Denuded Dwarfs Demystified: Gas Loss from dSph Progenitors and Implications for the Minimum Mass of Galaxies

Abstract

The placement of early-type dwarf galaxies (dSphs and dEs) with respect to the Potential Plane defined by pressure-supported late-type dwarf galaxies (dIs and BCDs) has been determined from surface brightness profiles. dEs and the most luminous dSphs lie on the Plane, suggesting that they emerged from late-type dwarfs that converted most of their gas into stars. However, there is a critical value of the potential at which dSphs start to fall systematically below the Plane, with the deviation growing as the potential becomes shallower. The displacements are attributed to depletion of baryons through gas loss, smaller galaxies having lost proportionately more gas. The critical potential corresponds to an escape velocity of 50 $\pm$ 8 km/s, which is what is expected for gas with a temperature of 13,000 $\pm$ 4,000 K, typical of a low-metallicity HII region. This suggests that photoionization was responsible for instigating the loss of gas by galaxies with potentials shallower than the critical value, with evacuation occurring over a few tens of millions of years. Extreme ratios of dynamical to luminous masses observed for the smallest dSphs are an artifact of mass loss. Because the efficiency with which gas was converted into stars was lower for dSphs with shallower potentials, there should be a minimum baryonic mass for a galaxy below which the stellar mass is negligible. Gross extrapolation of the trend of inferred gas masses with stellar masses suggests a value between 500 and 10,000 M$_\odot$. The corresponding dynamical mass is below 10$^6$ M$_\odot$.