Dense Molecular Gas Properties of the Central Kiloparsec of Nearby Ultraluminous Infrared Galaxies Constrained by ALMA Three Transition-line Observations

We report the results of Atacama Large Millimeter/submillimeter Array (ALMA) 1-2 kpc resolution, three rotational transition-line (J = 2-1, J = 3-2, and J = 4-3) observations of multiple dense molecular gas tracers (HCN, HCO+, and HNC) for 10 nearby (ultra)luminous infrared galaxies ((U)LIRGs). Following the matching of beam sizes to 1-2 kpc for each (U)LIRG, the high-J-to-low-J transition-line flux ratios of each molecule and the emission-line flux ratios of different molecules at each J transition are derived. We conduct RADEX non-LTE model calculations and find that, under a wide range of gas density and kinetic temperature, the observed HCN-to-HCO+ flux ratios in the overall (U)LIRGs are naturally reproduced with enhanced HCN abundance compared to HCO+. Thereafter, molecular gas properties are constrained primarily through the use of HCN and HCO+ data and the adoption of fiducial values for the HCO+ column density and HCN-to-HCO+ abundance ratio. We quantitatively confirm the following: (i) molecular gas at the (U)LIRGs' nuclei is dense (greater than or similar to 10(3-4) cm(-3)) and warm (greater than or similar to 100 K), (ii) the molecular gas density and temperature in nine ULIRGs' nuclei are significantly higher than those of one LIRG's nucleus, (iii) molecular gas in starburst-dominated sources tends to be less dense and cooler than ULIRGs with luminous AGN signatures. For six selected sources, we also apply a Bayesian approach by freeing all parameters and support the above main results. Our ALMA 1-2 kpc resolution, multiple transition-line data of multiple molecules are a very powerful tool for scrutinizing the properties of molecular gas concentrated around luminous energy sources in nearby (U)LIRGs' nuclei.