Dense Molecular Gas in Nearby Gas-rich Active Galaxies

TL;DR

This study uses 3 mm observations to analyze molecular gas emissions in nearby active galaxies, revealing how molecular line ratios correlate with infrared luminosity and suggesting a link between gas density and galaxy luminosity.

Contribution

First detection of multiple molecular lines in a diverse sample of nearby active galaxies, exploring their relation to infrared luminosity and gas density.

Findings

HCN/HCO+ ratio increases with infrared luminosity

C2H/HCN and C2H/HCO+ ratios decrease with LIR

Optical depth of HCN is low to moderate (tau ~ 1-5)

Abstract

We present 3 mm observations obtained with the IRAM 30-m telescope for ten nearby gas-rich active galaxies spanning three orders of magnitude in infrared luminosity. Emission lines of HCO+(1-0), HCN(1-0), and C2H(1-0) were simultaneously detected in most galaxies of our sample. We also tentatively detected the rare isotopic species, H13CN, HC15N and H13CO+ in up to four galaxies (IC 694, NGC 3690, NGC 4258 and NGC 6240). Our estimation shows that the optical depth of HCN is low to moderate in these galaxies (tau ~ 1-5). After comparing the intensity ratios of different molecular emission lines (HCN/HCO+, C2H/HCN, and C2H/HCO+) with the infrared luminosity (LIR), we find that in the infrared luminous sample of five composite (AGN+Starburst) galaxies, these ratios vary with LIR: the HCN/HCO+ ratio increases with LIR, which is consistent with previous studies, while there is a slight trend in the composite galaxies that both C2H/HCN and C2H/HCO+ ratios decrease with increasing LIR. Although our sample is limited, this trend may possibly imply a relation between the overall gas density and the infrared luminosity of galaxies.