Tracing circumnuclear dense gas in H2O maser galaxies

TL;DR

This study investigates the relationship between dense molecular gas tracers and water maser emission in galaxies, revealing strong correlations especially with HCO+ and emphasizing the importance of dense gas in maser activity.

Contribution

It provides the first comprehensive analysis of HCO+ as a dense gas tracer in relation to H2O masers, expanding previous HCN studies and highlighting HCO+'s superior correlation.

Findings

Strong correlation between H2O maser luminosity and HCO+ line luminosity.

Weaker correlation between H2O maser luminosity and HCN line luminosity.

HCO+ is a better dense gas tracer than HCN for maser studies.

Abstract

A sample of 30 H2O extra-galactic maser galaxies with their published HCN(J=1-0) and HCO+(J=1-0) observations has been compiled to investigate the dense gas correlation with H2O maser emission. Our sample number exceeds the size of the previous HCN samples studied so far by a factor of three, and it is the first study on the possible relation with the dense gas tracer HCO+. We find a strong correlation between normalized H2O maser emission luminosity (LH2O\LCO) and normalized HCO+ line luminosity (LHCO+\LCO). Moreover, a weak correlation has been found between LH2O\LCO and normalized HCN line luminosity (LHCN\LCO). The sample is also studied after excluding Luminous and Ultraluminous infrared galaxy (U)LIRG sources, and the mentioned correlations are noticeably stronger. We show that 'Dense gas' fractions as obtained from HCN and HCO+ molecules tightly correlate with maser emission, especially for galaxies with normal IR luminosity(LIR< 10^11Lsun) and we show that HCO+ is a better 'dense gas' tracer than HCN. Further systematic studies of these dense gas tracers with higher transition level lines are vital to probe megamaser physical conditions and to accurately determining how maser emission interrelates with the dense gas.