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

Abstract

Abstract We present the results of ALMA ∼2 mm, ≲1″-resolution observations of 10 (ultra)luminous infrared galaxies ([U]LIRGs; infrared luminosity ≳1011.7 L ⊙) at z < 0.15, targeting dense (>104 cm−3) molecular (HCN, HCO+, and HNC J = 2–1) and 183 GHz H2O 31,3–22,0 emission lines. Active galactic nucleus (AGN)-important ULIRGs tend to show higher HCN/HCO+ J = 2–1 flux ratios than starburst-classified sources. We detect 183 GHz H2O emission in almost all AGN-important ULIRGs, and elevated H2O 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 H2O emission largely comes from the entire nuclear regions (∼1 kpc), rather than an AGN-origin megamaser at the very center (≪1 kpc). Nuclear (∼1 kpc) dense molecular gas mass derived from HCO+ J = 2–1 luminosity is ≳ a few × 108 M ⊙, and its depletion time is estimated to be ≳106 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 ∼2 mm data of 10 (U)LIRGs, two continuum sources are serendipitously detected within ∼10″, which are likely to be an infrared-luminous dusty galaxy at z > 1 and a blazar.