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Issue
A&A
Volume 643, November 2020
Article Number A124
Number of page(s) 5
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202038928
Published online 11 November 2020

© ESO 2020

1. Introduction

The Local Volume of the Universe amounts to almost a thousand galaxies having distance estimates within 11 Mpc1 (Karachentsev & Nasonova 2013). Near the far edge of this volume at a distance of 9.55 Mpc (McQuinn et al. 2016) there is a bright galaxy M 104 (also known as NGC 4594 or the Sombrero galaxy). With an apparent K-band magnitude of m K = 5 . m 0 $ m_K = 5{{\overset{\text{ m}}{.}}}0 $ it has the luminosity of LK/L⊙ = 11.32 dex, which is four times higher than the luminosity of the Milky Way (10.70 dex) or M 31 (10.73 dex). Thanks to its luminosity and, by inference, to its stellar mass the Sombrero is the most outstanding galaxy of the Local Volume.

Over recent years several attempts have been undertaken to determine the total (virial) mass of Sombrero using radial velocities and projected separations of its companions. Estimations of MT/M⊙ vary widely: 10.90 dex (Makarov & Karachentsev 2011), 13.17 dex (Karachentsev & Nasonova 2013), 13.45 dex (Karachentsev & Kudrya 2014), and 13.96 dex (Kourkchi & Tully 2017). The main reason of the scatter in estimates of the total mass is caused by the uncertainty on the gravitational binding of Sombrero with galaxies neighbouring in the projection. The Sombrero galaxy is located near the equator of the Local Supercluster where galaxies are concentrated in the filamental structure, the Virgo Southern Extension (VirgoSE; Tully 1982; Kourkchi & Tully 2017). Many galaxies in the VirgoSE have radial velocities similar to that of Sombrero, but lie at greater distances typical of the Virgo cluster (15–20 Mpc). At a distance of 8 Mpc from the core of the Virgo cluster, the Sombrero galaxy lies at the edge of the zero velocity surface bounding the cluster infall domain, a property shared by other galaxies in the VirgoSE over an extended range in distances.

To reveal the true satellites of Sombrero among the neighbouring galaxies we need to measure their distances, preferably with an error ΔD <∼ 1 Mpc. At present there is only one galaxy, KKSG 29, in the Sombrero vicinity with an accurately measured distance (9.82 ± 0.32 Mpc; Karachentsev et al. 2018), determined via the tip of the red giant branch (TRGB) method. This distance places the dwarf galaxy KKSG 29 as a physical satellite of the Sombrero galaxy.

In this work we present measurements of TRGB distances for two more dwarf galaxies, UGCA 307 (or DDO 153) and KKSG 30 (or LEDA 3097708) situated close to Sombrero. The distances of both the galaxies agree well with their belonging to the family of Sombrero satellites. The new distance measurements together with other less reliable distance estimates give us a possibility to make more precise value of the virial (orbital) mass of Sombrero.

2. TRGB distances to UGCA 307 and KKSG 30

The dwarf galaxies UGCA 307 ( 12 h 53 m 56 . s 8 $ 12^{\mathrm{h}}53^{\mathrm{m}}56{{\overset{\text{ s}}{.}}}8 $–12° 06′21″) and KKSG 30 ( 12 h 37 m 35 . s 9 $ 12^{\mathrm{h}}37^{\mathrm{m}}35{{\overset{\text{ s}}{.}}}9 $–08° 52′01″) have apparent B magnitudes of 14 . m 6 $ 14{{\overset{\text{ m}}{.}}}6 $ and 16 . m 3 $ 16{{\overset{\text{ m}}{.}}}3 $, respectively, and projected separations of ∼3° with respect to Sombrero. Their radial velocities in the Local Group rest frame, 731 km s−1 (UGCA 307) and 918 km s−1 (KKSG 30), are close to the radial velocity of Sombrero, 892 km s−1. The galaxies were observed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST) on December 5, 2019, and March 13, 2020, as a part of the SNAP project 15922 (PI R.B. Tully).

Two exposures for each object were made in a single orbit with the filters F606W (750 s) and F814W (750 s). The combined images of the galaxies are presented in Fig. 1. We used the ACS module of the DOLPHOT package by Dolphin (2002) to perform photometry of resolved stars based on the recommended recipe and parameters. Only stars with good quality photometry were included in the analysis. We selected the stars with a signal-to-noise ratio S/N >  4 in both filters, and with DOLPHOT parameters crowdF606W + crowdF814W <  0.8, (sharpF606W + sharpF814W)2 <  0.075. Artificial stars were inserted and recovered using the same reduction procedures to accurately estimate photometric errors. Subsequent analysis included only those image regions that contain stars of the galaxies themselves. We selected the region of 1.6 × 1.6 arcmin around UGCA 307 and 2.8 × 1.5 arcsec around KKSG 30. The resulting colour-magnitude diagrams in F606W–F814W versus F814W are plotted in Fig. 2. A maximum-likelihood method by Makarov et al. (2006) was applied to estimate the magnitude of the TRGB. We found F814W(TRGB) to be 25 . 76 − 0.11 + 0.19 $ 25.76^{+0.19}_{-0.11} $ for UGCA 307 and 25 . 96 − 0.07 + 0.08 $ 25.96^{+0.08}_{-0.07} $ for KKSG 30. Following the zero-point calibration of the absolute magnitude of the TRGB developed by Rizzi et al. (2007), we obtained M(TRGB) values of –4.09 (UGCA 307) and –4.08 (KKSG 30). Assuming values of foreground reddening, E(B − V), 0.049 (UGCA 307) and 0.028 (KKSG 30) from Schlafly & Finkbeiner (2011), we derived the true distance modulus of ( m − M ) 0 = 29 . 78 − 0.12 + 0.20 $ (m- M)_0 = 29.78^{+0.20}_{-0.12} $ or the distance D = 9 . 03 − 0.51 + 0.84 $ D = 9.03^{+0.84}_{-0.51} $ Mpc for UGCA 307 and ( m − M ) 0 = 29 . 94 − 0.09 + 0.10 $ (m- M)_0 = 29.94^{+0.10}_{-0.09} $ or the distance D = 9 . 72 − 0.41 + 0.44 $ D = 9.72^{+0.44}_{-0.41} $ Mpc for KKSG 30.

thumbnail Fig. 1.

HST/ACS combined images of UGCA 307 and KKSG 30. The image size is 1 . ′ 6 × 1 . ′ 4 $ 1{{\overset{\prime}{.}}}6\times1{{\overset{\prime}{.}}}4 $. North is up and east is left.

thumbnail Fig. 2.

Colour–magnitude diagrams of UGCA 307 and KKSG 30. The TRGB position is indicated by the horizontal line.

3. Companions of Sombrero and background objects

Judging by the big stellar mass of the Sombrero galaxy, the virial radius of its halo can reach about 400 kpc. To search for assumed satellites of Sombrero we examined a region of radius rp = 6° around it that corresponds to the linear projected radius of Rp = 1.0 Mpc at the distance of 9.55 Mpc. In this area there are 48 galaxies with radial velocities VLG <  1400 km s−1. The data are presented in Table 1. The table columns contain (1) galaxy name; (2) equatorial coordinates J2000.0; (3) radial velocity in the Local Group rest frame (km s−1); (4) morphological type; (5) apparent B magnitude from the Lyon Extragalactic Database (LEDA, Makarov et al. 2014) or NASA Extra-galactic Database (NED); (6) distance to the galaxy in Mpc; (7) method used for the distance estimate; (8) reference to the source of distance.

Table 1.

Galaxies around Sombrero with rp <  6° and VLG <  1400 km s−1.

As seen from these data, 41 of the 48 galaxies have distance estimates. Most of them were made by the Tully-Fisher method, with uncertainties of 35–30% for these low luminosity galaxies. Accordingly, we consider only galaxies with distances D <  12 Mpc as probable members of the Sombrero group. The distance and radial velocity distribution of galaxies around Sombrero are given in Fig. 3. In total, 15 galaxies are probable satellites of Sombrero, with the luminosity of each of them more than an order of magnitude fainter than the luminosity of Sombrero. An empty volume (mini-void) is visible ahead of the group. The background galaxies have radial velocities substantially overlapping the Sombrero group velocities. Due to significant TF distance errors, ΔD of about 3–5 Mpc, the membership of some galaxies (UGCA 312, PGC 104868), whether in the group or background may be subject to revision.

thumbnail Fig. 3.

Distribution of assumed members of the Sombrero group (filled circles) and background galaxies (open circles) according to their distance and radial velocity in the Local Group rest frame. The galaxies satisfy the conditions VLG <  1400 km s−1 and projected separation rp <  6° with respect to Sombrero (asterisk).

The distribution of galaxies from Table 1 in supergalactic coordinates SGL, SGB is presented in Fig. 4. Indications of galaxies with different symbols are the same as in the previous figure. Sombrero’s satellites, as well as background galaxies, are distributed asymmetrically. In both subsamples there is a noticeable increase in galaxy number towards the supergalactic equator and towards the Virgo cluster centre (SGL = 103°, SGB = −2°). The reason for this asymmetry in the case of Sombrero group members remains unclear to us.

thumbnail Fig. 4.

Distribution of assumed Sombrero satellites (filled circles) and background galaxies (open circles) in supergalactic coordinates. All galaxies have VLG <  1400 km s−1.

Figure 5 presents the distribution of assumed satellites of Sombrero (solid circles) and background galaxies (open circles) according to angular projected separation, rp, and absolute value of radial velocity difference, |ΔV|. Sombrero satellites dominate within rp <  2.4° (i.e. 400 kpc), and at larger distances assumed Sombrero satellites are lost among the numerous background galaxies. Such a confusion of two categories of galaxies makes it difficult to estimate the virial mass of the Sombrero galaxy.

thumbnail Fig. 5.

Radial velocity difference and angular projected separation for assumed members of the Sombrero group (filled circles) and background galaxies (open circles) taken with respect to the Sombrero galaxy.

4. Total (orbital) mass of Sombrero

The list of 15 assumed satellites of Sombrero with known radial velocities is presented in Table 2. The galaxies are ranked according to their projected separation from Sombrero. The average linear projected separations of the satellites is ⟨Rp⟩ = 431 kpc, the mean difference of their radial velocities with respect to the principal galaxy is +62 ± 54 km s−1, and the dispersion of radial velocities is σv = 204 km s−1.

Table 2.

Satellites of Sombrero with known radial velocities.

We estimated the virial (orbital) mass of the Sombrero galaxy assuming Keplerian motion of satellites, as test particles, around the massive central body. At random orientation of the satellite orbits with the mean eccentricity of ⟨e2⟩ = 1/2 (Barber et al. 2014) the estimate of orbital mass can be written (Karachentsev & Kudrya 2014) as

M orb = ( 16 / π ) G − 1 ⟨ Δ V 2 R p ⟩ , $$ \begin{aligned} M_{\rm orb} = (16/\pi ) G^{-1} \langle \Delta V^2 R_{\rm p}\rangle , \end{aligned} $$(1)

where G is the gravitational constant. If Rp is expressed in kpc and ΔV expressed in km s−1, then

log ( M orb / M ⊙ ) = log ⟨ Δ V 2 R p ⟩ + 6.07 . $$ \begin{aligned} \log (M_{\rm orb}/M_{\odot }) = \log \langle \Delta V^2 R_{\rm p}\rangle + 6.07. \end{aligned} $$(2)

Using all 15 assumed satellites from Table 2 we obtain the mean value of orbital mass to be (17.2 ± 6.1) × 1012 M⊙. For three satellites with accurate TRGB distances this quantity is (15.3 ± 8.1) × 1012 M⊙, while for five dwarfs with TRGB and SBF distances the average orbital mass drops to (10.3 ± 5.4) × 1012 M⊙. Attributing to 15 satellite galaxies different weights (w = 1 for TRGB distances, w = 1/2 for SBF distances and w = 1/4 for TF and mem distances), we derive the weighted mean (15.5 ± 4.9) × 1012 M⊙. We adopt this quantity as the optimal estimate of the total mass of the Sombrero group.

As seen from Table 2, the early-type galaxies are concentrated towards Sombrero much more tightly than spiral and irregular galaxies. Apart from the S0 galaxy NGC 4802, all other E-companions reside in the central zone Rp <  200 kpc. The known effect of morphological segregation is more pronounced if probable Sombrero satellites without radial velocities are taken into account. Table 3 lists 12 dwarf galaxies of low and very low surface brightness have been detected near Sombrero by different authors (Karachentsev et al. 2000, 2020; Javanmardi et al. 2016; Carlsten et al. 2020a,b). None of them is detected in the HI line, and all are classified as spheroidal dwarfs. These objects with quenched star formation have projected separations Rp <  200 kpc, increasing the segregation effect.

Table 3.

Assumed satellites of Sombrero without radial velocities.

5. Concluding remarks

Radial velocities and projected separations of 15 assumed satellites of Sombrero yield the weighted estimate of its total mass (MT/M⊙) = (1.55 ± 0.49) × 1013. At M*/LK = 1 M⊙/L⊙ (Bell et al. 2003) the stellar mass of Sombrero is 2.1 × 1011 M⊙. Accounting for the luminosity of all the satellites increases the stellar mass of the group to M *  = 2.4 × 1011 M⊙. Therefore, the Sombrero halo has a total-mass-to-stellar-mass ratio of MT/M *  = 65 ± 20, which is much higher than the cosmic baryonic ratio, Mhalo/Mb = 6.

Karachentseva et al. (2011) undertook a search for faint companions around 2MASS Isolated Galaxies (2MIG). They found 214 faint neighbours around 125 2MIG galaxies with radial velocity differences of |ΔV| < 500 km s−1 and projected separations of Rp < 500 kpc. For 60 companions around E,S0-galaxies the median ratio of Morb/M* turns out to be 63, while for the remaining 154 spiral galaxies the median ratio is only 17. A similar search for companions around late-type spiral galaxies without bulges was performed by Karachentsev & Karachentseva (2019). Based on 43 companions around 30 Sc-Scd-Sd galaxies, they found the mean ratio Morb/M * = 20 ± 3. The factor of three difference in halo-mass-to-stellar-mass ratio between early-type and late-type luminous galaxies attests to their different dynamical histories.

In the vicinity of Sombrero there are still more than a dozen galaxies with unreliable or even unknown distance estimates. Measurements of their TRGB distances with HST would help us to study the structure and dynamics of this group.

Acknowledgments

We are grateful to the referee for helpful advice. This work is based on observations made with the NASA/ESA Hubble Space Telescope. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. The work in Russia is supported by RFBR grant 18-02-00005.

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All Tables

Table 1.

Galaxies around Sombrero with rp <  6° and VLG <  1400 km s−1.

In the text
Table 2.

Satellites of Sombrero with known radial velocities.

In the text
Table 3.

Assumed satellites of Sombrero without radial velocities.

In the text

All Figures

thumbnail Fig. 1.

HST/ACS combined images of UGCA 307 and KKSG 30. The image size is 1 . ′ 6 × 1 . ′ 4 $ 1{{\overset{\prime}{.}}}6\times1{{\overset{\prime}{.}}}4 $. North is up and east is left.
In the text
thumbnail Fig. 2.

Colour–magnitude diagrams of UGCA 307 and KKSG 30. The TRGB position is indicated by the horizontal line.
In the text
thumbnail Fig. 3.

Distribution of assumed members of the Sombrero group (filled circles) and background galaxies (open circles) according to their distance and radial velocity in the Local Group rest frame. The galaxies satisfy the conditions VLG <  1400 km s−1 and projected separation rp <  6° with respect to Sombrero (asterisk).
In the text
thumbnail Fig. 4.

Distribution of assumed Sombrero satellites (filled circles) and background galaxies (open circles) in supergalactic coordinates. All galaxies have VLG <  1400 km s−1.
In the text
thumbnail Fig. 5.

Radial velocity difference and angular projected separation for assumed members of the Sombrero group (filled circles) and background galaxies (open circles) taken with respect to the Sombrero galaxy.
In the text

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