arXiv:1704.07648v1 [astro-ph.GA] 25 Apr 2017 MNRAS ul tal. et Tully omdit lnae ettdet h eeto ftwo ( of P Leo detection and the B to Antlia due : sextet ultra-dwarf elongated into formed of dimension a by characterized are ∼ dwarfs of associations 2013 threshold of Group B Sex Local distance A, the Sex a beyond 3109, at NGC just dwarfs: located of Antlia, quartet and a but is sky ( example the distances est on spatial only the not by other these also each in with galaxies associated dwarf are that nests confirmed (TRGB) Branch the ant with ( distances Sub- Telescope below. their Space Hubble or of galax- clouds measurements only Magellanic accurate containing like sequent luminosity, galaxies low of of systems ies of category cial n .J Shaya J. E. and Karachentsev, D. dwarf I. nearby of pair isolated an 319: galaxies UGCA and 161 DDO oa tla asof mass stellar total ⋆ e ntenab nvrewt ailvelocities radial with universe nearby the V in ies nearby the in predominantly detectable volume. are dwarfs of tions ai of ratio 7Jl 2018 July 27 INTRODUCTION 1 c 3 2 1 5 4 LG -al [email protected] E-mail: .M ekOsraoy 512 aaao w,Kmea I9 HI Kamuela, Hwy, Mamalahoa 65-1120 Observatory, Keck M. W. nttt o srnm,Uiest fHwi,28 Woodla 2680 Hawaii, of University Astronomy, for Institute pca srpyia bevtr fRS ihi Arkhyz, Nizhnij RAS, of Observatory Astrophysical Special srnm eatet nvriyo ayad olg Par College Maryland, of University Department, Astronomy anAtooia bevtr,Aaeyo cecso Ukra of Sciences of Academy Observatory, Astronomical Main 06TeAuthors The 2016 0 p,arda eoiydseso of dispersion velocity radial a kpc, 400 osdrn apeo bu 00 galax- 10000 about of sample a Considering ; ade l 2015 al. et Sand < ∼ 000 (300 ( , 50k s km 3500 2002 1 – − 5 ∼ 21)Pern 7Jl 08Cmie sn NA L MNRAS using Compiled 2018 July 27 Preprint (2016) rwatnint h xsec faspe- a of existence the to attention drew ) 00.Bcueo o uioiy associa- luminosity, low of Because 1000). 1 ∼ . − p.Ltr h ure a trans- was quartet the Later, Mpc. 4 .Acrigto According ). 1 10 5 HST 9 epc oteLclGroup Local the to respect M ⊙ 1 ul ta.2006 al. et Tully sn h i fteRdGi- Red the of Tip the using ) ⋆ n iilt-tla mass virial-to-stellar a and yaial elsprtdfo h ers egbr:K 7 (7 176 KK neighbors: nearest the from separated well dynamically oad h ig lse of cluster the towards G 08(5 5068 NGC n GA39 hi itne eemndvateTpo h e Giant Red the of Tip the via determined distances 6 Their 319. UGCA and ABSTRACT ∼ otn aais niiul D 6,UC 1,K 7,NC5068 NGC 176, KK 319, UGCA 161, DDO individual: galaxies: – content words: Key .N Makarova, N. L. . 03 3 ms km 330 ereport We − +0 0 . . ul tal. et Tully 29 21 p n 5 and Mpc ∼ − cun tal. et McQuinn 1 .Terclos- Their ). 0k s km 30 . ihrsett h lse center. cluster the to respect with 16 HST aais itne n esit aais wr aais stella galaxies: – dwarf galaxies: – and distances galaxies: ± ( 2006 0 ASosrain ftonab a-ihdafglxe:DO161 DDO galaxies: dwarf gas-rich nearby two of observations /ACS − . . 1 75 1Mc.Alfu aaishv uksailpcla velocity peculiar spatial bulk a have galaxies four All Mpc). 21 a , ), CR 617 Russia 369167, KChR, 1 nDie I982 USA 96822, HI Drive, wn ± ,M 04,USA 20743, MD k, n,2 kdmk aoonh t A013Ki,Ukraine Kyiv, UA-03143 St, Zabolotnoho Akademika 27 ine, .B Tully, B. R. 0 . 8Mc epciey h aaisfr nioae pair isolated an form galaxies The respectively. Mpc, 18 ∼ h Ugop n soitoso wrsdsusdby discussed dwarfs of between luminosity associations and be size and in to MU-groups gap of out the lack turns a noted ratio authors mass virial-to-stellar aso ru fdaf bu 3 about dwarfs of group of mass 1c Iln ihteGatMteaeRdoTelescope, Radio the Metrewave in Giant low the voids with of nearby line regions of HI gas-rich in cm population usually 21 reside the are Observing components to their dwarfs. prefer and dwarfs density, spatial of groups and is dwarfs of associations on eea utpegsrc wrso eylwmetal- low very of dwarfs gas-rich multiple several found Pustilnik & Chengalur al. et Tully sample, galaxies. this dwarf In binary observed ( luminosities. of large in abundance the clus- Makarov galaxies account Their & Karachentsev multiplicity. into of different took diversity of algorithm groups tering into lation Karachentsev & Makarov yia ieo hs Ugop is MU-groups only these is dispersion of velocity size typical A g fteuies,tema rsigtm o MU-groups for time systems. crossing only mean the is universe, the of age V etframe, rest 73 USA 6743, 2012 LG 158 shsbe oe by noted been has As < opldals 7ssesi h oueof volume the in systems 57 list a compiled ) ± 50k s km 3500 ∼ 7k s km 17 y,bigtpclfrdnmclyrelaxed dynamically for typical being Gyr, 3 ( 2006 2 .Rizzi, L. − aahnsv&Makarov & Karachentsev − .Hwvr hl cosn ie for time” ”crossing a while However, ). 1 1 omdb w rmr wr galaxies. dwarf more or two by formed nteLclGoprs rm and frame rest Group Local the in ( 2013 ∼ ∼ ( ( aao Uklein & Makarov y,ie oprbewt the with comparable i.e. Gyr, 9 2008 2011 3 0k s km 10 and ) .E Karachentseva E. V. oe h unexpected the noted ) lseie hspopu- this clusterized ) × hnau tal. et Chengalur − 10 1 . ttema stellar mean the At . 28 ∼ 8 aao Uklein & Makarov M A ± 0kc typical a kpc, 30 T ⊙ E 0 tl l v3.0 file style X hi typical their , . ( 9Mc and Mpc) 29 ( 2008 ∼ rnhare Branch 2012 0 The 40. and ) ,pairs ), ( 2017 r ) 4 2 I. D. Karachentsev, L. N. Makarova, R. B. Tully, L. Rizzi, V. E. Karachentseva & E. Shaya

Table 1. TRGB and distance measurements for the pair compo- Table 2. Global properties of the pair components nents. Parameter DDO 161 UGCA 319 TRGB parameters DDO161 UGCA319 R.A.,Dec.(J2000.0) 130316.8–172523 130214.4–171415 +0.08 F 814W 24.94−0.04 24.84 ± 0.03 Morphological type Sm Ir hF 606W − F 814W i 1.07 ± 0.01 1.01 ± 0.01 BT , mag 13.5 15.1 E(B − V ) 0.070 0.072 Holmberg diameter, arcmin 7.94 1.26 +0.10 −1 (m − M)0 28.90−0.07 28.80 ± 0.07 VLG, kms 543 555 (529) +0.29 −1 D,Mpc 6.03−0.21 5.75 ± 0.18 W50, kms 113 94 (30) −1 log(FHI ), Jy km s 1.88 0.88 (-0.20) m(F UV ) 14.6 17.2 licity. Presumably, a thorough HI-survey of nearby dwarfs Distance, Mpc 6.03 5.75 with the spatial resolution offered by interferometers could MB, mag –16.04 –14.17 significantly extend the existing collection of multiple dwarf log(M∗/M⊙) 8.75 8.01 log(MHI /M⊙) 8.83 7.76 (6.68) galaxies. −1 log(sSFR)F UV , yr –9.80 –10.30 In this Note, we report on measurements of accurate distances to dwarf galaxies DDO 161 = UGCA 320 and Projected separation, arcmin 18.6 − − UGCA 319 with radial velocities 543 km s 1 and 555 km s 1 Projected separation, kpc 32.7 9 Orbital mass,10 M⊙ 5.6 (7.6) respectively, which form a tight pair well separated from ∗ ∗ other neighboring galaxies. Morb/(M1 + M2 ) 8.3 (11.3) Tcross, Gyr 2.7 (2.3)

2 HST OBSERVATIONS AND DATA marked in Fig. 2 by the solid line. The absolute magni- REDUCTION tude of the TRGB in the HST flight system for F606W and F814W using ACS was estimated using the calibration The galaxies DDO 161 and UGCA 319 were observed with from Rizzi et al. (2007): the (HST ) using Advanced Camera for Surveys (ACS) on February 11 and 14, 2017 (cycle 24 M(F 814W )= −4.06 + 0.20[(F 606W − F 814W ) − 1.23]. proposal GO-14636, PI Karachentsev). Four exposures were The formula defines a zero-point calibration of the TRGB made in a single orbit per : F606W (broad- band V ), as a function of the stellar population colour, accounting F814W (broad-band I) filters at the same position as well as for variation due to metallicity and age. As a result, we F606W and F814W at a dithered position, about 3 arcsec +0.29 derived the distances: 6.03−0.21 Mpc and 5.75 ± 0.18 Mpc away. The total exposure time per galaxy for each filter was for DDO 161 and UGCA 319 respectively. These measure- 1030 seconds. The F606W images of the galaxies are shown ments assume foreground Galactic extinction, E(B − V ), in Fig. 1. The dimension of the fields is about 3.3 arcmin. according to Schlafly & Finkbeiner (2011). Based on the The photometry of resolved stars in the galaxies were accurate TRGB distance determinations, we conclude that performed with the ACS module of the DOLPHOT package DDO 161 and UGCA 319 are spatially associated each other (http://purcell.as.arizona.edu/dolphot/) for crowded field forming a physical system. The measured distances differ by photometry (Dolphin 2002) using the recommended recipe 280 ± 310 kpc. and parameters. Only stars with photometry of good qual- ity were included in the final compilation, following recom- mendations given in the DOLPHOT User’s Guide. Artificial 3 DISCUSSION stars were inserted and recovered using the same reduction procedures to accurately estimate photometric errors, in- NASA Extragalactic Database (=NED, cluding crowding and blending effects. A large library of arti- http://ned.ipac.caltech.edu) contains 10 distance estimates ficial stars was generated spanning the full range of observed for DDO 161 made by the Tully-Fisher (Tully & Fisher stellar magnitudes and colours to assure that the distribu- 1977) method in its different modifications. The average tion of the recovered photometry is adequately sampled. value among them, 6.1 Mpc, is perfectly consistent with Colour-magnitude diagrams (CMDs) for the detected the TRGB distance 6.03 Mpc measured by us. In contrast stars in DDO 161 and UGCA 319 are shown in Figure 2. In to DDO 161, NED presents for UGCA 319 only a single the case of DDO 161 we present the CMD for the complete distance estimate, 19.7 Mpc, via the infra-red Tully-Fisher galaxy body (left panel) and after excluding some central relation (Sorce et al. 2014) that is in sharp contradiction regions crowded by star formation complexes (middle panel). to our present TRGB measurement. The nature of this The method of deriving the distance from the Tip of the Red discrepancy is caused apparently by confusion of the HI-line Giant Branch (TRGB) magnitude rests on a well understood width of UGCA 319 with the strong HI-flux from the close physical basis (Madore et al. 1997). Observationally, there neighbor DDO 161. This region of sky has been imaged in is a sharp cutoff of the bright end of the red giant branch HI with Australian Telescope Compact Array (ATCA) by luminosity function at MI ≃−4.0 mag. Pisano et al. (2011). Answering our request, Daniel Pisano We determined the TRGB using a maximum-likelihood kindly re-estimated HI parameters for UGCA 319 from algorithm written and provided by Makarov et al. (2006). ATCA observations. The estimated values of the TRGB and their errors are The basic properties of DDO 161 and UGCA 319 taken presented in Table 1. The measured TRGB positions are from the NED and HyperLEDA (Makarov et al. 2014)

MNRAS 000, 1–5 (2016) An isolated pair of nearby dwarfs 3

Figure 1. HST /ACS images of DDO 161 (left panel) and UGCA 319 (right panel) in the F606W filter. The dimension of the fields is about 3.3 arcmin. North is straight out to the upper left corners of the images, and East is to the lower left ones.

Figure 2. DDO 161 = UGCA 320 and UGCA 319 colour-magnitude diagrams. Photometric errors are indicated by the bars at the right in the CMD. databases are presented in Table 2. The data for UGCA 319 Their present star formation rate is moderate, but sufficient from D.Pisano are given in parentheses. The table con- to reproduce the observed stellar mass of the galaxies over tains: equatorial coordinates; morphological type; total B- the age of universe, 13.7 Gyr. magnitude; Holmberg’s diameter; ; 21 cm line DDO 161 and UGCA 319 as a physical pair of dwarfs are width at the 50%-level; the HI line flux; apparent magni- present in the lists by Karachentsev & Makarov (2008) and tude in the Far Ultra-Violet (Gil de Paz et al. 2007); dis- Makarov & Uklein (2012). The linear projected separation tance measured via TRGB; absolute magnitude corrected of the components, Rp, is 32.7 kpc. At the radial velocity − for Galactic and internal extinction; stellar mass derived via difference ∆V = 12 (14) km s 1 the orbital mass of the pair, the K-band luminosity from the Local Volume Galaxy Data

Base (http://www.sao.ru/lv/lvgdb/) assuming M∗/LK = 2 Morb = (16/π × G) × Rp × ∆V 1×M⊙/L⊙; hydrogen mass and specific star formation rate 9 is 5.6 (7.6) × 10 M⊙, being 8.3 (11.3) times the sum of stellar masses of the components. These parameters of log(sSFR) = 2.78−0.4×m(FUV )+2×log(DMpc)−log(M∗/M⊙) the binary dwarf system are presented in the last rows of calculated via the FUV-flux (Lee et al. 2011). Table 2 along with the crossing time, tcross = Rp/∆V ∼ As one can see, the galaxies are gas-rich dwarf systems. 2.7 (2.3) Gyr.

MNRAS 000, 1–5 (2016) 4 I. D. Karachentsev, L. N. Makarova, R. B. Tully, L. Rizzi, V. E. Karachentseva & E. Shaya

−1 −1 −1 eter H0 = 73 km s Mpc , i.e.: +103 km s (DDO 161), Table 3. TRGB and global parameters for KK 176 and NGC 5068 − − +135 km s 1 (UGCA 319), +87 km s 1 (KK 176), and −1 Parameter KK 176 NGC 5068 +92 km s (NGC 5068). They are located in the Local su- pergalactic plane South from the center at F 814W 25.37 ± 0.06 24.68 ± 0.06 angular separations of (30 − 35)o. An apparent reason of hF 606W − F 814W i 1.08 ± 0.01 1.31 ± 0.03 their positive peculiar velocities is the infall of galaxies to- E(B − V ) 0.086 0.091 wards the Virgo cluster as the nearest massive attractor. As- (m − M)0 29.31 ± 0.09 28.56 D,Mpc 7.28 ± 0.29 5.16 ± 0.21 suming strictly radial infall towards Virgo, we get the com- R.A.,Dec.(J2000.0) 125956.3–192447 131855.3–210221 plete (spatial) vector of the mean peculiar velocity of the o Morphological type Ir Scd galaxy ”flock” towards the cluster: (104 ± 11)/cos(49 ) ≃ −1 BT , mag 16.5 10.5 (158 ± 17) km s . Here, observed velocities are given with Holmberg diameter, arcmin 1.48 10.0 respect to the Local Group centroid. −1 VLG, kms 618 469 According to Mei et al. (2007), the Virgo cluster cen- −1 W50, kms 37 74 ter, distant from the Local Group at 16.5 Mpc, has the log(F ), Jy km s−1 0.72 2.04 −1 HI mean velocity of VLG = 1034 ± 60 km s . It corresponds m(F UV ) 18.8 12.8 to the infall velocity of the Local Group towards the Virgo Distance, Mpc 7.28 5.16 −1 of 170 ± 60 km s , then the considered galaxy flock falls M , mag −12.97 −18.51 − B to the cluster with the velocity of ∼ 330 km s 1. Being at log(M∗/M⊙) 7.44 9.73 a distance of about 11.8 Mpc from the Virgo center, the log(MHI /M⊙) 7.81 8.82 −1 log(sSFR)F UV , yr −10.34 −10.06 galaxies locate outside the zero-velocity radius of the Virgo, R0 = 7.2 ± 0.7 Mpc (Karachentsev et al. 2014). Given their distance from Virgo and peculiar velocity, together with sim- Note that the first estimates of dynamical mass of ilar data on other targets, one can calculate the Virgo clus- DDO 161 have been derived by Fisher & Tully (1975) and ter mass, assuming spherical symmetry, with the formulae Karachentsev et al. (1981) from the HI line width W = in Tully & Shaya (1984). −1 2 × Vm = 114 ± 7 kms and from the optical rota- As it was noted above, gas-rich multiple dwarf systems, −1 tion amplitude of Vm ≃ 50 km s , respectively. The im- like DDO 161 + UGCA 319, are attractive targets for studies 9 plied internal dynamical mass, ∼ (1.5 − 4.7) × 10 M⊙, is of their structure and kinematics using HI-observations with in a reasonable agreement with the orbital mass estimate, interferometer. 9 (5.6 − 7.6) × 10 M⊙, presented in this Note.

ACKNOWLEDGEMENTS

4 THE PAIR ENVIRONMENT This work is based on observations made with the Apart from UGCA 319, in the wide surroundings of NASA/ESA Hubble Space Telescope, program GO-14636, DDO 161 up to the angular separation of ∼ 5o there are with data archive at the Space Telescope Science Insti- only two galaxies with radial velocities within ±200 km s−1 tute. STScI is operated by the Association of Universi- around the velocity of DDO 161: the dwarf galaxy KK 176 ties for Research in Astronomy, Inc. under NASA contract NAS 5-26555. The work in Russia is supported by the Rus- at a separation of 128 arcmin with the velocity of VLG = 618 km s−1, and the NGC 5068 at a separa- sian Science Foundation grant 14–12–00965. The authors −1 thank Daniel Pisano for re-estimating HI parameters for tion of 310 arcmin with the velocity of VLG = 469 km s . Pisano et al. (2011) assumed that DDO 161 together with UGCA 319 from ATCA observations and John Cannon for KK 176, NGC 5068 and another dwarf BCD galaxy MCG- providing VLA results that clearly resolve DDO 161 and −1 UGCA 319. 03-34-002 (VLG = 765 km s ) form a loose ”HIPASS”-group having the linear diameter of ∼ 600 kpc, radial velocity dis- − persion of 124 km s 1 and the virial mass of 5.1 ± 2.5 × 11 10 M⊙. REFERENCES We have observed the galaxies KK 176 and NGC 5068 Chengalur J. N., Pustilnik S. A., 2013, MNRAS, 428, 1579 with ACS on HST during February 1 and February 11, Chengalur J. N., Pustilnik S. A., Egorova E. S., 2017, MNRAS, 2017 in the same program with DDO 161 and UGCA 319. 465, 2342 The CM diagrams for both the galaxies are presented in Dolphin A. E., 2002, MNRAS, 332, 91 Fig. 3. The estimated values of the TRGB for KK 176 and Fisher J. R., Tully R. B., 1975, A&A, 44, 151 NGC 5068 as well as basic properties of them are given in Ta- Gil de Paz A., et al., 2007, ApJS, 173, 185 ble 3. The distance estimates: 7.28±0.29 Mpc for KK 176 and Karachentsev I. D., Makarov D. I., 2008, Astrophysical Bulletin, 5.16±0.21 Mpc for NGC 5068 do not give grounds for consid- 63, 299 ering these neighbors to be physically connected to the tight Karachentsev I. D., Karachentseva V. E., Mineva V. A., 1981, Pisma v Astronomicheskii Zhurnal, 7, 533 pair DDO 161/UGCA 319. The larger galaxy NGC 5068 is ± Karachentsev I. D., Tully R. B., Wu P.-F., Shaya E. J., Dolphin 540 kpc removed in projection and 730 310 kpc removed A. E., 2014, ApJ, 782, 4 in the line of sight. Lee J. C., et al., 2011, ApJS, 192, 6 It is interesting to note that all 4 galaxies with ac- Madore B. F., Freedman W. L., Sakai S., 1997, in Livio M., Don- curately measured distances have positive peculiar veloci- ahue M., Panagia N., eds, The Extragalactic Distance Scale. ties, Vpec = VLG − H0 × D assuming the Hubble param- pp 239–253

MNRAS 000, 1–5 (2016) An isolated pair of nearby dwarfs 5

Figure 3. KK 176 and NGC 5068 colour-magnitude diagrams. Photometric errors are indicated by the bars at the right in the CMD.

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MNRAS 000, 1–5 (2016)