2022ApJ...926..159I

We present the results of ALMA ∼2 mm, <=1''-resolution observations of 10 (ultra)luminous infrared galaxies ([U]LIRGs; infrared luminosity >=10^11.7 L _☉) at z < 0.15, targeting dense (>10⁴ cm^–3) molecular (HCN, HCO⁺, and HNC J = 2-1) and 183 GHz H₂O 3_1,3-2_2,0 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 x 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.