Tracing circumnuclear dense gas in H2O maser galaxies
  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.
