The Square Kilometre Array (SKA) will be a formidable instrument for the detailed study of neutral hydrogen (HI) in external galaxies and in our own Galaxy and Local Group. The sensitivity of the SKA, its wide receiver bands, and the relative freedom
from radio frequency interference at the SKA sites will allow the imaging of substantial number of high-redshift galaxies in HI for the first time. It will also allow imaging of galaxies throughout the Local Volume at resolutions of <100 pc and detailed investigations of galaxy disks and the transition between disks, halos and the intergalactic medium (IGM) in the Milky Way and external galaxies. Together with deep optical and millimetre/sub-mm imaging, this will have a profound effect on our understanding of the formation, growth and subsequent evolution of galaxies in different environments. This paper provides an introductory text to a series of nine science papers describing the impact of the SKA in the field of HI and galaxy evolution. We propose a nested set of surveys with phase 1 of the SKA which will help tackle much of the exciting science described. Longer commensal surveys are discussed, including an ultra-deep survey which should permit the detection of galaxies at z=2, when the Universe was a quarter of its current age. The full SKA will allow more detailed imaging of even more distant galaxies, and allow cosmological and evolutionary parameters to be measured with exquisite precision.
The SKA will be a unique instrument with which to study the evolution of the gas content of galaxies. A proposed deep (~8 Msec) pencil-beam survey is simulated using recently updated specifications for SKA sensitivity and survey speed. Almost 10^7 ga
laxies could be detected in the redshifted 21cm line, most at redshifts in excess of two. This will enable confident statements to be made about the evolution of the cosmic HI density and the HI mass function to z=3, corresponding to a lookback time of 11 Gyr. However, galaxies or groups of galaxies with masses the same as the most HI-massive galaxies at z=0 will be detectable at redshifts of 6, if they exist. The ideal instrument for studying HI evolution would have an instantaneous sensitivity at least a factor of two higher than current specifications in the critical frequency range 200-500 MHz, or A/T > 2x10^4 m^2/K. The capabilities of the SKA will be highly complementary to ALMA which will be able to study the evolution of the molecular gas component over the same redshift range.
The Australian SKA Pathfinder (ASKAP) will be a powerful instrument for performing large-scale surveys of galaxies. Its frequency range and large field of view makes it especially useful for an all-sky survey of Local Volume galaxies, and will probab
ly increase the number of known galaxies closer than 10 Mpc by a factor of two and increase, by at least an order of magnitude, the number detected in HI. Implications for our knowledge of the HI mass function for the very faintest galaxies and for the structure and dynamics of the Local Volume are discussed.
