Dense and warm molecular gas and warm dust in nearby galaxies

Abstract

We performed 12CO (1-0), 13CO (1-0), and HCN (1-0) single-dish observations (beam size ̃ 14′-18′) toward nearby starburst and non-starburst galaxies using the Nobeyama 45-m telescope. The 13CO (1-0) and HCN (1-0) emissions were detected from all seven starburst galaxies, with the intensities of both lines being similar (i.e., the ratios are around unity). On the other hand, for the case of non-starburst galaxies, 13CO (1-0) emission was detected from all three galaxies, while HCN (1-0) emission was weak, or not detected in past observations. This result indicates mat the HCN/13CO intensity ratios are significantly larger (̃ 1.15 ±0.32) in the starburst galaxy samples than the non-starburst galaxy samples (< 0.31 ±0.14). The large-velocity- gradient model suggests that the molecular gas in the starburst galaxies has warmer and denser conditions than that in the non-starburst galaxies, and the photon-dominated-region model suggests that denser molecular gas is irradiated by a stronger interstellar radiation field in starburst galaxies than that in non-starburst galaxies. In addition, HCN/,13CO ratios in our sample galaxies exhibit strong correlations with the IRAS 25 μm flux ratios. It is a well-established fact that there exists a strong correlation between dense molecular gas and star-formation activities, but our results suggest that the molecular gas temperature is also an important parameter. © 2010. Astronomical Society of Japan.