The effect of star formation on molecular clouds in dwarf irregular galaxies: IC 10 and NGC 6822

We have observed the (CO)-C-12 J = 2-1 and J = 3-2 lines at a few locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James Clerk Maxwell Telescope. In addition, we have observed the (CO)-C-13 J = 2-1 line for IC 10 and the first detection of the (CO)-C-13 J = 3-2 transition in a Local Group galaxy. The CO line ratios in IC 10 are uniform and are consistent with the average line ratios observed in M33 at the 1 sigma level. These low-metallicity environments appear to be porous to UV radiation and allow for more efficient heating of molecular gas by nearby H II regions. The (CO)-C-12 J = 3-2/J = 2-1 ratio for the molecular cloud in NGC 6822 is higher than those found for IC 10 and M33 and suggests that the (CO)-C-12 emission is optically thin in this region. This high line ratio is likely the result of its location inside a large H II region with low metallicity and low gas content, and requires a hydrogen density greater than 10(4) cm(-3) and a kinetic temperature greater than 100 K. The (CO)-C-12/(CO)-C-13 J = 3-2 line ratio in one of the molecular clouds in IC 10 indicates that the gas must have a rather high kinetic temperature of about 100 K. In IC 10 we observe structures on a variety of size scales that all appear to be gravitationally bound. This effect may help explain the rather high star formation rate in IC 10.