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

We perform a systematic Bayesian analysis of rotation versus dispersion support (v(rot)/sigma) in 40 dwarf galaxies throughout the local volume (LV) over a stellar mass range of 10(3.5)M circle dot < M star < 10(8)M circle dot. We find that the stars in similar to 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 v(rot)/sigma less than or similar to 1.0, while all have v(rot)/sigma less than or similar to 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 v(rot)/sigma and distance to the closest L star galaxy, nor between v(rot)/sigma and M star within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (10(9)M circle dot < M-vir < 10(10)M circle dot) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar v(rot)/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, dispersiondominated 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.