitude and morphological type (Melnyk et al. 2017). Calculating the satellite projected separation $R_{p}$, we use the galaxy distance $D=V_{LG}/H_{0}$ with the Hubble parameter $H_{0}=73$ km/s/Mpc. For nearby galaxies situated in the Local Volume we attracted their individual distance moduli from http://www.sao.ru/lv/lvgdb. The orbital (Keplerian) mass of a galaxy, $M_{orb}=(16/\pi G)\times\Delta V^{2}\times R_{p}$, is estimated assuming a random orientation of satellite orbits with the mean orbital eccentricity of $\langle e^{2}\rangle=1/2$ (Karachentsev & Kudrya 2014), where $G$ is the gravitation constant. Notes to columns of Table 1 are given below the Table.

The distribution of 43 satellites around their host face-on bulg-less galaxies on the modulus of radial velocity difference and the projected separation is presented in Fig.1. Here the mean-square of radial velocity difference is $\langle\Delta V^{2}\rangle^{1/2}=96$ km/s, and the mean projected separation is equal to $\langle R_{p}\rangle=130$ kpc.

As it is seen from Table 1, the majority of face-on galaxies with detected satellites belong to the Sc morphological type. The median of their luminosity, 10.71 dex, is comparable with the Milky Way luminosity. (This high value is caused by the fact that in searching for satellites we donated the face-on low luminosity galaxies with their shallow potential well.) The orbital mass values show a pronounced scatter that depends on prevailing character of satellites motions. At fixed value of the orbital eccentricity, the ensemble mean of $M_{orb}$ is the unbiased estimate of the mass of central galaxy. In the case of an arbitrary eccentricity $e$, the orbital Keplerian mass estimate is expressed as

[TABLE]

This value grows in three times from severely round motions, $e=0$, to the pure radial ones, $e=1$. We assumed the mean value of $\langle e^{2}\rangle=1/2$, corresponding the expected average obtained in the N-body simulations (Barber et al. 2014).

The mean value of the orbital-mass-to-K-band luminosity ratio for face-on bulge-less galaxies is $\langle M_{orb}/L_{K}\rangle=20\pm 3$ (see last column of Table 1). This ratio diminishes to $17\pm 3$ having regard to statistical correction for errors of the radial velocity measurements.

3 Satellites of the face-on bulge-less galaxies in the Local Volume.

The Local Volume limited by the radius of 11 Mpc, is unique in having an abundance of galaxies which distances are measured at the Hubble Space Telescope with accuracy of about 5%. The summary sample on the galaxy distances is presented in the Updated Nearby Galaxy Catalog = UNGC (Karachentsev et al. 2013) as well as in the regularly replenished data base http://www.sao.ru/lv/lvdb (Kaisina et al. 2012). In the Local Volume faint dwarf galaxies are seen which are lost in more distant and less researched regions of the universe. The knowledge of the high accuracy measured distances permits to separate the physical satellites of bright galaxies against foreground and background objects with more confidence. In the Local Volume there are seven face-on bulge-less galaxies from the list of Karachentsev & Karachentseva (2019): IC 342 (3.28 Mpc), NGC 5068 (5.15 Mpc), M 101 (6.95 Mpc), NGC 6946 (7.73 Mpc), NGC 3344 (9.82 Mpc), NGC 628 (10.19 Mpc), and NGC 3184 (11.12 Mpc). Here in parentheses the distances from observer are given. Of them, two low luminosity galaxies, NGC 3344 and NGC 5068 have no satellites with measured radial velocities. The remaining five galaxies have from 5 to 9 physical satellites, the data on which are presented in Table 2. Its columns contain: (1) galaxy name, (2) the total apparent $B$-magnitude, (3) the projected separation in kpc, (4) the difference between the radial velocities of the satellite and the host galaxy, in km/s.

The distribution of 38 satellites on their modulus velocity difference and projected separation is presented in Fig.2. As one can see, the satellite velocity differences do not exceed 150 km/s, justifying the above selection limit of $\mid\Delta V\mid<300$ km/s in the search for galaxies situated in more distant volume. About half of satellites are located outside of a characteristic virial radius $\sim 250$ kpc. Nethertheless, they all are inside the ”zero velocity sphere” ($R_{0}\sim 1$ Mpc), which separates the collapsing group members against the global cosmic expansion. Obviously, beyond the Local Volume, many distant satellites with $R_{p}>R_{vir}$ are lost among the general field galaxies.

Table 3 contains the basic parameters of the nearby groups. As seen from the data, even the brightest satellites appear to be fainter of the central galaxy at 3–5 magnitudes. Thus, the application of the model of test particles moving around central massive body is quite correct in evaluating the total mass of the central galaxy.

In the Local Volume there are two another groups, NGC 253 and NGC 5236, where the dominated member is a spiral bulge-less galaxy but oriented arbitrary. We give their data in the bottom of Table 3. The K-band luminosities and orbital mass estimates for these nearby groups are rather similar to ones from Table 1 obtained for more distant systems. Particularly, the mean total mass-to-luminosity ratio for the Local Volume bulge-less galaxies, $\langle M_{orb}/L_{K}\rangle=22\pm 5$, is practically the same as the value ($20\pm 3$) derived for the remote Sc-Sd galaxies.

It should be noted that according to the data by Karachentsev & Kudrya (2014), the Local Volume contains five groups: M 31, M 81, NGC 4258, NGC 4736, and NGC 3627, where the bulgy Sab–Sbc galaxies dominate, and also three groups: NGC 5128, NGC 4594, NGC 3115, where the objects are grouping around the E, S0, Sa galaxies. These groups are characterized of the mean values of total mass-to-luminosity ratio $\langle M_{orb}/L_{K}\rangle=41\pm 9$ and $\langle M_{orb}/L_{K}\rangle=88\pm 30$, respectively. In spite of poor statistics, these results can indicate that the dark mass-to-luminous mass ratio grows when the fraction of galaxy spheroidal stellar sub-system increases.

4 Concluding remarks.

As we noticed above, the thin spiral bulge-less galaxies reside in the low density regions, i.e. have a poor environment. The rare satellites of the Sc–Scd–Sd galaxies are, as a rule, dwarf galaxies containing gas and young stars. Our statistics of satellites around 220 face-on bulge-less galaxies shows that they have the average projected separation of $\langle R_{p}\rangle=130$ kpc and mean-square velocity difference of 96 km/s. The estimate of the total mass of bulge-less galaxies via orbital motions of their satellites yields the value of $\langle M_{orb}\rangle=12.14$ dex and $\langle M_{orb}/L_{K}\rangle=20\pm 3$. A similar value, $\langle M_{orb}/L_{K}\rangle=22\pm 5$, is obtained by us for the Local Volume Sc–Sd galaxies, where the separation of physical satellites and field galaxies is more reliable due to abundance of data on the galaxy distances. For comparison, the Sab–Sbc spirals in the Local Volume with much more developed bulges have the ratio $\langle M_{orb}/L_{K}\rangle=41\pm 9$, and a few in number galaxies of E, S0, Sa types are characterised by the ratio of $\langle M_{orb}/L_{K}\rangle=88\pm 30$. It seems unlikely that this difference would be caused by different manner of transition from the B-luminosity of a galaxy to its K-luminosity, or the different nature of the orbital motions of the satellites. Obviously, the conclusion about the reduced amount of dark matter per unit of K-band luminosity in the Sc–Sd galaxies needs confirmation on a richer observational material. It is quite possible that the presence of ”scraggy” halos in spiral bulge-less galaxies will be explained within the framework of the standard paradigm of the hierarchy galaxy clustering.

Acknowledgements.

We thank the referee for his/her valuable comments. The work is supported by the Russian Science Foundation grant 19-12-00145. The paper makes use of the data from NASA Extragalactic Database and from the HyperLEDA.

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