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

Abstract

We perform a systematic Bayesian analysis of rotation versus dispersion support (vrot/σ) in 40 dwarf galaxies throughout the local volume (LV) over a stellar mass range of 103.5 M☉ < M* < 108 M☉. We find that the stars in ∼80 per cent 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 vrot/σ ≲ 1.0, while all have vrot/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 discs, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between vrot/σ and distance to the closest L* galaxy, nor between vrot/σ and M* within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (109 M☉ < Mvir < 1010 M☉) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar vrot/σ ratios that are consistent with the observed population of dIrrs anddSphs 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 discs. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.