Dense molecular gas tracers in water megamaser galaxies

Abstract

Since their discovery in 1979, water megamasers (H2OMMs) have been of special interest for both astrophysicists and cosmologists. The unique environment they are emitted from is interesting, being located inside accretion disks surrounding SMBHs. H2OMMs have helped astrophysicists accurately calculate SMBH masses and determine the kinematics of gas falling into the central SMBHs. More important, H2OMMs gave the golden chance to find the distance to a special kind of H2OMM galaxies, i.e., disk masers, accurately and directly, without any need to use standard candles or the cosmic microwave background (CMB) radiation. Unfortunately, detecting H2OMMs is very difficult, only 180 galaxies have been found out of more than 6000 galaxies searched for H2OMMs. The low detection rate is motivating many researchers to find a method that would help to increase the detecting rate. As H2OMMs are believed to be created inside dense gas clouds at galaxy centers, a correlation between dense gas and maser emission was suggested. Until now, this correlation has been barely studied. Only one work has been done so far in 2014 with a small sample number. In this work, we tried to study this possible correlation. A sample of HCN(J=1-0) and HCO+(J=1-0) emission lines was successfully collected and used as dense gas tracers via normalizing them by CO(J=1-0) emission lines, to get dense gas fractions. Subsequently, a sample of 30 H2OMM galaxies along with the dense gas fractions has been statistically analysed. Both of the dense gas fractions show a correlation with water maser emission, with HCO+ being stronger than HCN. Moreover, after removing (Ultra)Luminous IR Galaxies (U)LIRG sources, the HCO+ correlation becomes very tight and HCN correlation becomes stronger. Our results agree with previous works on dense gas as a favorable parameter to be used in H2OMM surveys. According to our analyses, HCO+ is preferable to HCN as a dense gas tracer in MM sources.