2020MNRAS.499....1F

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 3, 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 (L_H2_O/L_CO) and normalized HCO + line luminosity (L_HCO +_/LCO_). Moreover, a weak correlation has been found between L_H2_O/L_CO and normalized HCN line luminosity (L_HCN/L_CO). 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 (L_IR < 10¹¹ L_☉) 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.