Dwarf Spheroidal Galaxies in the M101 group and behind it

TL;DR

This study uses Hubble Space Telescope images to identify and analyze dwarf spheroidal galaxies around M101, estimating the galaxy's total mass and satellite membership.

Contribution

It provides new distance measurements for low surface brightness objects and refines the mass estimate of M101 based on satellite dynamics.

Findings

Only two dwarf galaxies are confirmed as M101 satellites.

The total mass of M101 is estimated at approximately 8.5 x 10^{11} solar masses.

The mass-to-stellar mass ratio of M101 is about 16.

Abstract

We use images from the Hubble Space Telescope to determine the distances to a dozen objects with low surface brightness recently observed around the bright nearby spiral M101. Only two dwarf galaxies, M101-DwA and M101-Dw9, turn out to be actual satellites of M101 at distances of about 7 Mpc. The other objects are probably members of a distant group around the S0-galaxies NGC 5485/5473. Based on the radial velocities and projected separations of the 9 satellites, we obtain an estimate of $(8.5\pm3.0)\times 10^{11} M_{\odot}$ for the total mass of M101, which is consistent with a ratio $16\pm6$ for the total mass to the stellar mass of the galaxy.

Tables

Table 1. Table 1: Estimated distances for the two dwarf galaxies.

Galaxy	TRGB	$A_I$	${\left(m-M\right)}_0$	$D$,Mpc
M101-DwA	25.04	0.01	29.11$\pm$0.13	6.64
M101-Dw9	25.36	0.01	29.44$\pm$0.13	7.71

Table 2. Table 2: Major parameters of galaxies in the M101 group.

Galaxy	RA (2000.0) DEC	D	meth	$V_{LG}$	$\log M^{\ast }$	T	$r_p$	$R_p$	Ref.
		Mpc		km/s	$M_{\odot }$		deg	kpc
dw1343+58	134307.0+581340	6.95	mem	365	7.65	BCD	4.99	605
Holm IV	135445.1+535417	6.93	TRGB	272	8.60	Sm	1.34	163	[15]
GBT1355+54	135450.6+543850	6.95	mem	345	7.10	HIcl	1.25	152	[16]
M101-Dw9	135544.6+550845	7.71	TRGB	—	6.17	Sph	1.37	166
UGC8882	135714.6+540603	7.8	sbf	482	7.64	Sph	0.91	110	[9]
M101-df3	140305.7+533656	6.52	TRGB	—	7.27	Sph	0.80	97	[7]
M 101	140312.8+542102	6.95	TRGB	378 1	0.79	Scd	0.0	0	[17]
M101-df1	140345.0+535640	6.37	TRGB	—	6.17	dTr	0.44	53	[7]
PGC2448110	140457.8+534129	6.95	mem	392	6.91	BCD	0.76	92
NGC5474	140502.1+533947	6.82	TRGB	424	9.20	Sm	0.79	96	[15]
NGC5477	140533.1+542739	6.77	TRGB	451	8.26	Im	0.36	44	[15]
M101-DwA	140650.0+534429	6.64	TRGB	—	6.97	Sph	0.84	102
M101-df2	140837.5+541931	6.87	TRGB	—	6.74	Sph	0.79	96	[7]
NGC5585	141948.3+564349	7.76	TF	457	9.16	Sd	3.54	429	[17]
DDO194	143524.6+571524	6.30	TRGB	381	8.13	Sm	5.33	647	[15]
NGC5204	132936.4+582504	4.59	TRGB	339	8.84	Sm	6.33	—	1[7]
UGC8508	133044.4+545436	2.58	TRGB	181	7.53	dIr	4.73	—	[15]
NGC5238	133442.7+513650	4.21	TRGB	342	7.99	dIr	5.19	—	[5]
KKH87	141509.4+570515	9.40	TRGB	473	7.76	dIr	3.44	—	[15]

Table 3. Table 3: Parameters of the group of galxies behind M101.

Group	J2000.0	$N_v$	$V_{LG}$	${\sigma }_v$	D	$R_v$	$\log M^{\ast }$	$\log M_v$	$M_v/M^{\ast }$
			km/s	km/s	Mpc	kpc	$M_{\odot }$	$M_{\odot }$
NGC5422	140042.0+550952	12	1935	121	29.8	280	11.17	13.06	78
NGC5485	140711.3+550006	18	2162	94	27.9	277	11.20	12.72	33

Equations

$$⟨M_{orb}⟩=(16/\pi G)⟨\Delta V^2{R}_p⟩,$$

Full text

galaxies: dwarf galaxies

1 Introduction.

M101=NGC 5457 is one of the closest massive galaxies of the late Scd type in which there is essentially no spheroidal stellar component (bulge). These galaxies usually lie in regions of low cosmic density. The amount of dark matter in Sc and Sd galaxies per unit stellar mass is a factor of 2–4 smaller than in the spirals of earlier types (Sa,Sb) or elliptical galaxies [1]. Dwarf spheroidal satellites with a low surface brightness (dSph), lacking gas, and young stars are encountered comparatively rarely around Sc and Sd galaxies.

Until recently, only one dSph- type galaxy, UGC 8882 [2], was known within the vicinity of M101. A group of amateur astronomical photographers (Tief Belichtete Galaxies) completed a survey of the surroundings of M101 on small telescopes with very long exposure times and discovered 10 candidate dSph satellites of M101 [3]. Some of these objects were rediscovered independently by another team using a system of small high-throughput telescopes, the ’’Dragonfly Telephoto Array’’ [4]. A subsequent survey of the virial zone of M101 at the 3.6-m CFHT telescope [5] resulted in the detection of another 38 new candidate spheroidal satellites of M101 with typical apparent magnitudes $B\simeq(19-21)^{m}$ and angular diameters $\sim(10-20)^{\prime\prime}$. Another 5 candidate dSph satellites of M101 have been found [6] in a survey of a wider region of the sky.

Confirmation of the membership of spheroidal dwarfs in the M101 group by measuring the radial velocity is complicated observational task because of low surface brightness of these objects and the absence of gas and visible structural features in them. It turned out to be possible, however, to measure the distances of dSph galaxies on the Hubble Space Telescope (HST) based on the luminosity of the tip of the red giant branch (TRGB). Images of seven of the ten dwarf galaxies discovered in Refs. 3 and 4 were obtained with the ACS camera on the HST in the GO 13682 program (PI P. van Dokkum). An analysis of the data showed that three of the dwarf objects were real satellites of M101 [7], while the remaining four were diffuse dwarf systems not resolved into stars [8].

Carlsten et al. [9] have recently used data from the CFHT survey of the surroundings of M101 to determine the distances of the previously found spheroidal dwarfs by measuring the fluctuations in their surface brightness. They concluded that most of these objects (29 out of 43) are distant galaxies with no relationship to the M101 group. Besides the previously confirmed three dSph satellites, DF1, DF2, and DF3, and the brighter galaxy UGC 8882, only two of the spheroidal dwarfs, DwA and Dw9, were certified as new likely members of the M101 group.

2 The distances of two new satellites of M101.

The archives of the HST contain images of 19 dwarf galaxies in the neighborhood of M101 obtained with the HST ACS camera in F814W and F606W filters as part of the program GO 14796 (PI D. Crnojevich) with exposures, respectively, of 900 and 1200 s. We processed these images, taken from HST archive, by the standard procedure using the program package DOLPHOT [10].

During processing, the parameters recommended for elimination of nonstellar and blended objects were used. In most cases the galaxies did not manifest signs of resolution into stars. Only two of the galaxies, M101-DwA and M101-Dw9, show up as distinctly resolved into stars, including a red giant branch, which can be used to determine the distances of these galaxies. Images of the two galaxies are shown in Fig.1, where the two frames have an area of $35^{\prime\prime}\times 35^{\prime\prime}$.

The tip of the red giant branch was found by a maximum likelihood principle using modeling and subsequent recognition of artifical stars [11]. This approach made it possible to estimate the photometric errors and the completeness of the color-magnitude diagram (CMD) in fields with overlapping images of the stars. The absolute magnitude of the TRGB was calibrated in accordance with Ref. 12 taking the Galactic extinction from Ref.13 into account. CM diagrams for both objects are shown in Fig. 2. The horizontal line indicates the position of the TRGB and the crosses on the right are the photometric errors determined from the artifical stars.

These values of the TRGB, their errors, the magnitude of the assumed extinction, and the distance moduli of the two dwarf galaxies are listed in Table 1.

3 The group of galaxies around M101.

At present, the group of galaxies associated with M101 has 15 members. Data on them are given in Table 2. The columns of the table list: (1) the galaxies; (2) their equatorial coordinates; (3) distances in Mpc; (4) the method used to determine the distance: ’’TRGB’’ from the tip of the red giant branch, ’’sbf’’ from surface brightness fluctuations, ’’TF’’ from the Tully- Fisher relation [14], and ’’mem’’ from assumed membership in the M101 group; (5) radial velocity relative to the centroid of the Local group; (6) logarithm of the stellar mass from the database http://www.sao.ru/lv/lvgdb; (7) morphological type; (8,9) projected separation of the satellite in degrees and in kpc; (10) references for data on the distance. The lower part of the table includes 4 neighboring galaxies on the sky which are not physical members of the group in terms of the distances.

These date imply that the average projected separation for the 14 satellites of M101 is 204 kpc, which is typical of the virial radius of the halo of spiral galaxies of the type in the Milky Way and M31. The ’’zero velocity radius’’, which separates the group from the general cosmological expansion is equal to $\sim 1$ Mpc for galaxies of this virial mass [18]. Thus, even the most distant dwarfs, DDO 194, NGC 5585, and dw1343+58, can be regarded as physical members of the M101 group. The radial velocities were measured only for the 9 brightest satellites of M101. The mean square difference in the radial velocities is small for them and equals 64 km/s.

Even the brightest satellites have a stellar mass an order of magnitude and a half lower than the mass of the M101 itself. This suggests treating them as a system of test particles moving around the central massive galaxy. Assuming a random orientation of the orbits of the satellites with an average eccentricity e = 0.7, the average orbital estimate for the mass is given by

[TABLE]

where $G$ is the gravitational constant and $\Delta V$ is the radial velocity of the satellite relative to M101 [19]. This Keplerian approach gives an average value for the mass of the halo of M101 of $(8.5\pm 3.0)10^{11}M_{\odot}$. For a stellar mass of M101 equal to $5.3\times 10^{10}M_{\odot}$, the ratio of the total mass of the halo to the mass of the stellar component, $\langle M_{orb}\rangle/M^{*}=16\pm 6$ turns out to be a factor of two smaller than the typical value of  $\sim 32$ for M31, M81 and other nearby spiral galaxies with developed bulges [19].

4 Groups of galaxies in the far background.

The abundance of dwarf spheroidal galaxies in the vicinity of M101 that are not resolved into stars in the HST observations, requires an explanation. The distribution of galaxies on their radial velocities in this sky region has a substantial excess in the number of galaxies with radial velocities of $\sim 2000$ km/s. A catalog of groups of galaxies in the nearby universe [20] includes $\sim 350$ groups over the entire sky with average radial velocities $V_{LG}<3500$ km/s. Of these, there are 11 within a radius of 7 degrees around M101. For all these groups the average radial velocities lie within an interval of [1900 – 2400] km/s. These groups form a diffuse structure of cosmic ’’pancake’’ type with a high probability. The two most populated groups around the galaxies NGC 5422 and NGC 5473/85 lie so close in the sky to M101 that their zero velocity spheres encompass the galaxy M101 in projection. Table 3 lists the following data on these groups: equatorial coordinates of the group center, number of galaxies with the measured radial velocities, average radial velocity, dispersion of of the radial velocities, distance in Mpc, virial radius, integrated stellar and virial mass. The resulting ratios of the virial-to-stellar masses $M_{v}/M^{*}=78$ and 33 are typical for groups with these morphological populations.

Figure 3 is a fragment of the image of the central region of the group NGC 5473/85 obtained by F.Neyer on the 15-cm amateur telescope with an exposure time of 40 hours. Four dwarf galaxies of very low surface brightness that were unresolved into stars in the HST images are indicated on this figure. A system of faint peripheral streams indicated by arrows can be seen around the massive S0 galaxy NGC 5485. As members of the group at a distance of $D=27.9$ Mpc, the low-surface brightness dwarf objects have luminosities and linear diameters that are typical of dSph galaxies in the more studied nearby groups.

As a whole, the population of dwarf spheroidal galaxies has a large variety of luminosities and linear sizes [4,8]. The extreme manifestations of this class of objects are ultradiffuse and ultracompact dwarfs whose surface brightnesses differ by hundreds of times. The large dispersion in the integrated parameters of dSph galaxies makes it extremely difficult to estimate their distances if they are far away and not resolved into separate stars.

This work was supported by RFFI grant No. 18–02–00005. Data from the archive of the Hubble Space Telescope have been used in this paper.

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