出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Hydrogen is the most common element in the Universe and is a powerful diagnostics for the
kinematics and the conditions of the interstellar medium in galaxies. It provides key information
on formation and evolution of galaxies, dark matter content and, for radio loud objects, on the
conditions around the super massive black hole. Future radio telescopes, and in particular SKA,
will use HI observations to solve some of the fundamental questions in astronomy.
This paper presents an overview of 21-cm HI observations. This goes from the parameters that
need to be setup, how carry out the observations to the data reduction and visualization. The
paper also briefly presents a few recent results on extragalactic HI studies. These results can be
taken as examples of how important is to improve the sensitivity of HI observations. The still
open questions associated to HI extragalactic studies are among the one driving the designs of
the new SKA pathfinders, and ultimately, of SKA. The need for improved sensitivity but also
for large field of view – with focal plane array or aperture arrays –is discussed
