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

Abstract

We present the results of our ALMA 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.

Planck’s Dusty GEMS

“…For instance, Tan et al (2018) obtained lower ratios for galaxies with strong AGN contributions in their sample of six nearby starforming galaxies. Using 500-pc resolution ALMA line interferometry of nearby ULIRGs, Imanishi et al (2019) found a 15-20% decrease of HCO + /HCN J = 3-2 ratios over compact A45, page 13 of 18 regions located mainly in the central nuclear cores, and systematically lower integrated HCO + /HCN ratios 1 for those galaxies showing hints of strong dust-obscured AGN. Generally, HCO + /HCN 1 therefore seems to indicate AGN-dominated systems (in agreement with ratios of García-Burillo et al 2014, integrated over large radii) 9 .…”

“…Observational studies of the dense-gas reservoirs in lowredshift star-forming galaxies started about three decades ago (e.g., Nguyen et al 1992), followed by extended surveys of HCN line emission in luminous and ultra-luminous infrared galaxies (ULIRGs) and normal spirals (Gao & Solomon 2004a,b). Since then, HCN and HCO + detections have been extensively used to characterize the dense interstellar medium (ISM) of local (U)LIRGs (e.g., Graciá-Carpio et al 2008;Juneau et al 2009;García-Burillo et al 2012), including diagnostics for the physical conditions of the gas from line excitation modeling (see, e.g., Papadopoulos et al 2014, for NGC 6240 and Arp 193) and spatially-resolved studies with the NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA) interferometers (Chen et al 2017;König et al 2018;Imanishi et al 2019). The reliability of each molecule as a proxy of the overall dense-gas content was discussed in Papadopoulos (2007).…”

Subaru Infrared Adaptive Optics-assisted High-spatial-resolution Imaging Search for Luminous Dual Active Galactic Nuclei in Nearby Ultraluminous Infrared Galaxies

“…However, ALMA (sub)millimeter observations reveal that the NE nucleus of IRAS 12112+0305 is brighter than the SW nucleus in the ∼0.9 mm and ∼1.2 mm continuum as well as dense molecular rotational J-transition lines (i.e., HCN and HCO + ) (Imanishi et al 2016b(Imanishi et al , 2018(Imanishi et al , 2019. The infrared fainter, but (sub)millimeter brighter, NE nucleus shows (1) (sub)millimeter dense molecular rotational Jtransition line flux ratios (i.e., HCN and HCO + ) often seen in luminous AGNs and (2) signatures of fairly strong vibrationally excited HCN J=3-2 and J=4-3 emission lines (HCN-VIB) at ∼1.2 mm and ∼0.9 mm, respectively (Imanishi et al 2016b(Imanishi et al , 2018(Imanishi et al , 2019, which are naturally explained by mid-infrared (∼14 µm) radiative pumping by AGN-heated hot dust emission (Sakamoto et al 2010;Aalto et al 2015;Imanishi et al 2016aImanishi et al , 2017. The stronger (sub)millimeter continuum and brighter molecular emission lines of plausible AGN origin suggest that a (sub)millimeter-detectable buried AGN with higher intrinsic luminosity may be present in the infrared fainter NE nucleus of IRAS 12112+0305.…”

Deeply Buried Nuclei in the Infrared-Luminous Galaxies NGC 4418 and Arp 220: I. ALMA Observations at $λ= $1.4-0.4 mm and Continuum Analysis

“…At the same time, the continuum emission carries information about the dust opacity and hence the obscuring column density. Fourth, rotational lines from vibrationally excited HCN at sub/millimeter wavelengths can be an indicator and probe of the NGC 4418/Arp 220-type nuclei with significant obscuration (Sakamoto et al 2009;Aalto et al 2015Aalto et al , 2019Imanishi et al 2019). These nuclei are opaque enough to trap infrared photons to increase their inner temperature (i.e., the greenhouse effect), and the enhanced infrared radiation vibrationally excites molecules such as HCN (González-Alfonso & Sakamoto 2019).…”