The opaque heart of the galaxy IC 860: Analogous protostellar, kinematics, morphology, and chemistry

¹ Department of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
e-mail: mark.gorski@chalmers.se
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 7 October 2022
Accepted: 14 December 2022

Abstract

Compact Obscured Nuclei (CONs) account for a significant fraction of the population of luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These galaxy nuclei are compact, with radii of 10–100 pc, with large optical depths at submm and far-infrared wavelengths, and characterized by vibrationally excited HCN emission. It is not known what powers the large luminosities of the CON host galaxies because of the extreme optical depths towards their nuclei. CONs represent an extreme phase of nuclear growth, hiding either a rapidly accreting supermassive black hole or an abnormal mode of star formation. Regardless of their power source, the CONs allow us to investigate the processes of nuclear growth in galaxies. Here we apply principal component analysis (PCA) tomography to high-resolution (0″​​.06) ALMA observations at frequencies 245 to 265 GHz of the nearby CON (59 Mpc) IC 860. PCA is a technique to unveil correlation in the data parameter space, and we apply it to explore the morphological and chemical properties of species in our dataset. The leading principal components reveal morphological features in molecular emission that suggest a rotating, infalling disk or envelope, and an outflow analogous to those seen in Galactic protostars. One particular molecule of astrochemical interest is methanimine (CH₂NH), a precursor to glycine, three transitions of which have been detected towards IC 860. We estimate the average CH₂NH column density towards the nucleus of IC 860 to be ∼10¹⁷cm⁻², with an abundance exceeding 10⁻⁸ relative to molecular hydrogen, using the rotation diagram method and non-LTE radiative transfer models. This CH₂NH abundance is consistent with those found in hot cores of molecular clouds in the Milky Way. Our analysis suggests that CONs are an important stage of chemical evolution in galaxies, that are chemically and morphologically similar to Milky Way hot cores.