The Carina Nebula and Gum 31 molecular complex: II. The distribution of
  the atomic gas revealed in unprecedented detail
  We report high spatial resolution observations of the HI 21cm line in the
Carina Nebula and the Gum 31 region obtained with the Australia Telescope
Compact Array. The observations covered $\sim$ 12 deg$^2$ centred on $l=
287.5\deg,b = -1\deg$, achieving an angular resolution of $\sim $ 35
arcseconds. The HI map revealed complex filamentary structures across a wide
range of velocities. Several "bubbles" are clearly identified in the Carina
Nebula Complex, produced by the impact of the massive star clusters located in
this region. An HI absorption profile obtained towards the strong extragalactic
radio source PMN J1032--5917 showed the distribution of the cold component of
the atomic gas along the Galactic disk, with the Sagittarius-Carina and Perseus
spiral arms clearly distinguishable. Preliminary calculations of the optical
depth and spin temperatures of the cold atomic gas show that the HI line is
opaque ($\tau \gtrsim$ 2) at several velocities in the Sagittarius-Carina
spiral arm. The spin temperature is $\sim100$ K in the regions with the highest
optical depth, although this value might be lower for the saturated components.
The atomic mass budget of Gum 31 is $\sim35 \%$ of the total gas mass. HI self
absorption features have molecular counterparts and good spatial correlation
with the regions of cold dust as traced by the infrared maps. We suggest that
in Gum 31 regions of cold temperature and high density are where the atomic to
molecular gas phase transition is likely to be occurring.
