Giant molecular clouds in the Galaxy - The distribution of CO-13 emission in the galactic plane

Abstract

A survey of the carbon monoxide isotope (/sup 13/CO) is reported for the galactic equator at l< or =44/sup 0/. Since the /sup 13/CO lines are expected to have low opacity (tauapprox.1), their intensity variations as a function of galactic radius are more readily interpreted in terms of gas density variations than are the saturated, nearly thermalized /sup 12/CO lines. The distribution of /sup 13/CO emissivity as a function of galactic radius shows a peak at R=4 to 8 kpc nearly identical to that previously reported for /sup 12/CO. J (/sup 12/CO)/J (/sup 13/CO) =5.5 +- 0.63 determined from analysis of the emissivity at ..delta..R=0.5 kpc intervals over the range 3< or =R< or =9 kpc. We thus verify the presence of a high density ring of giant molecular clouds midway between the Sun and the galactic center. The density of molecular hydrogen in this structure exceeds by a factor of 10 that of atomic hydrogen in the same region. Typical column densities of gas (H/sub 2/) observed looking through this ring at lapprox.30/sup 0/ are 4 x 10/sup 22/ cm/sup -2/ or A/sub v/approx.40 mag. These results validate the use of /sup 12/CO line observations for galactic mapping ofmore » the distribution of molecular gas, since the /sup 13/CO and /sup 12/CO emissivities show similar variations on a galactic scale. The similarity of the distribution from the two lines, which are quite disparate in optical depth, suggests that the /sup 12/CO line-integrated intensity in the galactic plane is a measure primarily of the number of clouds, and the temperature in the majority of the galactic plane H/sub 2/ is fairly similar (T/sub k/approx.10 K). The /sup 12/CO//sup 13/CO emissivity ratio changes by less than 40% between R=4 and R=8 kpc, although it is consistent with a constant isotopic abundance ratio within the error limits.« less