ALMA Sub-arcsec-resolution 183 GHz H2O and Dense Molecular Line Observations of Nearby Ultraluminous Infrared Galaxies

Abstract

We present the results of ALMA $\sim$2 mm, $\lesssim$1$''$-resolution observations of ten (ultra)luminous infrared galaxies ([U]LIRGs; infrared luminosity $\gtrsim$ 10$^{11.7}$L$_{\odot}$) at $z <$ 0.15, targeting dense ($>$10$^{4}$ cm$^{-3}$) molecular (HCN, HCO$^{+}$, and HNC J=2-1) and 183 GHz H$_{2}$O 3$_{1,3}$-2$_{2,0}$ emission lines. Higher HCN to HCO$^{+}$ J=2-1 flux ratios are observed in some, but not all, AGN-important ULIRGs than in starburst-classified sources. We detect 183 GHz H$_{2}$O emission in almost all AGN-important ULIRGs, and elevated H$_{2}$O emission is found in two sources with elevated HCN J=2-1 emission, relative to HCO$^{+}$ J=2-1. Except one ULIRG (the Superantennae), the H$_{2}$O emission largely comes from the entire nuclear regions ($\sim$1 kpc), rather than AGN-origin megamaser at the very center ($<<$1 kpc). Nuclear ($\sim$1 kpc) dense molecular gas mass derived from HCO$^{+}$ J=2-1 luminosity is $\gtrsim$a few $\times$ 10$^{8}$M$_{\odot}$, and its depletion time is estimated to be $\gtrsim$10$^{6}$ yr in all sources. Vibrationally excited J=2-1 emission lines of HCN and HNC are detected in a few (U)LIRGs, but those of HCO$^{+}$ are not. It is suggested that in mid-infrared-radiation-exposed innermost regions around energy sources, HCO$^{+}$ and HNC are substantially less abundant than HCN. In our ALMA $\sim$2 mm data of ten (U)LIRGs, two continuum sources are serendipitously detected within $\sim$10$''$, which are likely to be an infrared luminous dusty galaxy at $z >$ 1 and a blazar.