Smooth HI Low Column Density Outskirts In Nearby Galaxies
  The low column density gas at the outskirts of galaxies as traced by the 21
cm hydrogen line emission (HI) represents the interface between galaxies and
the intergalactic medium, i.e., where galaxies are believed to get their supply
of gas to fuel future episodes of star formation. Photoionization models
predict a break in the radial profiles of HI at a column density of 5x10E+19
cm^-2 due to the lack of self-shielding against extragalactic ionizing photons.
To investigate the prevalence of such breaks in galactic disks and to
characterize what determines the potential "edge" of the HI disks, we study the
azimuthally-averaged HI column density profiles of 17 nearby galaxies from The
HI Nearby Galaxy Survey (THINGS) and supplemented in two cases with published
Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) data. To detect potential faint
HI emission that would otherwise be undetected using conventional moment map
analysis, we line up individual profiles to the same reference velocity and
average them azimuthally to derive stacked radial profiles. To do so, we use
model velocity fields created from a simple extrapolation of the rotation
curves to align the profiles in velocity at radii beyond the extent probed with
the sensitivity of traditional integrated HI maps. With this method, we improve
our sensitivity to outer-disk HI emission by up to an order of magnitude.
Except for a few disturbed galaxies, none show evidence for a sudden change in
the slope of the HI radial profiles, the alleged signature of ionization by the
extragalactic background.
