arxiv:1710.09040v1 [astro-ph.ga] 25 oct 2017draft version october 26, 2017 preprint typeset using...

Draft Version October 26, 2017Preprint typeset using LATEX style emulateapj v. 12/16/11

UGC 4703 INTERACTING PAIR NEAR TO AN ISOLATED SPIRAL GALAXY NGC 2718: A MILKY WAYMAGELLANIC CLOUD ANALOGUE

Sanjaya Paudel1, C. Sengupta 2

1 Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Korea2 Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea

Draft Version October 26, 2017

ABSTRACT

We present an analysis of physical and morphological properties of an interacting pair of dwarfgalaxies UGC 4703 located at the vicinity of an isolated Milky-Way (MW) type spiral galaxy NGC2718. Based on the comparison of physical and morphological properties with that of the Large andSmall Magellanic Clouds (LMC and SMC), we report that the UGC 4703 pair-NGC 2718 system isprobably an LMC-SMC-MW analogue. Located at a sky projected distance of 81 kpc from NGC 2718,we find that UGC 4703 is clearly interacting with its nearby smaller mass companion UGC 4703Bforming a bridge of stellar stream between them. Total B-band luminosity of UGC 4703 and itscompanion is -17.75 and -16.25 mag, respectively. We obtained HI 21-cm line data of UGC 4703 usingthe GMRT to get a more detailed view of neutral hydrogen (HI) emission. The HI image revealedevidence of interaction between the dwarf galaxy pair but no extended emission, like Magellanic Stream(MS), was detected between NGC 2781 and the UGC 4703 dwarf pair. We also detected star-formingregions along the UGC 4703/ 4703B bridge with stellar mass exceeding 107 M�. While comparingthe optical and HI morphology of the interacting dwarf pairs (UGC 4703-4703B and LMC-SMC) wediscuss possible difference in interaction histories of these systems.

Subject headings: galaxies: dwarf - galaxies: evolution - galaxies: individual (UGC 4703) - galax-ies: interactions - galaxies: groups: individual (NGC 2718) - galaxies: MagellanicClouds

1. INTRODUCTION

Unusual morphology of the Large and Small Magel-lanic clouds (LMC and SMC) in optical and radio obser-vations has been used to interpret their close encounter(Gardiner & Noguchi 1996), and the resultant triggeredstar formation (Harris & Zaritsky 2009; Glatt et al.2010). Besla et al. (2007) suggested that both dwarfgalaxies may be entering the Milky-Way (MW) system asa binary pair for the first time. Magellanic clouds are theonly bright and star-forming satellites of MW. Numberof studies have shown that a satellite pair of LMC-SMCmass around MW mass host is neither common in ob-servation, nor in numerical simulation with hierarchicalaccretion (Boylan-Kolchin et al. 2010; Robotham et al.2012; Tollerud et al. 2011). Analyzing a result of theMillennium-II Simulation, Boylan-Kolchin et al. (2010)predicted that there is a less than 10 per cent chancethat MW mass halo hosts two subhalos of mass of Mag-ellanic clouds. Similarly, analyzing the Galaxy And MassAssembly (GAMA) catalog of galaxies, Robotham et al.(2012) calculated the probability of such system is lessthan 5%. Following this statistics, the merger probabilityof LMC-SMC morphology dwarf galaxy satellites aroundthe MW mass host maybe even smaller.

It is commonly believed that, by having a shallow po-tential well, low-mass dwarf satellites should be moreinfluenced by tidal potential of parent halos, and lessso by the merging events (Paudel et al. 2013; Paudel& Ree 2014). Nevertheless, mounting evidence suggeststhat merger of dwarf galaxies might not be as rare as

2 Email: [email protected]

it was previously thought. Apart from the classic LMC-SMC interaction, there are several cases of merging dwarfgalaxy candidates reported in recent studies (Martınez-Delgado et al. 2012; Johnson 2013; Crnojevic et al. 2014;Paudel et al. 2015; Stierwalt et al. 2015). In our previ-ous work, we studied merging of gas-rich dwarf galaxies,UGC 6741 pair, which is located in a low density en-vironment, i.e outskirt of the NGC 3853 group (Paudelet al. 2015). We found UGC 6741 has a similar visualmorphology of ARP 104.

Here we report one more system of merging dwarfgalaxies near to the MW type isolated host galaxy, NGC2718. This resembles the LMC-SMC-MW system in var-ious ways. This paper is organized as follows. In Sec-tion 2, we introduce the system and the environment.Section 3 describes the photometric characterization andmorphology, while Section 4 is dedicated to discussing itsimportance.

2. IDENTIFICATION

As our primary interest is to perform a detailed studyof merging system of dwarf galaxies in various environ-ments, we carried out a systematic search of such objectsvisually inspecting the SDSS color image in the local vol-ume (z < 0.02). In this work, we present UGC 4703, aninteracting pair of dwarf galaxies. It is similar to our pre-viously studied galaxy UGC 6741, but located near MWmass spiral galaxy NGC 2718. The main aim of thiswork is to report the similarity between the UGC 4703interacting pair around NGC 2718 and the LMC-SMCinteraction around MW.

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Stellar population of UCDs. 5

Glatt, K., Grebel, E. K., & Koch, A. 2010, A&A, 517, A50Harris, J. & Zaritsky, D. 2009, AJ, 138, 1243Johnson, M. 2013, AJ, 145, 146Jordi, K., Grebel, E. K., & Ammon, K. 2006, A&A, 460, 339Kallivayalil, N., Besla, G., Sanderson, R., & Alcock, C. 2009,

ApJ, 700, 924Kennicutt, Jr., R. C. 1998, ARA&A, 36, 189Martin, D. C., Fanson, J., Schiminovich, D., Morrissey, P.,

Friedman, P. G., Barlow, T. A., Conrow, T., Grange, R.,Jelinsky, P. N., Milliard, B., Siegmund, O. H. W., Bianchi, L.,& Byun, Y.-I. 2005, ApJ, 619, L1

Martınez-Delgado, D., Romanowsky, A. J., Gabany, R. J.,Annibali, F., Arnold, J. A., Fliri, J., Zibetti, S., van der Marel,R. P., Rix, H.-W., Chonis, T. S., Carballo-Bello, J. A., Aloisi,A., Maccio, A. V., Gallego-Laborda, J., Brodie, J. P., &Merrifield, M. R. 2012, ApJ, 748, L24

Paudel, S., Duc, P.-A., Cote, P., Cuillandre, J.-C., Ferrarese, L.,Ferriere, E., Gwyn, S. D. J., Mihos, J. C., Vollmer, B., Balogh,M. L., Carlberg, R. G., Boissier, S., Boselli, A., Durrell, P. R.,Emsellem, E., MacArthur, L. A., Mei, S., Michel-Dansac, L., &van Driel, W. 2013, ApJ, 767, 133

Paudel, S., Duc, P. A., & Ree, C. H. 2015, AJ, 149, 114Paudel, S. & Ree, C. H. 2014, ApJ, 796, L14

Paudel, S., Smith, R., Duc, P.-A., Cote, P., Cuillandre, J.-C.,Ferrarese, L., Blakeslee, J. P., Boselli, A., Cantiello, M., Gwyn,S. D. J., Guhathakurta, P., Mei, S., Mihos, J. C., Peng, E. W.,Powalka, M., Sanchez-Janssen, R., Toloba, E., & Zhang, H.2016, ArXiv e-prints

Robotham, A. S. G., Baldry, I. K., Bland-Hawthorn, J., Driver,S. P., Loveday, J., Norberg, P., Bauer, A. E., Bekki, K.,Brough, S., Brown, M., Graham, A., Hopkins, A. M., Phillipps,S., Power, C., Sansom, A., & Staveley-Smith, L. 2012, MNRAS,424, 1448

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Tollerud, E. J., Boylan-Kolchin, M., Barton, E. J., Bullock, J. S.,& Trinh, C. Q. 2011, ApJ, 738, 102

0 0.43 0.85 1.3 1.7 2.1 2.6 3 3.4 3.9 4.3

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Paudel, S., Duc, P. A., & Ree, C. H. 2015, AJ, 149, 114Paudel, S. & Ree, C. H. 2014, ApJ, 796, L14Paudel, S., Smith, R., Duc, P.-A., Cote, P., Cuillandre, J.-C.,

Ferrarese, L., Blakeslee, J. P., Boselli, A., Cantiello, M., Gwyn,S. D. J., Guhathakurta, P., Mei, S., Mihos, J. C., Peng, E. W.,Powalka, M., Sanchez-Janssen, R., Toloba, E., & Zhang, H.2016, ArXiv e-prints






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Fig. 1.— We show the SDSS r−band image of an area around NGC 2718 and its satellites with a field of view ofRain 9′×5′. In thebottom panel, we show a zoom in view of interacting dwarf galaxies. The left is a color image cutout directly obtained from the SDSSsky-server. Right, we show co-added g, r and i-band images with an arcsinh scaling. Some interesting features, like S1 and S2 and TDG,are highlighted.

and a redshift of z = 0.01199, we found a rare pair ofstar-forming dwarf galaxies with a connecting bridge ofstellar stream. The brighter galaxy in the pair, UGC4703, is slightly fainter than LMC with an r−band ab-solute magnitude Mr = -18.25 mag and the fainter com-panion, here after UGC 4703B, has an r−band absolutemagnitude Mr = -16.76 mag. The pair is located North-West of NGC 2718, at an angular distance of ≈5.2′. As-suming the distance to the NGC 2718 group to be 54.5Mpc2, the physical projected separation between NGC2718 and UGC 4703 is 81 kpc. A relative line of sight re-dial velocity between the two is 263 km/s. According toavailable notes in the NED3, UGC 4703 is already iden-tified as a star-forming pair of galaxies interconnected by

2 Based on Hubble flow with a redshift z= 0.0123 http://ned.ipac.caltech.edu

a thin straight bridge.

3. DATA ANALYSIS

This work benefits from a substantial multi-wavelengthdata available in public archives. This allowed us to per-form the required detailed analysis of morphology andstellar population properties of our sources. We reporta multi-wavelength study of the system based on thearchival images from the Sloan Digital Survey (SDSS),Galaxy Evolutionary Explorer (GALEX) and the Spitzerspace telescope which covers a wavelength range from FarUltraviolet to Infra Red. Additionally, we observed thesystem in HI 21-cm line using the Giant Metrewave Ra-dio Telescope (GMRT).

3.1. Analysis of archival data

UGC 4703 pair: an LMC-SMC analog. 3

TABLE 1Results of aperture photometry

Galaxy Ra Dec FUV NUV u g r i z [3.6] [4.5] log(SFR) log(M∗)h:m:s d:m:s mag mag mag mag mag mag mag mJy mJy M�/yr M�

UGC 4703 08:58:29.869 06:19:17.07 17.1 16.8 16.31 15.60 15.26 15.19 15.08 2.060 1.540 -0.64 9.27UGC 4703B 08:58:25.024 06:20:06.44 19.6 18.9 18.25 17.10 16.76 16.50 16.38 0.390 0.230 -1.49 8.74

TDG 08:58:28.424 06:19:34.76 20.4 20.3 20.05 19.25 19.32 19.20 19.30 0.010 0.005 -2.05 7.28The UV magnitudes are from the GALEX images and the optical are from the SDSS. Spitzer space telescope images are used to deriveIR flux. UV and Optical magnitudes are corrected for galactic extinction. We used Schlafly & Finkbeiner (2011) extinction map andwe calculated the extinction in UV using Seibert et al. (2005).The stellar mass are derived from Spitzer [3.6] and [4.5] channel flux andstar-formation rates are derived from the GALEX FUV flux.

In Figure 1, we show the SDSS r−band image of anarea around NGC 2718 and its satellites with a fieldof view of 9′×5′. NGC 2718 is a well studied isolatedgalaxy (Hernandez-Toledo et al. 2010; Karachentsevaet al. 2010). We find no bright galaxy (Mr < -19 mag)around it within one Mpc sky-projected radius and a ra-dial velocity ±500 km/s. Our NED query found no otherdwarf (Mr > -19 mag) companion other than the two,UGC 4703 and UGC 4703B, around NGC 2718. NGC2718 is a nuclear star-burst galaxy and in RC3 catalog,it is classified as a face-on SAB type. It has a total stel-lar mass of Log(M∗/M�) = 10.7 and total HI mass ofLog(MHI/M�) = 10.05 (Chang et al. 2015; Courtois &Tully 2015). Only, NGC 2718 and UGC 4703 were tar-geted by the SDSS spectroscopy survey, and measuredredshift for them are 0.0127 and 0.0119, respectively. Un-fortunately, the SDSS target selection picks up a back-ground red galaxy located on the edge of UGC 4703B,however the NED cataloged redshift of UGC 4703B is0.0118.

A stellar bridge connecting UGC 4703 and its compan-ion UGC 4703B can be clearly seen in the SDSS image.A sky-projected separation between the two is 20 kpc.In Figure 1 lower panel, we show a more detailed view ofthe interacting pair. In the left, we show a color imagecutout directly obtained from the SDSS sky-server4. Inthe g − r − i combined color image, we can see a fewextremely blue knots in the central region of UGC 4703which indicate ongoing burst of star-formation. The Hα

emission line equivalent width measured from the SDSSoptical spectrum of UGC 4703 is 181 A. This gives astar-formation age of order of a few Myr (Leitherer et al.1999). The optical morphology of UGC 4703B however ismuch smooth where no distinct blue star-forming knotsis visible. From a detailed inspection of UGC 4703 mor-phology in the Figure 1, bottom right panel, it seems likea spiral galaxy and now one of it’s spiral arms is outflungin the direction of NGC 2718.

The stellar bridge connecting the interacting pair is nothomogeneous in surface brightness and color, and seemsbifurcated (see Figure 1 lower right panel for more de-tail). Interestingly, each one of these streams (S1 and S2)seems to originate from different galaxies. It is impossi-ble to reckon the difference between these two in termsof stellar population properties from the shallow SDSSimages, but the stellar bridge, overall, is bluer than theboth interacting galaxies. We find a bright star-formingclump nearly at the end of the stellar stream startingfrom UGC 4703B, i.e S2. It may be a potential candi-date of Tidal Dwarf Galaxy (TDG) in formation5 In the

4 http://skyserver.sdss.org5 Exact definition of TDG is vague. Here, a working definition

left color image, we can see that it is much bluer thanthe surrounding stellar stream.

We performed the aperture photometry to derive totalbrightness of the objects of interest, i.e UGC 4703, UGC4703B and the potential TDG candidate. The sizes ofapertures were selected visually where we used a wideenough aperture that secure all the flux. We used a sim-ilar approach to subtract the sky-background count as inPaudel et al. (2015). Before the doing aperture photome-try, we masked all unrelated foreground and backgroundobjects manually. The GALEX all-sky survey (Martinet al. 2005) archival images were used to derive the UVmagnitudes. We used the SDSS-III (Abazajian et al.2009) image to derive the optical band magnitudes. IRfluxes were measured from the Spitzer space telescopewhich were obtained from IRSA6 archive. We list theresults of aperture photometry in various band from UVto IR in Table 3.

We find the both the dwarfs, UGC 4703 and UGC4703B, have similar g− r color indices of 0.34 mag. Theputative TDG g − r color index is -0.07 mag whereasan average g − r color index of the stellar bridge is 0.1mag. We derived the star-formation rate from the FUVflux using a calibration provided by Kennicutt (1998).Eskew et al. (2012) calibration was used to convert theSpitzer [3.6] and [4.5] channel flux to stellar mass. Thetotal r−band luminosity and stellar mass of UGC 4703,UGC 4703B and TDG candidate were estimated to be-18.25, -16.75 and -14.19 mag and 1.9×109, 5.5×108 and1.9×107 M�, respectively.

3.2. Radio 21-cm observation

To get a detailed view of the HI distribution of theUGC 4703 system and its connection to NGC 2718, wecarried out HI interferometric observations of UGC 4703using the GMRT. The system was observed on June 15th,2017. A baseband bandwidth of 16 MHz was used forthe observations yielding a velocity resolution ∼7 km/s.The GMRT primary beam at L band is 24′ and the syn-thesised beam of the images presented in the paper is45.4′′ × 38.0′′. The data was analysed using the soft-ware AIPS7 and the procedure followed was similar tothat explained in Sengupta et al. (2017). Figure 2, toppanel shows the contours from the integrated HI mapoverlaid on the SDSS g, r and i-band combined image.While an HI bridge is visible validating an interactionbetween UGC 4703 and UGC 4703B, it is clear thatthere is no apparent sign of interaction between the MW

is any stellar system of mass range & 107 M∗/M� born out ofexpelled debris of interacting galaxies.

6 http://irsa.ipac.caltech.edu7 http://www.aips.nrao.edu

4 Paudel et al.


Glatt, K., Grebel, E. K., & Koch, A. 2010, A&A, 517, A50Harris, J. & Zaritsky, D. 2009, AJ, 138, 1243Johnson, M. 2013, AJ, 145, 146Jordi, K., Grebel, E. K., & Ammon, K. 2006, A&A, 460, 339Kallivayalil, N., Besla, G., Sanderson, R., & Alcock, C. 2009,

ApJ, 700, 924Kennicutt, Jr., R. C. 1998, ARA&A, 36, 189Martin, D. C., Fanson, J., Schiminovich, D., Morrissey, P.,

Friedman, P. G., Barlow, T. A., Conrow, T., Grange, R.,Jelinsky, P. N., Milliard, B., Siegmund, O. H. W., Bianchi, L.,& Byun, Y.-I. 2005, ApJ, 619, L1



Paudel, S., Duc, P. A., & Ree, C. H. 2015, AJ, 149, 114Paudel, S. & Ree, C. H. 2014, ApJ, 796, L14

Paudel, S., Smith, R., Duc, P.-A., Cote, P., Cuillandre, J.-C.,Ferrarese, L., Blakeslee, J. P., Boselli, A., Cantiello, M., Gwyn,S. D. J., Guhathakurta, P., Mei, S., Mihos, J. C., Peng, E. W.,Powalka, M., Sanchez-Janssen, R., Toloba, E., & Zhang, H.2016, ArXiv e-prints






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Fig. 2.— Top: integrated HI contours from the GMRT low-resolution map overlaid on the SDSS g-r-i combined tricolor image. The H icolumn density levels are N(HI) = 1019 (2.4, 4.1, 6.6, 9.9, 13.3, 16.6, 19.9, 23.3, 26.7, 30.0) cm−2

Bottom: we show a zoom-in view of area around UGC 4703 and its companion. In right, we show HI velocity field from the GMRTlow-resolution cube for HI emission >3σ. The blue contour traces the SDSS r-band light where the outermost is of surface brightness 24.5mag/arsec2. The black circle represents position of TDG.

type NGC 2718 and the dwarf satellites in both opticaland radio HI images. In the interacting satellites, theHI emission is mainly concentrated around UGC 4703with an extension of tenuous emission in the direction ofUGC 4703B along the stellar bridge. In UGC 4703, thepeak HI column density (1.6 × 1020 cm−2) overlaps withits bright central star-forming region. Interestingly, HIemission around UGC 4703B does not overlap with theoptical counterpart. The emission is not strong, the HIcolumn density levels at UGC 4703B are similar to thebridge (between 2 - 6 × 1019 cm−2) with no prominentHI column density concentration on the galaxy. The in-tegrated HI line flux density of the UGC 4703/ 4703Bsystem estimated from our GMRT data is ∼ 1.4 Jy km/scompared to the uncorrected flux density estimated fromthe Arecibo spectrum from NED ∼ 2.0 Jy km/s. Twofactors contributed to this flux discrepancy. Our dataquality of that day was not good and heavy flagging re-

sulted in losing crucial UV coverage. Additionally, theGMRT synthesised beam could have resolved out somediffuse emission from the extended, low column densitybridge area of the UGC 4703 system. Thus the Areciboflux density was used to estimate the total HI mass of thesystem, UGC 4703/ UGC 4703B combined, which is 1.4× 109 M�. Lack of better spatial resolution and the dis-turbed, interacting HI morphology forbids us from esti-mating more accurate individual HI masses of UGC 4703and UGC 4703B. However, from the HI image and HI col-umn density contours, we can conclude that gas contentin UGC 4703B is significantly lower than in UGC 4703.It seems that such low gas mass content in UGC 4703Bis intrinsic and which is not due to the detection limit ofour observation. Figure 2 lower right panel shows the ve-locity field of the UGC 4703/ 4703B system. We detecthint of a modest velocity gradient in NGC 4703 whichbecomes irregular along the bridge and UGC 4703B re-

UGC 4703 pair: an LMC-SMC analog. 5

gion though lack of better spatial resolution prevents usfrom making stronger claims about the gradient in UGC4703.

4. DISCUSSION

4.1. Comparison with LMC-SMC-MW System

Visible to the naked eye in southern sky, the LMC andSMC are the best-studied star-forming galaxies and thusthe system is regarded as an important laboratory tostudy the evolution of dwarf galaxies. They are currentlysuffering different scales of tidal force from both the mas-sive central host galaxy, MW, and their own lower masscompanions. Understanding the origin of LMC-SMC sys-tem in vicinity of MW has been an active area of researchand a widely discussed mechanism is binary infall (Beslaet al. 2007, 2010; Diaz & Bekki 2011; Kallivayalil et al.2009). Particularly, Besla et al. (2007) shows that LMC-SMC pair might be entering the MW halo for the firsttime and has not yet completed an orbit. This model wasupdated by Besla et al. (2012) to explore the morphologyof the stream produced from a head-on collision betweenthe Clouds, specifically by the SMC moving in a highlyeccentric orbit around the LMC, far from the MW poten-tial. Here we report another system similar in geometryand morphology to LMC-SMC and compare their prop-erties to gain a better understanding of the system withdwarf-dwarf interaction in proximity of a massive halo.

We list a direct comparison of physical parameters be-tween the LMC-SMC-MW system and the UGC 4703pair-NGC 2718 system in Table 2. NGC 2718 and MWhave a similar stellar mass of ≈5×1010 M�. NGC 2718is located in a fairly isolated environment. Its nearestbright (MB <-19 mag) galaxy is NGC 2731 at a sky-projected distance of 1.18 Mpc and relative line of sightvelocity between the two is 1260 km/s. In contrast, MWis part of the Local group where the nearest bright neigh-bor is M33 at a distance of 0.86 Mpc.

Since we do not have resolved distances of each indi-vidual galaxies of the NGC 2718 group, the derived geo-metric properties are sky-projected. For the LMC-SMC-MW system, we use three-dimensional geometric prop-erties which we obtained from D’Onghia & Fox (2016).The geometrical configuration of both systems look sim-ilar. The interacting dwarfs are located within a ∼100kpc distance from the massive hosts, i.e NGC 2718 andMW, and the relative line of sight radial velocities be-tween the hosts and dwarfs are <300km/s. The relativeof line of sight velocity between UGC 4703 and UGC4703B is 15 km/s and between LMC and SMC is 105km/s. Sky-projected separation between UGC 4703 andUGC 4703B is 21 kpc and between LMC and SMC is23 kpc. The current star-formation rates of individualgalaxies in UGC 4703 pair are also similar to the LMC-SMC pair. Stellar mass ratios of both interacting pairs,UGC 4703-UGC 4703B and LMC-SMC, are ≈5:1.

Although the tidal features of LMC-SMC pair is quiteprominent in gas structure, recent observations have re-vealed presence of stellar substructure in the outskirts ofthe SMC (Belokurov & Koposov 2016; Belokurov et al.2017; Besla et al. 2016). The presence of a gaseous bridgebetween the LMC and SMC proves that they must havehad at least one close encounter in the recent past Kalli-vayalil et al. (2013). Additionally, Nidever et al. (2013)

TABLE 2Comparison with LMC-SMC-MW

Galaxy MB D M∗ SFR HImass

mag kpc M� M�/yr M�MW -21.17 0 5×1010 0.68 - 1.45 8×109

LMC -18.60 50 2.3×109 0.2 4.4×108

SMC -17.20 60 5.3×108 0.04 4×108

NGC 2718 -21.01 0 7×1010 0.97 1.12×1010

UGC 4703 -18.0 81 1.8 ×109 0.2 1.4 × 109∗

UGC 4703B -16.5 104 5.4×108 0.03

D is distance from the host galaxy to the dwarfs. In ourcase, it is sky-projected while the values for LMC and SMC are3-dimensional. The LMC and SMC parameters are obtained fromD’Onghia & Fox (2016). ∗Combined HI mass of UGC 4703 andUGC 4703B.

identified a ≈55 kpc stellar stream located to the eastof SMC, a likely stellar counterpart of the HI Magel-lanic Bridge that was tidally stripped from the SMC(Subramanian et al. 2017). In comparison to the stel-lar streams observed in the LMC-SMC pair, the stellarstreams around UGC 4703 pair is more prominent. Wealso find a star-forming region (the putative TDG) in thestellar bridge which have a stellar mass log(M∗/M�) =7.25 and a star-formation rate 8.9×10−3 M�/yr. Over-all, the stellar bridge is significantly bluer than both thedwarfs (UGC 4703, UGC 4703B) and is fairly alignedwith the HI-bridge. This suggests, at least some, if notall, stellar population in the bridge may have formedvery recently from the tidally stripped gas. Evidence ofyoung stellar population has also been identified aroundthe gaseous bridge connecting the LMC-SMC (Belokurov& Koposov 2016; Belokurov et al. 2017).

As far as our detection limit permits no sign of an ex-tended HI tail is observed around the UGC 4703-pair un-like the LMC-SMC system where a long trailing gaseousstream (well known as Magellanic Stream MS) is promi-nent. Origin of the MS is a debate and a frequent, but notconclusive, explanation is that it is a product of tidal in-teractions between either LMC-SMC-MW or LMC-SMConly (Fujimoto & Sofue 1976; Guglielmo et al. 2014).There are other scenarios like ram-pressure stripping,that could also explain the origin of MS (Meurer et al.1985; Moore & Davis 1994) if the interacting pair had al-ready crossed the MW outer disk at some point in theirpast orbit. Even in a purely tidal interaction model, itis not clear how much MW potential affect the dynam-ical history of the pair or, in other words, whether thepair is bound to MW or not. Lack of three dimensionaldistance of NGC 2718 and UGC 4703 pair, prevents usfrom estimating if the UGC 4703 and UGC 4703B arebound to NGC 2718 or not.

We find that UGC 4703B is relatively gas poor andthe HI map shows that the gas distribution is displacedfrom its optical counterpart while both LMC and SMCrelatively are gas rich. This maybe a hint that the in-teraction between UGC 4703 and UGC 4703B is rela-tively advanced compare to LMC-SMC. Being the minorcompanion, UGC 4703B has experienced tidal stretch-ing from UGC 4703 which probably has displaced mostof the gas mass and deformed the stellar disk creating astellar bridge. One main difference in geometry of LMC-SMC and UGC 4703-UGC 4703B is relative line of sightvelocity between the pair, where the later has signifi-cantly lower value, i.e 15 km/s. In contrast to the previ-

6 Paudel et al.

ous hypothesis, this may also suggest that the encounterin UGC 4703-UGC 4703B is slower than encounter inLMC-SMC and that leads to a stronger tidal distortionof participating galaxies in UGC 4703-UGC 4703B com-pared to relatively fast interacting LMC-SMC. However,a comparative study with a result of numerical simula-tion with full orbital histories of interactions is requiredto make a firm conclusion.

In summary, we present UGC 4703 pair- NGC 2718system as a LMC-SMC-MW analog. Both the systemshave a similar physical (geometry, star-formation rate,total gas mass and stellar mass) and morphological prop-erties. Our GMRT observations detected HI in NGC2718 and UGC 4703 pair as well as in the bridge be-tween the dwarf pair. However no extended HI, simi-lar to the MS, is detected between NGC 2718 and UGC4703 pair. We also detected star-forming regions along

the UGC 4703/ 4703B bridge with stellar mass exceed-ing 107 M�. A comparison of optical and HI morphologyof interacting dwarfs pairs (UGC 4703-4703B and LMC-SMC) suggests interaction between UGC 4703 and UGC4703B is either slow or at relatively advanced stage com-pare to LMC-SMC interaction.

Paudel S. acknowledges the support by Samsung Sci-ence & Technology Foundation under Project NumberSSTF-BA1501-0. We thank the staff of the GMRT thatmade these observations possible. GMRT is run by theNational Centre for Radio Astrophysics of the Tata In-stitute of Fundamental Research. This work is basedarchival data from the SDSS-III, Spitzer telescope andGALEX.

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