Mon. Not. R. Astron. Soc. 000, 1–?? (2012) Printed 20 September 2018 (MN LATEX style file v2.2) Pre-existing dwarfs, tidal knots and a tidal dwarf galaxy: an unbiased H i study of the gas-rich interacting galaxy group NGC 3166/9 K. Lee-Waddell1⋆, K. Spekkens1, M. P. Haynes2, S. Stierwalt3, J. Chengalur4, P. Chandra1 and R. Giovanelli2 1Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON K7K 7B4, Canada 2Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA 3Spitzer Science Center, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA 4National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411 007, India Accepted 2012 September 11. Received 2012 September 10; in original form 2012 April 7 ABSTRACT We present Arecibo Legacy Fast ALFA (ALFALFA) and follow-up Giant Metrewave Radio Telescope (GMRT) H i observations of the gas-rich interacting group NGC 3166/9. The sensitive ALFALFA data provide a complete census of H i-bearing sys- tems in the group while the high-resolution GMRT data elucidate their origin, enabling one of the first unbiased physical studies of gas-rich dwarf companions and the subse- quent identification of second-generation, tidal dwarf galaxies in a nearby group. The ALFALFA maps reveal an extended H i envelope around the NGC 3166/9 group core, which we mosaic at higher resolution using six GMRT pointings spanning ∼1 square degree. A thorough search of the GMRT datacube reveals eight low-mass objects with 7 8 gas masses ranging from 4 × 10 to 3 × 10 M⊙ and total dynamical masses up to 9 1.4 × 10 M⊙. A comparison of the H i fluxes measured from the GMRT data to those measured in the ALFALFA data suggests that a significant fraction (∼ 60%) of the H i is smoothly distributed on scales greater than an arcminute (∼7 kpc at the NGC 3166/9 distance). We compute stellar masses and star formation rates for the eight low-mass GMRT detections, using ancillary SDSS and GALEX data, and use these values to constrain their origin. Most of the detections are likely to be either pre-existing dwarf irregular galaxies or short-lived, tidally formed knots; however, one candidate, AGC 208457, is clearly associated with a tidal tail extending below NGC 3166, exhibits a dynamical to gas mass ratio close to unity and has a stellar content and star formation rate that are broadly consistent with both simulated as well as arXiv:1209.3519v1 [astro-ph.CO] 16 Sep 2012 candidate tidal dwarf galaxies from the literature. Our observations therefore strongly suggest that AGC 208457 is a tidal dwarf galaxy. Key words: galaxies: interactions – galaxies: dwarf 1 INTRODUCTION ies, they should have total to baryonic mass ratios close to unity (Barnes & Hernquist 1992; Bournaud et al. 2004). Many present-day galaxies can be found in group environ- Not only do TDGs contain populations of recently formed ments where tidal interactions within these systems play stars that were produced during the interaction event, important roles in galactic dynamics (e.g. Tago et al. 2008). but they also have older pre-enriched stellar populations Certain interactions in gas-rich groups can form tidal bridges (Duc et al. 2000). Overall, the formation of any tidal fea- and tails as well as second-generation “tidal” dwarf galax- ture or galaxy greatly constrains the type of interaction and ies (TDGs), which differ from first-generation “pre-existing” the properties of the original objects involved in the pro- dwarfs by their lack of dark matter and higher metallic- cess (Duc 2011). In groups, the prevalence of tidal galaxies ity content (Hunter, Hunsberger & Roye 2000). Since TDGs relative to their pre-existing counterparts probes the mech- form from the material from the outer disks of larger galax- anisms that drive galaxy evolution in these environments (Bournaud & Duc 2006). ⋆ E-mail: [email protected] (KLW) Although TDGs are frequently produced in galaxy in- c 2012 RAS 2 K. Lee-Waddell et al. teraction simulations and over the last two decades several sitive single-dish observations to locate H i-bearing systems probable candidates have been identified in ∼20 interacting on degree scales and high-resolution interferometric follow- systems, very few of these objects have been widely accepted up to elucidate their origin. In this paper, we target low-mass as authentic (see Weilbacher et al. 2003; Sheen et al. 2009). ALFALFA H i detections in the nearby gas-rich group NGC Tidally formed knots that assemble along tidal filaments 3166/9 with the GMRT affording one of the first blind, sys- 6 8 tend to have total masses between 10 M⊙ and 10 M⊙; tematic studies of both first- and second- generation systems however, in order to survive into long-lived dwarf galaxies in a group environment. – with life-times > 2 Gyr – these objects typically require NGC 3166/9 consists of three main spiral galaxies: NGC 8 a total mass > 10 M⊙ (Bournaud & Duc 2006). Models 3165, NGC 3166 and NGC 3169 and is located in the Sex- predict that the most probable TDGs are found in the high tans constellation. The heliocentric velocity difference be- −1 density tips of tidal tails where large tidal knots are typi- tween these galaxies is small – cz⊙ = 1340 ± 5 km s cally formed (Bournaud et al. 2004). Genuine TDGs should (Schneider et al. 1990), 1345 ± 5 km s−1 (van Driel et al. also be self-gravitating (Duc et al. 2007). Nevertheless, even 2001) and 1238 ± 4 kms−1 (van Driel et al. 2001) for NGC high-resolution studies of the neutral hydrogen (H i) dy- 3165, NGC 3166 and NGC 3169 respectively – and they namics of well-known TDG candidates in the southern tail share a common H i distribution at arcminute resolution of NGC 4038/9 failed to prove that the objects are rotat- (see below). The close proximity of these galaxies to one an- ing – a clear indicator of self-gravitation – and that they are other as well as a tail-like feature extending below NGC therefore kinematically distinct from the tidal tail, which has 3166 in the ultraviolet (UV; discussed in § 5), indicates given rise to much debate and speculation about their na- previous and on-going interactions within the group. The ture (Hibbard et al. 2001). As can be seen, deciding whether average distance to NGC 3169, derived from the type 1A a given object is a tidal dwarf or not is a non trivial issue supernova SN2003cg, is 22.6 Mpc (Wood-Vasey et al. 2008; and generally requires a host of corroborating observations. Mandel et al. 2009), which is assumed for all group members Given the properties of (young) TDGs as well as throughout this paper. the environments within which they form, H i obser- Recently, the extended H i emission of this group, ′ vations are a reasonable preliminary search tool. H i first observed by Haynes (1981), was mapped at 4 (30 traces the location of tidally formed features, can indi- kpc at the distance of NGC 3169) resolution by ALFALFA cate regions of potential star formation and has been (Giovanelli et al. 2005; Fig. 1). We present the ALFALFA routinely used to map the gas distribution in and data for the NCG 3166/9 region in § 2. Most of the de- around gas-rich systems (see Freeland, Stilp & Wilcots tected H i is located around NGC 3169 with various fea- 2009; Kent et al. 2009; Stierwalt et al. 2009; tures extending outwards; however, the spatial resolution of Borthakur, Yun & Verdes-Montenegro 2010 for recent the ALFALFA data is too low to characterize the structure examples). H i measurements can be used to estimate gas of individual group members. The high H i content and clear masses and total masses (discussed further in § 4). With signs of interactions in the NGC 3166/9 group make it an sufficient resolution, the internal structure and dynamics ideal candidate for high-resolution follow-up to distinguish of TDG candidates can also be constrained (Duc et al. first- and second-generation objects, which should result in 2007). Interferometric data from instruments such as a better understanding of galaxy evolution in groups. the Very Large Array (VLA; e.g. Freeland et al. 2009) We obtained interferometric H i observations using the and the Australia Telescope Compact Array (ATCA; GMRT, as a follow-up to the ALFALFA data, in an at- e.g. Pisano et al. 2011) become essential for resolving the tempt to resolve the putative dwarf galaxy population in structure of more compact features. The Faint Irregular NGC 3166/9. We have been able to make higher resolution Galaxies GMRT Survey (FIGGS; Begum et al. 2008) shows maps of the H i features resulting in the detection of sev- that the Giant Metrewave Radio Telescope (GMRT) is also eral low-mass objects (§ 4). We computed H i masses and particularly useful in surveying gas-rich dwarfs as its fixed total masses, from the GMRT data, for each detection (§ antenna configuration allows for maps at different angular 4). Stellar masses and star formation rates (SFRs) were also resolutions to be produced by tapering the data during estimated for each object using ancillary optical and UV ob- imaging. servations (§ 5). The combination of mass ratio estimates, Considerable effort has been devoted to interferomet- SFRs and other galactic properties have been used to cat- ric H i mapping of nearby interacting systems in order egorize the low-mass H i features and identify a probable to search for TDGs (e.g.