ALMA Spatially Resolved Dense Molecular Gas Survey of Nearby Ultraluminous Infrared Galaxies

Abstract

We present the results of our Atacama Large Millimeter Array HCN J  = 3–2 and HCO + J  = 3–2 line observations of a uniformly selected sample (>25) of nearby ultraluminous infrared galaxies (ULIRGs) at z  < 0.15. The emission of these dense molecular gas tracers and continuum are spatially resolved in the majority of observed ULIRGs for the first time with achieved synthesized beam sizes of ≲0.″2 or ≲500 pc. In most ULIRGs, the HCN-to-HCO + J  = 3–2 flux ratios in the nuclear regions within the beam size are systematically higher than those in the spatially extended regions. The elevated nuclear HCN J  = 3–2 emission could be related to (a) luminous buried active galactic nuclei, (b) the high molecular gas density and temperature in ULIRG’s nuclei, and/or (c) mechanical heating by spatially compact nuclear outflows. A small fraction of the observed ULIRGs display higher HCN-to-HCO + J  = 3–2 flux ratios in localized off-nuclear regions than those of the nuclei, which may be due to mechanical heating by spatially extended outflows. The observed nearby ULIRGs are generally rich in dense (>10 5 cm −3 ) molecular gas, with an estimated mass of >10 9 M ⊙ within the nuclear (a few kpc) regions, and dense gas can dominate the total molecular mass there. We find a low detection rate (<20%) regarding the possible signature of a vibrationally excited ( v 2  = 1f) HCN J  = 3–2 emission line in the vicinity of the bright HCO + J  = 3–2 line that may be due, in part, to the large molecular line widths of ULIRGs.