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 (≳10 3–4 cm −3 ) and warm (≳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.