Molecular gas in compact galaxies

New observations of eleven compact galaxies in the $\,{\rm ^{12}CO}$ $J = 2{-}1$ and $J = 3{-}2$ transitions are presented. From these observations and literature data accurate line ratios in matched beams have been constructed, allowing the modelling of physical parameters. Matching a single gas component to observed line ratios tends to produce physically unrealistic results, and is often not possible at all. Much better results are obtained by modelling two distinct gas components. In most observed galaxies, the molecular gas is warm ($T_{\rm k} = 50{-}150$ K) and at least partially dense ($n({\rm H}_{2}) \geq 3000 \,{\rm cm^{-3}}$). Most of the gas-phase carbon in these galaxies is in atomic form; only a small fraction (~$5\%$) is in carbon monoxide. Beam-averaged CO column densities are low (of the order of $10^{16}\,{\rm cm^{-2}}$). However, molecular hydrogen column densities are high (of the order of $10^{22} \, {\rm cm^{-2}}$) confirming large CO-to-$\,{\rm H_{2}}$ conversion factors (typically $X = 10^{21}{-}10^{22} \,{\rm cm^{-2}}/\,{\rm {K\,km\,s^{-1}}}$) found for low-metallicity environments by other methods. From CO spectroscopy, three different types of molecular environment may be distinguished in compact galaxies. Type I (high rotational and isotopic ratios) corresponds to hot and dense molecular clouds dominated by star-forming regions. Type II has lower ratios, similar to the mean found for infrared-luminous galaxies in general, and corresponds to environments engaged in, but not dominated by, star-forming activity. Type III, characterized by low $\,{\rm ^{12}CO}$ (2–1)/(1–0) ratios, corresponds to mostly inactive environments of relatively low density