The Baryon Content of Extremely Low Mass Dwarf Galaxies

We investigate the gas content and baryonic Tully-Fisher relationship forextremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consistsof 101 galaxies for which we have obtained follow-up HI observations using theArecibo Observatory and Green Bank Telescope. This represents the largesthomogeneous sample of dwarfs at low luminosities with well-measured HI andoptical properties. The sample spans a range of environments, from dense groupsto truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,significantly exceeding that of typical gas-rich galaxies at higherluminosities. Dwarf galaxies are therefore less efficient at turning gas intostars over their lifetimes. The strong environmental dependence of the gasfraction distribution demonstrates that while internal processes can reduce thegas fractions to roughly f_gas=0.4, external processes are required to fullyremove gas from a dwarf galaxy. The average rotational velocity of our sampleis vrot=50 km/s. Including more massive galaxies from the literature, we fit abaryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slopecompares well with CDM models that assume an equal baryon to dark matter ratioat all masses. While gas stripping or other processes may modify the baryon todark matter ratio for dwarfs in the densest environments, the majority of dwarfgalaxies in our sample have not preferentially lost significant baryonic massrelative to more massive galaxies.