Dense and Warm Molecular Gas and Warm Dust in Nearby Galaxies

TL;DR

This study compares molecular gas properties in nearby starburst and non-starburst galaxies, revealing that starbursts have warmer, denser gas and stronger radiation fields, with implications for star formation activity.

Contribution

It provides new observational evidence linking molecular gas conditions to starburst activity and highlights the importance of gas temperature in star formation processes.

Findings

Starburst galaxies have higher HCN/13CO ratios (~1.15) than non-starbursts (<0.31).

Molecular gas in starbursts is warmer and denser, influenced by stronger radiation fields.

HCN/13CO ratios correlate with IRAS 25 micron flux ratios.

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 the seven starburst galaxies, with the intensities of both lines being similar (i.e., the ratios are around unity). On the other hand, for case of the non-starburst galaxies, the 13CO(1-0) emission was detected from all three galaxies, while the HCN(1-0) emission was weakly or not detected in past observations. This result indicates that 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 have warmer and denser conditions than that in the non-starburst galaxies, and the photon-dominated-region model suggests that the denser molecular gas is irradiated by stronger interstellar radiation field in the starburst galaxies than that in the non-starburst galaxies. In addition, HCN/13CO in our sample galaxies exhibit strong correlations with the IRAS 25 micron 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 molecular gas temperature is also an important parameter.