Exploring Neutral Hydrogen and Galaxy Evolution with the SKA
  One of the key science drivers for the development of the SKA is to observe
the neutral hydrogen, HI, in galaxies as a means to probe galaxy evolution
across a range of environments over cosmic time. Over the past decade, much
progress has been made in theoretical simulations and observations of HI in
galaxies. However, recent HI surveys on both single dish radio telescopes and
interferometers, while providing detailed information on global HI properties,
the dark matter distribution in galaxies, as well as insight into the
relationship between star formation and the interstellar medium, have been
limited to the local universe. Ongoing and upcoming HI surveys on SKA
pathfinder instruments will extend these measurements beyond the local universe
to intermediate redshifts with long observing programmes. We present here an
overview of the HI science which will be possible with the increased
capabilities of the SKA and which will build upon the expected increase in
knowledge of HI in and around galaxies obtained with the SKA pathfinder
surveys. With the SKA1 the greatest improvement over our current measurements
is the capability to image galaxies at reasonable linear resolution and good
column density sensitivity to much higher redshifts (0.2 < z < 1.7). So one
will not only be able to increase the number of detections to study the
evolution of the HI mass function, but also have the sensitivity and resolution
to study inflows and outflows to and from galaxies and the kinematics of the
gas within and around galaxies as a function of environment and cosmic time out
to previously unexplored depths. The increased sensitivity of SKA2 will allow
us to image Milky Way-size galaxies out to redshifts of z=1 and will provide
the data required for a comprehensive picture of the HI content of galaxies
back to z~2 when the cosmic star formation rate density was at its peak.
