DDO 161 and UGCA 319: an isolated pair of nearby dwarf galaxies

TL;DR

This study presents detailed observations of two nearby gas-rich dwarf galaxies, DDO 161 and UGCA 319, establishing their distances, isolation, and peculiar velocities, contributing to understanding dwarf galaxy environments.

Contribution

The paper provides precise distance measurements and analyzes the dynamical isolation and velocities of the dwarf galaxy pair, highlighting their unique status among local galaxies.

Findings

Distances of 6.03 Mpc and 5.75 Mpc for DDO 161 and UGCA 319

The galaxies form an isolated pair with significant peculiar velocities

All four galaxies have velocities towards the Virgo cluster

Abstract

We report HST/ACS observations of two nearby gas-rich dwarf galaxies: DDO 161 and UGCA 319. Their distances determined via the Tip of the Red Giant Branch are 6.03 (-0.21/+0.29) Mpc and 5.75+-0.18 Mpc, respectively. The galaxies form an isolated pair dynamically well separated from the nearest neighbors: KK 176 (7.28+-0.29 Mpc) and NGC 5068 (5.16+-0.21 Mpc). All four galaxies have a bulk spatial peculiar velocity towards the Virgo cluster of ~158+-17 km/s in the Local Group rest frame and ~330 km/s with respect to the cluster center.

Tables

Table 1. Table 1: TRGB and distance measurements for the pair components.

TRGB parameters	DDO161	UGCA319
$F814W$	${24.94}_{-0.04}^{+0.08}$	$24.84\pm 0.03$
$⟨F606W-F814W⟩$	$1.07\pm 0.01$	$1.01\pm 0.01$
$E\left(B-V\right)$	$0.070$	$0.072$
${\left(m-M\right)}_0$	${28.90}_{-0.07}^{+0.10}$	$28.80\pm 0.07$
D, Mpc	${6.03}_{-0.21}^{+0.29}$	$5.75\pm 0.18$

Table 2. Table 2: Global properties of the pair components

Parameter	DDO 161	UGCA 319
R.A.,Dec.(J2000.0)	130316.8–172523	130214.4–171415
Morphological type	Sm	Ir
$B_T$, mag	13.5	15.1
Holmberg diameter, arcmin	7.94	1.26
$V_{LG}$, km s-1	543	555 (529)
$W_{50}$, km s-1	113	94 (30)
$\log \left(F_{HI}\right)$, Jy km s-1	1.88	0.88 (-0.20)
$m\left(FUV\right)$	14.6	17.2
Distance, Mpc	6.03	5.75
$M_B$, mag	–16.04	–14.17
$\log (M_{\ast }/$$M_{☉}$$)$	8.75	8.01
$\log (M_{HI}$/$M_{☉}$$)$	8.83	7.76 (6.68)
$\log {\left(sSFR\right)}_{FUV}$, yr-1	–9.80	–10.30
Projected separation, arcmin	18.6
Projected separation, kpc	32.7
Orbital mass,${10}^9$ $M_{☉}$	5.6 (7.6)
$M_{orb}/\left(M_1^{\ast }+M_2^{\ast }\right)$	8.3 (11.3)
$T_{cross}$, Gyr	2.7 (2.3)

Table 3. Table 3: TRGB and global parameters for KK 176 and NGC 5068

Parameter	KK 176	NGC 5068
$F814W$	$25.37\pm 0.06$	$24.68\pm 0.06$
$⟨F606W-F814W⟩$	$1.08\pm 0.01$	$1.31\pm 0.03$
$E\left(B-V\right)$	$0.086$	$0.091$
${\left(m-M\right)}_0$	$29.31\pm 0.09$	$28.56$
D, Mpc	$7.28\pm 0.29$	$5.16\pm 0.21$
R.A.,Dec.(J2000.0)	125956.3–192447	131855.3–210221
Morphological type	Ir	Scd
$B_T$, mag	$16.5$	$10.5$
Holmberg diameter, arcmin	$1.48$	$10.0$
$V_{LG}$, km s-1	$618$	$469$
$W_{50}$, km s-1	$37$	$74$
$log\left(F_{HI}\right)$, Jy km s-1	$0.72$	$2.04$
$m\left(FUV\right)$	$18.8$	$12.8$
Distance, Mpc	$7.28$	$5.16$
$M_B$, mag	$-12.97$	$-18.51$
$log\left(M_{\ast }/M_{☉}\right)$	$7.44$	$9.73$
$log\left(M_{HI}/M_{☉}\right)$	$7.81$	$8.82$
$log{\left(sSFR\right)}_{FUV}$, yr-1	$-10.34$	$-10.06$

Equations

$$M(F814W)=-4.06+0.20[(F606W-F814W)-1.23].$$

$$log(sSFR)=2.78-0.4\times m(FUV)+2\times log(D_{Mpc})-log(M_{*}/\mbox{$M_{\sun}$}{})$$

$$M_{orb}=(16/\pi \times G)\times R_p\times \Delta V^2$$

Full text

††pagerange: DDO 161 and UGCA 319: an isolated pair of nearby dwarf galaxies–DDO 161 and UGCA 319: an isolated pair of nearby dwarf galaxies††pubyear: 2016

1 Introduction

Tully et al. (2002) drew attention to the existence of a special category of systems of galaxies containing only galaxies of low luminosity, like Magellanic clouds or below. Subsequent accurate measurements of their distances with the Hubble Space Telescope (HST) using the Tip of the Red Giant Branch (TRGB) confirmed that dwarf galaxies in these nests are associated with each other not only on the sky but also by the spatial distances (Tully et al., 2006). Their closest example is a quartet of dwarfs: NGC 3109, Sex A, Sex B and Antlia, located just beyond the Local Group threshold at a distance of $\sim 1.4$ Mpc. Later, the quartet was transformed into elongated sextet due to the detection of two ultra-dwarf galaxies: Antlia B and Leo P (McQuinn et al., 2013; Sand et al., 2015). According to Tully et al. (2006), associations of dwarfs are characterized by a dimension of $\sim 400$ kpc, a radial velocity dispersion of $\sim 30$ km s*-1*, a total stellar mass of $\sim 10^{9}$ $M_{\sun}$ and a virial-to-stellar mass ratio of $\sim(300-1000)$. Because of low luminosity, associations of dwarfs are detectable predominantly in the nearby volume.

Considering a sample of about 10000 galaxies in the nearby universe with radial velocities $V_{LG}<3500$ km s*-1* respect to the Local Group rest frame, Karachentsev & Makarov (2008) and Makarov & Karachentsev (2011) clusterized this population into groups of different multiplicity. Their clustering algorithm took into account the large observed diversity of galaxies in luminosities. In this sample, Karachentsev & Makarov (2008) noted the unexpected abundance of binary dwarf galaxies. Makarov & Uklein (2012) compiled a list 57 systems in the volume of $V_{LG}<3500$ km s*-1* formed by two or more dwarf galaxies. A typical size of these MU-groups is $\sim 30$ kpc, a typical velocity dispersion is only $\sim 10$ km s*-1*. At the mean stellar mass of group of dwarfs about $3\times 10^{8}$ $M_{\sun}$, their typical virial-to-stellar mass ratio turns out to be $\sim 40$. The authors noted a lack of gap in size and luminosity between the MU-groups and associations of dwarfs discussed by Tully et al. (2006). However, while a ”crossing time” for associations of dwarfs is $\sim 9$ Gyr, i.e. comparable with the age of the universe, the mean crossing time for MU-groups is only $\sim 3$ Gyr, being typical for dynamically relaxed systems.

As has been noted by Makarov & Uklein (2012), pairs and groups of dwarfs prefer to reside in regions of low spatial density, and their components are usually gas-rich dwarfs. Observing the population of nearby voids in the 21 cm HI line with the Giant Metrewave Radio Telescope, Chengalur & Pustilnik (2013) and Chengalur et al. (2017) found several multiple gas-rich dwarfs of very low metallicity. Presumably, a thorough HI-survey of nearby dwarfs with the spatial resolution offered by interferometers could significantly extend the existing collection of multiple dwarf galaxies.

In this Note, we report on measurements of accurate distances to dwarf galaxies DDO 161 = UGCA 320 and UGCA 319 with radial velocities 543 km s*-1* and 555 km s*-1* respectively, which form a tight pair well separated from other neighboring galaxies.

2 HST observations and data reduction

The galaxies DDO 161 and UGCA 319 were observed with the Hubble Space Telescope (HST) using Advanced Camera for Surveys (ACS) on February 11 and 14, 2017 (cycle 24 proposal GO-14636, PI Karachentsev). Four exposures were made in a single orbit per galaxy: F606W (broad- band $V$), F814W (broad-band $I$) filters at the same position as well as F606W and F814W at a dithered position, about 3 arcsec away. The total exposure time per galaxy for each filter was 1030 seconds. The F606W images of the galaxies are shown in Fig. 1. The dimension of the fields is about 3.3 arcmin.

The photometry of resolved stars in the galaxies were performed with the ACS module of the DOLPHOT package (http://purcell.as.arizona.edu/dolphot/) for crowded field photometry (Dolphin, 2002) using the recommended recipe and parameters. Only stars with photometry of good quality were included in the final compilation, following recommendations given in the DOLPHOT User’s Guide. Artificial stars were inserted and recovered using the same reduction procedures to accurately estimate photometric errors, including crowding and blending effects. A large library of artificial stars was generated spanning the full range of observed stellar magnitudes and colours to assure that the distribution of the recovered photometry is adequately sampled.

Colour-magnitude diagrams (CMDs) for the detected stars in DDO 161 and UGCA 319 are shown in Figure 2. In the case of DDO 161 we present the CMD for the complete galaxy body (left panel) and after excluding some central regions crowded by star formation complexes (middle panel). The method of deriving the distance from the Tip of the Red Giant Branch (TRGB) magnitude rests on a well understood physical basis (Madore et al., 1997). Observationally, there is a sharp cutoff of the bright end of the red giant branch luminosity function at $M_{I}\simeq-4.0$ mag.

We determined the TRGB using a maximum-likelihood algorithm written and provided by Makarov et al. (2006). The estimated values of the TRGB and their errors are presented in Table 1. The measured TRGB positions are marked in Fig. 2 by the solid line. The absolute magnitude of the TRGB in the HST flight system for F606W and F814W using ACS was estimated using the calibration from Rizzi et al. (2007):

[TABLE]

The formula defines a zero-point calibration of the TRGB as a function of the stellar population colour, accounting for variation due to metallicity and age. As a result, we derived the distances: $6.03_{-0.21}^{+0.29}$ Mpc and $5.75\pm 0.18$ Mpc for DDO 161 and UGCA 319 respectively. These measurements assume foreground Galactic extinction, $E(B-V)$, according to Schlafly & Finkbeiner (2011). Based on the accurate TRGB distance determinations, we conclude that DDO 161 and UGCA 319 are spatially associated each other forming a physical system. The measured distances differ by $280\pm 310$ kpc.

3 Discussion

NASA Extragalactic Database (=NED, http://ned.ipac.caltech.edu) contains 10 distance estimates for DDO 161 made by the Tully-Fisher (Tully & Fisher, 1977) method in its different modifications. The average value among them, 6.1 Mpc, is perfectly consistent with the TRGB distance 6.03 Mpc measured by us. In contrast to DDO 161, NED presents for UGCA 319 only a single distance estimate, 19.7 Mpc, via the infra-red Tully-Fisher relation (Sorce et al., 2014) that is in sharp contradiction to our present TRGB measurement. The nature of this discrepancy is caused apparently by confusion of the HI-line width of UGCA 319 with the strong HI-flux from the close neighbor DDO 161. This region of sky has been imaged in HI with Australian Telescope Compact Array (ATCA) by Pisano et al. (2011). Answering our request, Daniel Pisano kindly re-estimated HI parameters for UGCA 319 from ATCA observations.

The basic properties of DDO 161 and UGCA 319 taken from the NED and HyperLEDA (Makarov et al., 2014) databases are presented in Table 2. The data for UGCA 319 from D.Pisano are given in parentheses. The table contains: equatorial coordinates; morphological type; total B-magnitude; Holmberg’s diameter; radial velocity; 21 cm line width at the 50%-level; the HI line flux; apparent magnitude in the Far Ultra-Violet (Gil de Paz et al., 2007); distance measured via TRGB; absolute magnitude corrected for Galactic and internal extinction; stellar mass derived via the K-band luminosity from the Local Volume Galaxy Data Base (http://www.sao.ru/lv/lvgdb/) assuming M_{*}/L_{K}=1\times$$M_{\sun}/L_{\sun}; hydrogen mass and specific star formation rate

[TABLE]

calculated via the FUV-flux (Lee et al., 2011).

As one can see, the galaxies are gas-rich dwarf systems. Their present star formation rate is moderate, but sufficient to reproduce the observed stellar mass of the galaxies over the age of universe, 13.7 Gyr.

DDO 161 and UGCA 319 as a physical pair of dwarfs are present in the lists by Karachentsev & Makarov (2008) and Makarov & Uklein (2012). The linear projected separation of the components, $R_{p}$, is 32.7 kpc. At the radial velocity difference $\Delta V$ = 12 (14) km s*-1* the orbital mass of the pair,

[TABLE]

is $5.6\,(7.6)\times 10^{9}$ $M_{\sun}$, being 8.3 (11.3) times the sum of stellar masses of the components. These parameters of the binary dwarf system are presented in the last rows of Table 2 along with the crossing time, $t_{cross}=R_{p}/\Delta V\sim 2.7\,(2.3)$ Gyr.

Note that the first estimates of dynamical mass of DDO 161 have been derived by Fisher & Tully (1975) and Karachentsev et al. (1981) from the HI line width $W=2\times V_{m}=114\pm 7$ km s*-1* and from the optical rotation amplitude of $V_{m}\simeq 50$ km s*-1*, respectively. The implied internal dynamical mass, $\sim(1.5-4.7)\times 10^{9}$ $M_{\sun}$, is in a reasonable agreement with the orbital mass estimate, $(5.6-7.6)\times 10^{9}$ $M_{\sun}$, presented in this Note.

4 The pair environment

Apart from UGCA 319, in the wide surroundings of DDO 161 up to the angular separation of $\sim 5^{o}$ there are only two galaxies with radial velocities within $\pm 200$ km s*-1* around the velocity of DDO 161: the dwarf galaxy KK 176 at a separation of 128 arcmin with the velocity of $V_{LG}=618$ km s*-1*, and the spiral galaxy NGC 5068 at a separation of 310 arcmin with the velocity of $V_{LG}=469$ km s*-1*. Pisano et al. (2011) assumed that DDO 161 together with KK 176, NGC 5068 and another dwarf BCD galaxy MCG-03-34-002 ($V_{LG}=765$ km s*-1*) form a loose ”HIPASS”-group having the linear diameter of $\sim 600$ kpc, radial velocity dispersion of $124$ km s*-1* and the virial mass of $5.1\pm 2.5\times 10^{11}$ $M_{\sun}$.

We have observed the galaxies KK 176 and NGC 5068 with ACS on HST during February 1 and February 11, 2017 in the same program with DDO 161 and UGCA 319. The CM diagrams for both the galaxies are presented in Fig. 3. The estimated values of the TRGB for KK 176 and NGC 5068 as well as basic properties of them are given in Table 3. The distance estimates: $7.28\pm 0.29$ Mpc for KK 176 and $5.16\pm 0.21$ Mpc for NGC 5068 do not give grounds for considering these neighbors to be physically connected to the tight pair DDO 161/UGCA 319. The larger galaxy NGC 5068 is 540 kpc removed in projection and $730\pm 310$ kpc removed in the line of sight.

It is interesting to note that all 4 galaxies with accurately measured distances have positive peculiar velocities, $V_{pec}=V_{LG}-H_{0}\times D$ assuming the Hubble parameter $H_{0}=73$ km s*-1* Mpc*-1*, i.e.: $+103$ km s*-1* (DDO 161), $+135$ km s*-1* (UGCA 319), $+87$ km s*-1* (KK 176), and $+92$ km s*-1* (NGC 5068). They are located in the Local supergalactic plane South from the Virgo cluster center at angular separations of $(30-35)^{o}$. An apparent reason of their positive peculiar velocities is the infall of galaxies towards the Virgo cluster as the nearest massive attractor. Assuming strictly radial infall towards Virgo, we get the complete (spatial) vector of the mean peculiar velocity of the galaxy ”flock” towards the cluster: $(104\pm 11)/cos(49^{o})\simeq(158\pm 17)$ km s*-1*. Here, observed velocities are given with respect to the Local Group centroid.

According to Mei et al. (2007), the Virgo cluster center, distant from the Local Group at 16.5 Mpc, has the mean velocity of $V_{LG}=1034\pm 60$ km s*-1*. It corresponds to the infall velocity of the Local Group towards the Virgo of $170\pm 60$ km s*-1*, then the considered galaxy flock falls to the cluster with the velocity of $\sim 330$ km s*-1*. Being at a distance of about 11.8 Mpc from the Virgo center, the galaxies locate outside the zero-velocity radius of the Virgo, $R_{0}=7.2\pm 0.7$ Mpc (Karachentsev et al., 2014). Given their distance from Virgo and peculiar velocity, together with similar data on other targets, one can calculate the Virgo cluster mass, assuming spherical symmetry, with the formulae in Tully & Shaya (1984).

As it was noted above, gas-rich multiple dwarf systems, like DDO 161 + UGCA 319, are attractive targets for studies of their structure and kinematics using HI-observations with interferometer.

Acknowledgements

This work is based on observations made with the NASA/ESA Hubble Space Telescope, program GO-14636, with data archive at the Space Telescope Science Institute. 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 the Russian Science Foundation grant 14–12–00965. The authors thank Daniel Pisano for re-estimating HI parameters for UGCA 319 from ATCA observations and John Cannon for providing VLA results that clearly resolve DDO 161 and UGCA 319.