We report high spatial resolution observations of the H \sevensize I 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 \degr,b = -1 \degr , achieving an angular resolution of \sim 35 \arcsec .
The H \sevensize I 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 H \sevensize I 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 H \sevensize I line is opaque ( \tau \gtrsim 2 ) at several velocities in the Sagittarius-Carina spiral arm .
The spin temperature is \sim 100 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 \sim 35 \% of the total gas mass .
H \sevensize I 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 .