ALMA 0.5 kpc Resolution Spatially Resolved Investigations of Nuclear Dense Molecular Gas Properties in Nearby Ultraluminous Infrared Galaxies Based on HCN and HCO+ Three Transition Line Data

Abstract

We present the results of our ALMA ≲0.5 kpc resolution dense molecular line (HCN and HCO+ J = 2–1, J = 3–2, and J = 4–3) observations of 12 nearby (ultra)luminous infrared galaxies ([U]LIRGs). After matching beam sizes of all molecular line data to the same values in all (U)LIRGs, we derive molecular line flux ratios by extracting spectra in the central 0.5, 1, and 2 kpc circular regions and in 0.5–1 and 1–2 kpc annular regions. Based on non–local thermal equilibrium model calculations, we quantitatively confirm that the innermost (≲0.5 kpc) molecular gas is very dense (≳105 cm−3) and warm (≳300 K) in ULIRGs, and that in one LIRG, it is also modestly dense (104–5 cm−3) and warm (∼100 K). We then investigate the spatial variation of the HCN-to-HCO+ flux ratios and high-J to low-J flux ratios of HCN and HCO+. A subtle sign of a decreasing trend in these ratios from the innermost (≲0.5 kpc) to the outer nuclear (0.5–2 kpc) region is discernible in a significant fraction of the observed ULIRGs. For two ULIRGS hosting an active galactic nucleus (AGN), which display the trend most clearly, we find based on a Bayesian approach that the HCN-to-HCO+ abundance ratio and gas kinetic temperature systematically increase from the outer nuclear to the innermost region. We suggest that this trend comes from potential AGN effects because no such spatial variation is found in a starburst-dominated LIRG.