Molecular gas in compact galaxies

Abstract

New observations of eleven compact galaxies in the 12CO 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 (Tk = 50-150 K) and at least partially dense (n(H2) ≥ 3000cm -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 1016 cm-2). However, molecular hydrogen column densities are high (of the order of 10 22 cm-2) confirming large CO-to- H2 conversion factors (typically X = 1021 - 102 cm-2 / 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 12CO (2-1)/(1-0) ratios, corresponds to mostly inactive environments of relatively low density. © ESO 2005.