An important question of galaxy evolution is, how the neutral atomic gas transitions into the essential molecular gas
for star formation. A key to this is mapping the temperature and density variations of the cold neutral medium (CNM)
component of the inter stellar medium (ISM). Neutral hydrogen (HI) self absorption (HISA) clouds are ideal places to
study the CNM. They are found throughout the Galaxy where cold HI clouds are located in front of warmer HI emission
and they reveal the spatial structure of the CNM. However, to get a good handle on the physical properties of the
cold gas HI absorption measurements against bright background sources are needed. To do this, we mapped the
Riegel-Crutcher cloud, one of the most famous HISA regions located in front of the Galactic Centre, in HI absorption.
I will present our HI absorption survey against ~40 background continuum sources and discuss what we can learn from
these observations.
|
