Title:Tracing H2 density with CI and CO isotopologues

Abstract:
We present the first results of neutral carbon ([C I] at 492 GHz) and carbon monoxide (CO) 
mapping of the Vela Molecular Ridge cloud C and G333 giant molecular cloud complexes with 
the NANTEN2 and Mopra telescopes. For the mapped regions so far, we found that [C I] has 
very similar emission profiles to 13CO with comparable line widths. [C I] has opacity of 0.1 - 
1.3, the [C I]/13CO peak brightness temperature ratio is between 0.2 to 0.8 across the mapped 
regions. [C I] column density is an order of magnitude lower than that of 13CO. H2 column 
density derived from C I is comparable to values obtained from 12CO. Our data show C I is 
mainly found among gas with low temperatures (below 20 K), this possibly explains the 
comparable H2 column density that both [C I] and 12CO under estimate column density, as the 
C I in the warmer gas is in a higher energy state ([C I] at 810 GHz), and thus it is likely that 
observations of both [C I] transitions are needed to recover the total H2 column density.