  We use both an HI-selected and an optically-selected galaxy sample to
directly measure the abundance of galaxies as a function of their "baryonic"
mass (stars + atomic gas). Stellar masses are calculated based on optical data
from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated
using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA
(ALFALFA) survey. By using the technique of abundance matching, we combine the
measured baryonic function (BMF) of galaxies with the dark matter halo mass
function in a LCDM universe, in order to determine the galactic baryon fraction
as a function of host halo mass. We find that the baryon fraction of low-mass
halos is much smaller than the cosmic value, even when atomic gas is taken into
account. We find that the galactic baryon deficit increases monotonically with
decreasing halo mass, in contrast with previous studies which suggested an
approximately constant baryon fraction at the low-mass end. We argue that the
observed baryon fractions of low mass halos cannot be explained by reionization
heating alone, and that additional feedback mechanisms (e.g. supernova blowout)
must be invoked. However, the outflow rates needed to reproduce our result are
not easily accommodated in the standard picture of galaxy formation in a LCDM
universe.
