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Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

  • Author(s): Wheeler, Coral Rose
  • Advisor(s): Bullock, James S.
  • et al.

We study dwarf satellite galaxy quenching using observations from the Geha et al. (2012) NSA/SDSS catalog together with CDM cosmological simulations to facilitate selection and interpretation. We show that fewer than 30% of dwarfs (M* ~ 10^8.5-10^9.5 Msun) identified as satellites within massive host halos (Mhost ~ 10^12.5-10^14 Msun) are quenched. We conclude that whatever the action triggering environmental quenching of dwarf satellites, the process must be highly inefficient. We investigate a series of simple, one-parameter quenching models in order to understand what is required to explain the low quenched fraction and conclude that either the quenching timescale is very long (> 9.5 Gyr, a “slow starvation” scenario) or that the environmental trigger is not well matched to accretion within the virial volume.

We further present FIRE/Gizmo hydrodynamic zoom-in simulations of isolated dark matter halos, two each at the mass of classical dwarf galaxies (Mvir ~ 10^10 Msun) and ultra-faint galaxies (Mvir ~ 10^9 Msun). The resulting central galaxies lie on an extrapolated abundance matching relation from M* ~ 10^6 to 10^4 Msun without a break. Our dwarfs with M* ~ 10^6 Msun each have 1-2 well-resolved satellites with M* = 3 - 200 x 10^3 Msun. Even our isolated ultra-faint galaxies have star-forming subhalos. We combine our results with the ELVIS simulations to show that targeting the ~ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ~35% compared to random pointings.

The well-resolved ultra-faint galaxies in our simulations (M* ~ 3 - 30 x 10^3 Msun) form within

Mpeak ~ 0.5 - 3 x 10^9 Msun halos. Each has a uniformly ancient stellar population (> 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that Mhalo ~ 5 x 10^9 Msun is a probable dividing line between halos hosting reionization “fossils” and those hosting dwarfs that can continue to form stars in isolation after reionization.

Finally, we perform a systematic Bayesian analysis of rotation vs. dispersion support (vrot/sigma) in 40 dwarf galaxies throughout the Local Volume (LV) over a stellar mass range 10^3.5 Msun < M* < 10^8 Msun. We find that the stars in 80% of the LV dwarf galaxies studied – both satellites and isolated systems – are dispersion-supported. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion supported stars. We apply the same Bayesian analysis to four of the FIRE/Gizmo hydrodynamic zoom-in simulations of isolated dwarf galaxies (10^9 Msun < Mvir < 10^10 Msun) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar vrot/sigma ratios that are consistent with the observed population of dIrrs and dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-dominated systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.

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