Searching for Diffuse Light in the M96 Group
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Draft version June 30, 2016 Preprint typeset using LATEX style emulateapj v. 5/2/11 SEARCHING FOR DIFFUSE LIGHT IN THE M96 GALAXY GROUP Aaron E. Watkins1, J. Christopher Mihos1,Paul Harding1,John J. Feldmeier2 Draft version June 30, 2016 ABSTRACT We present deep, wide-field imaging of the M96 galaxy group (also known as the Leo I Group). Down to surface brightness limits of µB = 30:1 and µV = 29:5, we find no diffuse, large-scale optical counterpart to the “Leo Ring”, an extended HI ring surrounding the central elliptical M105 (NGC 3379). However, we do find a number of extremely low surface-brightness (µB & 29) small-scale streamlike features, possibly tidal in origin, two of which may be associated with the Ring. In addition we present detailed surface photometry of each of the group’s most massive members – M105, NGC 3384, M96 (NGC 3368), and M95 (NGC 3351) – out to large radius and low surface brightness, where we search for signatures of interaction and accretion events. We find that the outer isophotes of both M105 and M95 appear almost completely undisturbed, in contrast to NGC 3384 which shows a system of diffuse shells indicative of a recent minor merger. We also find photometric evidence that M96 is accreting gas from the HI ring, in agreement with HI data. In general, however, interaction signatures in the M96 Group are extremely subtle for a group environment, and provide some tension with interaction scenarios for the formation of the Leo HI Ring. The lack of a significant component of diffuse intragroup starlight in the M96 Group is consistent with its status as a loose galaxy group in which encounters are relatively mild and infrequent. Subject headings: galaxies: elliptical – galaxies: groups: individual(Leo I) – galaxies: interactions – galaxies: stellar content – galaxies: structure 1. INTRODUCTION subtle ways in their outskirts, where restoring forces are low In the current ΛCDM cosmological paradigm, massive and dynamical times are long. Morphological signatures of galaxies form hierarchically over time, via the continual ac- accretion such as tidal features, warps, and disk asymme- cretion of smaller objects (Searle & Zinn 1978; Davis et al. tries (Toomre & Toomre 1972; Bullock & Johnston 2005; 1985; Frenk et al 1988; White & Frenk 1991; Jenkins et Mart´ınez-Delgado et al. 2010) may trace the record of past en- al. 2001; Springel et al. 2005). Massive galaxies may in- counters, while changes in the structural properties and stellar teract with one another as well, leaving behind readily ob- populations of the outer disk can be probed via quantitative servable signatures in the form of tidal streams or drastic photometry and color profiles (e.g. van der Kruit 1979; Pohlen changes in morphology, stellar populations, and kinematics et al. 2002; Trujillo et al. 2009; Mihos et al. 2013a). Finding (e.g., Arp 1966; Toomre & Toomre 1972; Toomre 1977; Hern- and categorizing such signatures thus becomes an important quist 1992; Barnes 1992, 2002). Even after the initial con- step in constraining theories of galaxy formation. struction of galaxies has ended, however, cosmological sim- Unfortunately, the extended outskirts of galaxies are ex- ulations indicate that there should remain a wealth of low- tremely dim and diffuse. However, with the advent of deep mass satellites that continue to interact with the parent galaxy wide-field imaging techniques, over the past decade a wide over time (e.g., Frenk et al 1988; Gelb & Bertschinger 1994; variety of faint tidal features have been identified around Springel et al. 2005, 2008). Observations of the Local Group nearby galaxies. These features span a wide range of mor- indeed show a number of low-luminosity satellites around the phologies, from loops, plumes, and shells (Forbes et al. 2003; Milky Way and M31 (Hodge 1971; Feitzinger & Galinski Mart´ınez-Delgado et al. 2008, 2009, 2010; Atkinson et al. 1985; Mateo 1998; Belokurov et al. 2006; Weisz et al. 2011; 2013; Mihos et al. 2013a) to extended streams, diffuse stel- McConnachie 2012), lending credence to the idea. However, lar halos, and large-scale intracluster light (Uson et al. 1991; important discrepancies between theory and observation re- Scheick & Kuhn 1994; Gonzalez et al. 2000; Feldmeier et al. 2004; Adami et al. 2005; Mihos et al. 2005; Rudick 2010b). arXiv:1406.6982v2 [astro-ph.GA] 29 Jun 2016 main, such as the ‘missing satellites problem’ (Klypin et al. 1999; Moore et al. 1999), demonstrating the importance of Gaseous accretion events also appear to influence the proper- continuing to test the theory observationally in a variety of ties of disks (Sancisi et al. 2008), fueling new star formation ways. in their low density outskirts and driving the formation of ex- Probing the low surface brightness outskirts of galaxies via tended ultraviolet (XUV) disks (Thilker et al. 2005; Gil de deep surface photometry provides one such observational test. Paz et al. 2005; Thilker et al. 2007; Lemonias et al. 2011). Under proposed “inside-out” galaxy formation models (Mat- This extended star formation will in turn affect the age and teucci & Franc¸ois 1989; Bullock & Johnston 2005; Naab & metallicity distributions, and hence the colors, of the underly- Ostriker 2006; Hopkins et al. 2009; van Dokkum et al. 2010; ing stellar populations in these regions. Meanwhile, internal Kauffmann et al. 2012), even apparently undisturbed galax- processes within disks may scatter stars outwards, again re- ies should show signs of recent accretions or interactions in sulting in changes in the structure and stellar content of the outer disk (Sellwood & Binney 2002; Debattista et al. 2006; 1 Department of Astronomy, Case Western Reserve University, Cleve- Roskar et al. 2008a,b). land, OH 44106, USA Local environment also must play a role in the accretion 2 Department of Physics and Astronomy, Youngstown State University, history of galaxies and the evolution of their outer regions. Youngstown, OH 44555, USA 2 Watkins et al. In massive clusters, for example, frequent and rapid interac- IGL fractions found may not be reflective of the entire group. tions between galaxies tend to produce plumes and streams of However, in stark contrast, McGee & Balogh (2010) have es- starlight, which are shredded over time by interactions within +16 timated that 47%−15 of the stellar mass in galaxy groups is in the cluster potential to form the more diffuse intracluster light the IGL component through detections of spectroscopically (ICL) seen in clusters such as Virgo (Rudick et al. 2009). In confirmed hostless intragroup SNe Ia that were within the group environments, where the velocity dispersions are gener- Sloan Digital Sky Survey (SDSS; York et al. 2000) SN sur- ally lower, slow encounters lead to strong dynamical friction vey. and rapid merging (Hickson et al. 1977; Barnes 1989; Taranu To improve this situation, more searches for IGL need to et al. 2013). Contrary to cluster environments, then, galax- be undertaken in galaxy groups over a large range of mass. ies in groups may exhibit long-lived accretion shells or tidal The M96 Group (sometimes referred to as the Leo I Group), tails in their outskirts (e.g., Quinn 1984; Hernquist & Quinn shown in Figure 1, helps serve this purpose well. It is the 1987; Law et al. 2005). Meanwhile, the low density envi- nearest loose group4 that contains both early and late type ronment around isolated field galaxies should lead to fewer massive galaxies (de Vaucouleurs 1975), and hosts a rather recent interactions and accentuate secular evolution processes unusual feature – a roughly 200 kiloparsec diameter broken that are internal to galaxies (e.g. Kraljic et al. 2012). A com- ring of neutral hydrogen, first discovered by Schneider et al. plete survey of the processes that shape galaxies thus requires (1983). This ring is fairly diffuse; from Arecibo data, column an investigation across all environments. densities range from 6:4×1019 cm−2 to as low as 2×1018 cm−2 Of particular interest is the group environment, the most (Schneider et al. 1989), although smaller clumps with densi- common environment for galaxies to reside. Intragroup ties of roughly 4 × 1020 cm−2 were also detected using the starlight (IGL) in loose groups3 is of significant interest across Very Large Array (Schneider et al. 1986). These higher den- many studies of galaxy and galaxy cluster evolution. The frac- sity clumps appear to be confined to the southernmost portion tion of baryons within galaxy groups and clusters constrains of the ring, and include a bridge-like feature apparently con- models of star formation and cluster assembly, and there is necting the ring to M96 (NGC 3368), the most massive spiral strong evidence that the amount of intragroup and intracluster galaxy in the group. Two other galaxies – the E1 galaxy M105 starlight may be a significant contributor to the baryon frac- (NGC 3379) and the S0 galaxy NGC 3384 – appear to lie near tion of these systems (e.g. Gonzalez et al. 2013; Budzynski the center of the ring, implying some relation between these et al. 2014). The loose group environment may also dynami- three galaxies and the intergalactic HI. A fourth galaxy, the cally “preprocess” galaxies that later fall into galaxy clusters barred spiral M95 (NGC 3351), is associated with the group and produce ICL (Mihos 2004), and substructure within the as well, but due to its relatively large projected separation may diffuse IGL should trace this process in dynamically evolving not be associated with the ring complex itself. The group also galaxy groups. lies near in the sky (∼8◦ away) and at a somewhat similar dis- However, the mean fraction of stars in an IGL compo- tance to the M66 Group, leading to speculation that the two nent, and how that fraction varies with group/cluster mass, groups are actually each part of a larger complex of galaxies is substantially uncertain.