TiNy Titans  

1. TINY TITANS: THE ROLE OF DWARF-DWARF INTERACTIONS IN THE EVOLUTION OF LOW MASS GALAXIES

 Stierwalt et al. (2015)

 Accepted to ApJ, February 2015 [arXiv, ADS]

We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. This is a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction.

 Pearson et al. (2016)

 Accepted to MNRAS, March 2016 [arXiv, ADS]

We introduce the Local Volume TiNy Titans sample (LV-TNT), which consists of 10 dwarf galaxy pairs in the Local Universe with resolved synthesis maps of their neutral hydrogen. They are located in a range of environments and captured at various interaction stages, enabling us to do a comparative study of the diffuse gas in dwarf-dwarf interactions in isolation and near massive galaxies.

 Besla et al. (2016)

 Accepted to ApJ, July 2016 [arXiv, ADS]

We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts of the stellar disk of the LMC (<10° from the LMC center). These data highlight the existence of stellar arcs and multiple spiral arms in the northern periphery, with no comparable counterparts in the south. By comparing these data to detailed simulations of the LMC disk outskirts, we conclude that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar structures seen in the periphery of the LMC. More generally, we find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for 1–2 Gyr even after the secondary merges entirely with the primary.

 Stierwalt et al. (2016)

 Accepted to Nature Astronomy, January 2017 [Nature Astronomy, arXiv, ADS]

The demographics of dwarf galaxy populations have long been in tension with predictions from the Cold Dark Matter (CDM) paradigm. If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark matter subhaloes, the most massive of which may have undergone star formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are observed as satellites of more massive galaxies, and there is observational and theoretical evidence to suggest that these satellites at z=0 were captured by the massive host halo as a group. However, the evolution of dwarf galaxies is highly susceptible to environment making these satellite groups imperfect probes of CDM in the low mass regime. We have identified one of the clearest examples to date of hierarchical structure formation at low masses: seven isolated, spectroscopically confirmed groups with only dwarf galaxies as members. Each group hosts 3-5 known members, has a baryonic mass of ~4.4 x 10^9 to 2 x 10^10 Msun, and requires a mass-to-light ratio of <100 to be gravitationally bound. Such groups are predicted to be rare theoretically and found to be rare observationally at the current epoch and thus provide a unique window into the possible formation mechanism of more massive, isolated galaxies.

 Privon et al. (2017)

 Accepted to ApJ, August 2017 [arXiv, ADS]

Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the buildup of stellar mass in low-metallicity systems. We present the first VLT/MUSE optical IFU observations of the interacting dwarf pair dm1647+21, selected from the TiNy Titans survey. The Hα emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 Msun yr^−1, 2.7 times higher than the SFR inferred from SDSS data. The implied specific SFR (sSFR) for the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of > 50. Examining the spatially-resolved maps of classic optical line diagnostics, we find the ISM excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies: rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus be more distributed.

 Pearson et al. (2018)

 Accepted to MNRAS, July 2018 [arXiv, ADS]

Discoveries of low mass galaxy pairs and groups are increasing. Studies indicate that dwarf galaxy pairs are gas rich in the field and exhibit elevated star formation rates, suggestive of interactions. Lacking are dynamical models of observed dwarf galaxy pairs to disentangle the physical processes regulating their baryon cycles. We present new optical data and the first detailed theoretical model of an observed tidal encounter between two isolated low mass galaxies, NGC 4490 & NGC 4485. This system is an isolated analog of the Magellanic Clouds and is surrounded by a ˜50 kpc extended HI envelope. We use hybrid N-body and test-particle simulations along with a visualization interface (Identikit) to simultaneously reproduce the observed present-day morphology and kinematics. Our results demonstrate how repeated encounters between two dwarf galaxies can "park" baryons at very large distances, without the aid of environmental effects. Our best match to the data is an 8:1 mass ratio encounter where a one-armed spiral is induced in the NGC 4490-analog, which we postulate explains the nature of diffuse starlight presented in the new optical data. We predict that the pair will fully merge in ˜370 Myr, but that the extended tidal features will continue to evolve and return to the merged remnant over ˜5 Gyr. This pre-processing of baryons will affect the efficiency of gas stripping if such dwarf pairs are accreted by a massive host. In contrast, in isolated environments this study demonstrates how dwarf-dwarf interactions can create a long-lived supply of gas to the merger remnant.

 Besla et al. (2018)

 Accepted to MNRAS, July 2018 [arXiv, ADS]

We quantify the frequency of companions of low redshift (0.013 < z < 0.0252), dwarf galaxies (2 × 108 M⊙ < Mstar < 5 × 109 M⊙) that are isolated from more massive galaxies in SDSS and compare against cosmological expectations using mock observations of the Illustris simulation. Dwarf multiples are defined as 2 or more dwarfs that have angular separations >55″, projected separations rp < 150 kpc and relative line-of-sight velocities ΔVLOS < 150 km/s. While the mock catalogs predict a factor of 2 more isolated dwarfs than observed in SDSS, the mean number of observed companions per dwarf is Nc ˜ 0.04, in good agreement with Illustris when accounting for SDSS sensitivity limits. Removing these limits in the mock catalogs predicts Nc ˜ 0.06 for future surveys (LSST, DESI), which will be complete to Mstar = 2 × 108 M⊙. The 3D separations of mock dwarf multiples reveal a contamination fraction of ˜40% in observations from projection effects. Most isolated multiples are pairs; triples are rare and it is cosmologically improbable that bound groups of dwarfs with more than 3 members exist within the parameter range probed in this study. We find that <1% of LMC-analogs in the field have an SMC-analog companion. The fraction of dwarf "Major Pairs" (stellar mass ratio >1:4) steadily increases with decreasing Primary stellar mass, whereas the cosmological "Major Merger rate" (per Gyr) has the opposite behaviour. We conclude that cosmological simulations can be reliably used to constrain the fraction of dwarf mergers across cosmic time.

TNT in the press