Hierarchical formation models predict substructure should exist on all scales, not just around relatively massive galaxies like the Milky Way. Thus, exploration of the extended stellar structures of dwarf galaxies should reveal similar tidal features to those previously discussed. We first discuss the observational evidence for halos around dwarf galaxies and, in particular, for substructure in dwarf galaxy halos. Second, we discuss the impact that the creation of such substructures would have on dwarf galaxies. Understanding the formation and evolution of dwarf galaxies is particularly vital to form realistic “initial conditions” for simulations and to make highly detailed predictions of substructure properties. For many reasons, extra-galactic systems are best suited for these explorations.
5.1. Observational evidence for substructure at dwarf galaxy scales
In the process of hierarchical structure formation, it is likely that some of the most massive satellite dwarf galaxies themselves host their own, even smaller satellites (dwarf spheroidals –“dSphs” – or globular clusters). In the Milky Way, we know that the most massive classical dwarf galaxies have globular clusters associated with them – specifically the Large Magellanic Cloud, the Small Magellanic Cloud, and the Fornax dSph (e.g., Forbes et al. 2000), as well as the Sagittarius tidal stream (e.g., Law & Majewski 2010b). The most massive Milky Way satellite, the Large Magellanic Cloud (LMC), is itself part of a bound pair of dwarf irregular galaxies (with the Small Magellanic Cloud, or SMC). Thus, we would expect to find evidence of tidal interaction around dwarf galaxies, analogous to the features we see in halos of more massive galaxies. The majority of the dwarf galaxies in the Local Group are satellites, which have experienced interactions with their more massive host. Thus, it can be difficult to disentangle effects on the dwarf created during the accretion by its parent from those it experienced before falling in. Studying external, isolated dwarf galaxies may prove a more effective means of understanding the role of hierarchical formation, including the role interactions may have in forming the Hubble sequence at low masses.
In fact, many of the unique features of the LMC-SMC pair can be explained by their binary interaction (e.g., Besla et al. 2012), including the spectacular 200∘ Magellanic Stream in HI (Nidever et al. 2010). Binary pairs are somewhat rare, as Robotham et al. (2012) find only two MW+LMC+SMC analog systems among the Galaxy Mass Assembly (GAMA) galaxies. While only 3.4% percent of GAMA galaxies are MW+LMC+SMC analogs, 12% of SDSS galaxies have LMC-like companions (i.e., luminous satellite within 75 kpc; Tollerud et al. 2011), which suggests that about one in four LMC-like satellites have a smaller SMC-like companion. The relatively low fraction of LMC-SMC binary satellite systems supports a “transient” nature, as detailed numerical simulations of the LMC-SMC suggest they may not remain a bound pair for long (Besla et al. 2012). In fact, it has been suggested (e.g., D'Onghia & Lake 2008) that satellites should often fall in as pairs or in groups, rather than individually, and the dwarf galaxies found at the edge of the Local Group (representing future accretions) are grouped (e.g., Mateo 1998, McConnachie 2012). Thus, we would expect to find evidence of tidal interaction around dwarf galaxies, analogous to the features we see in halos of more massive galaxies.
Wide-field optical and near-infrared imaging has revealed stellar halos around many star-forming dwarf galaxies in and beyond the Local Group (see Stinson et al. 2009 for an extensive listing of many of these). While Stinson et al. determined that these extended stellar envelopes are not likely to arise due to tidal interactions with the (larger) host galaxies, it has not been determined whether they are the result of interactions with smaller satellites. Large area surveys of the most massive dwarf galaxies in the Local Group also indicate the presence of extended, “halo-like” stellar populations at large effective radii, including M33 (McConnachie et al. 2009), the LMC (Nidever et al. 2007), and the SMC (Nidever et al. 2011). Further characterization, including full kinematic profiles, is necessary to determine if these extended structures are lower mass versions of the halos found around Milky Way sized galaxies.
One of the many probable dwarf galaxies discovered by Karachentsev et al. (2007) was an elongated feature near the dwarf irregular galaxy NGC 4449 in Digitized Sky Survey (POSS-II) plates (denoted as object “d1228+4358” in their catalog). NGC 4449 is a dwarf starburst galaxy with an irregular morphology, with luminosity (MV = 18.6) similar to that of the LMC, but with much stronger and more widespread ongoing star-formation activity. Its cold gas and HII regions exhibit peculiar kinematics (Hartmann et al. 1986, Hunter et al. 1998), suggesting that it may have recently interacted with another galaxy. Using deep, wide-field imaging around NGC 4449, Martínez-Delgado et al. (2012) definitively identified the Karachentsev et al. feature as a dwarf galaxy undergoing accretion by NGC 4449 (see Figure 8). This new dwarf galaxy was also seen by Rich et al. (2012) in a similar deep-imaging survey, which revealed the dwarf (dubbed NGC 4449B) and its S-shaped morphology that is characteristic of disrupting satellites. After fitting and subtracting a halo model, Rich et al. showed additional arcs and possible disk ripple features in the residual stellar surface brightness maps, along with evidence for a break in the surface brightness profile of the NGC 4449 stellar halo. The morphology, size, luminosity, and surface brightness profile of the newly discovered stream/dwarf, along with evidence of tidal features in the NGC 4449 halo and outer disk, was suggested by Rich et al. (2012) to result from a dwarf (NGC 4449B) that is on its first passage, and passed near the center of its host ∼ 108 yr ago. Thus, NGC 4449 is the first direct evidence of hierarchical structure formation similar to that seen in Milky Way-type galaxies, but on the mass scale of dwarf galaxies.
Figure 8. The stellar stream around the dwarf galaxy NGC 4449. Left: greyscale image from Martínez-Delgado et al. (2012) with a color inset for the galaxy, which clearly indicates the presence of an S-shaped tidal stream approximately 7 kpc in length. There is no clear association with any of the existing HI gas features (Hunter et al. 1998). Right: Subaru telescope sub-arcsec resolution image of the stellar stream (Martínez-Delgado et al. 2012). This is one of the few extra-galactic stellar streams that has been resolved into individual stars, which provides direct probes of its stellar populations.
5.2. Implications of dwarf-dwarf interactions
It is worth noting that the first stream from a dwarf-galaxy accretion event was found around one of the most intensely star-forming nearby galaxies. This leads one to wonder whether such accretion events are common among dwarf galaxies in recent epochs. It is possible that exact analogs to this stream have not been noticed in POSS or SDSS images because they are uncommon. However, another explanation is that the majority of such structures are fainter, more diffuse, or at a larger radius than the NGC 4449 stream, and thus await future detection. If streams as in NGC 4449 are common around dwarfs, they re-ignite classic ideas about galaxy interactions triggering starbursts. Given the high rates of star formation in dwarf galaxies, it is natural to ask if satellites are responsible. Surveys along these lines have produced mixed results (Noeske et al. 2001, Brosch et al. 2004, Li et al. 2008), but these studies were not looking for objects like the detected dwarf satellite of NGC 4449 – a gas poor, low-surface brightness analog to the Local Group dSphs – and did not probe appropriate depths to find these objects. Regardless of the implications for starbursts, evidence from NGC 4449 and the Fornax dSph, which shows traces of having swallowed a smaller dSph (Coleman et al. 2005), suggests that accretion of even smaller building blocks is a viable avenue for direct assembly of dwarf galaxy stellar halos.
It has been proposed that dSphs orbiting massive galaxies such as the Milky Way may be the result of “pre-processing” which is a term for the effects of interactions within groups of dwarf galaxies (e.g., D'Onghia et al. 2009). More specifically, it has been suggested that the dSphs were once gas-rich, rotationally supported objects like the field galaxies (i.e., similar to NGC 4449) whose properties were modified into the gas-free, dispersion supported dSphs via interactions with companions. This is a compelling scenario for the class of dwarf galaxies in the Local Group known as “dwarf Transition” objects (see Mateo 1998, Grebel 1999, among others), whose properties are intermediate between those of dIrrs and dSphs. Evidence of a dwarf-dwarf interaction in NGC 4449, in conjunction with the known HI streams around this dwarf galaxy, may thus demonstrate such a process in action. Moreover, Nidever et al. (2013) identified an HI filament associated with the M31 dwarf satellite IC 10. The dynamics and orientation of the stream are inconsistent with the orbital parameters of IC 10, and Nidever et al. (2013) suggest that the stream and other atypical HI features in the IC 10 disk could be explained via interaction with a “stealth” companion.
Wetzel et al. (2015) used the ELVIS simulations to explore the frequency of “pre-processing” for satellites within a simulated Milky Way host and found that nearly half of all satellites with stellar masses less than 106 solar masses were pre-processed in a more massive satellite halo. More generally, satellites with lower stellar masses or those closer to their host are more likely to have undergone pre-processing. Recent observational work for the Milky Way and M31 also provides hints of associated satellites or debris, including potential satellites with similar line-of-sight velocities (e.g., Chapman et al. 2007, Martin et al. 2009, Tollerud et al. 2012) and potential kinematic associations between substructures (e.g., Deason et al. 2014). While these initial probes are tantalizing, full phase space realizations of these objects that are only permitted via precision distances and proper motions are required to fully explore these associations locally. Moreover, though it is tempting to explain morphological transitions with dwarf-dwarf interactions, there is significant degeneracy with other physical processes that can alter HI morphologies even at large radius in a group environment, for instance the effects of ram pressure from the hot gaseous halo can be quite dramatic (see case studies in McConnachie et al. 2007, Kenney et al. 2014) and many isolated gas-poor dwarfs could be the result of “fly-by” interactions with their host (see Teyssier et al. 2012). However, the HI debris created by dwarf-dwarf interactions and ram pressure are different, and finding more dwarf galaxies in “distress” will reveal the relative importance of these processes, which also have implications for their halo substructures.
A dramatic example of an ongoing dwarf-dwarf interaction was seen by Paudel et al. (2015), who found a pair of dwarf galaxies connected by a 15 kpc stellar bridge. The HI disk for one of the galaxies is “completely destroyed” and there are several knots of star formation that have global properties similar to either young globular clusters or ultra-compact dwarf galaxies. The Paudel et al. (2015) dwarf-dwarf merger bears a striking resemblance to a scaled down version of equal mass mergers at larger total masses. The importance of dwarf interactions in shaping stellar populations of low-mass galaxies is highlighted in a recent systematic, multi-wavelength study of the relative star formation rates in interacting pairs of dwarf galaxies (TiNy Titans, or TNTs) by Stierwalt et al. (2014). This work found clear evidence of star formation enhancement (by a factor of ∼ 2.3 ± 0.7) among paired dwarfs relative to their unpaired counterparts. This enhancement occurs even in interacting pairs that are isolated by D > 1.5 Mpc from their nearest massive neighbor, showing that galaxy interactions are a frequent driver of enhanced star formation even outside the influence of larger galaxies. The Stierwalt et al. 2014 study also finds a factor of ∼ 3 increase in the fraction of paired dwarfs that are starbursting relative to single dwarfs, further highlighting the role of interactions in triggering star forming episodes. Thus far, though, stellar tidal debris signatures of these dwarf-dwarf interactions have not been identified. [Note that this study is limited to pairs of mass ratio (M1 / M2)* < 10, while the disrupting companion to NGC 4449 has about 1/50th its stellar mass (Martínez-Delgado et al. 2012).]