Completing the Hydrogen Census in the Circumgalactic Medium at z~0
  Over the past decade, Lyman-alpha and metal line absorption observations have
established the ubiquity of a gas-rich circumgalactic medium (CGM) around
star-forming galaxies at z~0.2 potentially tracing half of the missing baryonic
mass within galaxy halos. Unfortunately, these observations only provide a
statistical measure of the gas in the CGM and do not constrain the spatial
distribution and kinematics of the gas. Furthermore, we have limited
sensitivity to Lyman-alpha at z~0 with existing instruments. As such, we remain
ignorant of how this gas may flow from the CGM onto the disks of galaxies where
it can fuel ongoing star-formation in the present day. Fortunately, 21-cm HI
observations with radio telescopes can map HI emission providing both spatial
and kinematic information for the CGM in galaxies at z=0. Observations with
phased array feeds, radio cameras, on single-dish telescopes yield unmatched
surface brightness sensitivity and survey speed. These observations can
complete the census of HI in the CGM below N(HI)<10^17 cm^-2 and constrain how
gas accretion is proceeding in the local universe, particularly when used in
concert with UV absorption line data.
