2018ApJ...856..143I

We present the results of our ALMA observations of 11 (ultra)luminous infrared galaxies ((U)LIRGs) at J = 4-3 of HCN, HCO⁺, and HNC and J = 3-2 of HNC. This is an extension of our previously published HCN and HCO⁺ J = 3-2 observations to multiple rotational J-transitions of multiple molecules, to investigate how molecular emission line flux ratios vary at different J-transitions. We confirm that ultraluminous infrared galaxies (ULIRGs) that contain or may contain luminous obscured active galactic nuclei (AGNs) tend to show higher HCN-to-HCO⁺ flux ratios than starburst galaxies, both at J = 4-3 and J = 3-2. For selected HCN-flux-enhanced AGN-important ULIRGs, our isotopologue H¹³CN, H¹³CO⁺, and HN¹³C J = 3-2 line observations suggest a higher abundance of HCN than HCO⁺ and HNC, which is interpreted to be primarily responsible for the elevated HCN flux in AGN-important galaxies. For such sources, the intrinsic HCN-to-HCO⁺ flux ratios after line opacity correction will be higher than the observed ratios, making the separation between AGNs and starbursts even larger. The signature of the vibrationally excited (v₂ = 1f) HCN J = 4-3 emission line is seen in one ULIRG, IRAS 12112-0305 NE. P Cygni profiles are detected in the HCO⁺ J = 4-3 and J = 3-2 lines toward IRAS 15250+3609, with an estimated molecular outflow rate of ∼250-750 M_☉ yr^–1. The SiO J = 6-5 line also exhibits a P Cygni profile in IRAS 12112+0305 NE, suggesting the presence of shocked outflow activity. Shock tracers are detected in many sources, suggesting ubiquitous shock activity in the nearby ULIRG population.