The no-spin zone: rotation versus dispersion support in observed and simulated dwarf galaxies

Abstract

We perform a systematic Bayesian analysis of rotation vs. dispersion support (v rot /σ) in 40 dwarf galaxies throughout the Local Volume (LV) over a stellar mass range 10 3.5 M < M < 10 8 M . We find that the stars in ∼ 80% of the LV dwarf galaxies studied -both satellites and isolated systems -are dispersion-supported. In particular, we show that 6/10 isolated dwarfs in our sample have v rot /σ 1.0, while all have v rot /σ 2.0. 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 see no clear trend between v rot /σ and distance to the closest L galaxy, nor between v rot /σ and M within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (10 9 M < M vir < 10 10 M ) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar v rot /σ 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.

The Effects of Ram-pressure Stripping and Supernova Winds on the Tidal Stirring of Disky Dwarfs: Enhanced Transformation into Dwarf Spheroidals

“…In the aforementioned scenario, transition-type dwarfs should possess appreciable stellar rotation, having been only partially transformed. Although recent studies have reported significant rotation in both Pegasus (Wheeler et al 2017) and Phoenix (Kacharov et al 2017) with V rot /σ * ≈1, irrefutable evidence…”

A unified model for age–velocity dispersion relations in Local Group galaxies: disentangling ISM turbulence and latent dynamical heating

“…The gas velocity dispersion as a function of galaxy mass is estimated following the theoretical disk instability arguments from W15, in conjunction with the empirical trends of gas fraction as a function of galaxy stellar mass, M * from Whitaker et al (2014). Specifically at redshift zero, …”

Mergers and the outside-in formation of dwarf spheroidals

“…Whereas there is little evidence for systemic rotation in dSph galaxies, a number of dIrr (or at least their gas and young stars) show clear signs of ordered rotation, at least in their gas component and young stars (e.g., Leaman et al 2012;Kirby et al 2014). Although finding rotating subcomponents in a dSph would be harder than in dIrr, where recently-formed stars are more easily identified, it is generally agreed that the kinematic evidence suggests a scenario where some event in the past history of dSphs has been responsible for both terminating their star formation and for reducing the importance of centrifugal support (see, however, Wheeler et al 2015, for a more nuanced view).…”