The Opaque Heart of the Galaxy IC 860: Analogous Protostellar, Kinematics, Morphology, and Chemistry

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. Here we apply principal component analysis (PCA) tomography to high-resolution (0.06${}^{\prime \prime }$) 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 (CH${}_2$NH), a precursor to glycine, three transitions of which have been detected towards IC 860. We estimate the average CH${}_2$NH column density towards the nucleus of IC~860 to be $\sim 10^{17}$cm${}^{-2}$, with an abundance exceeding $10^{-8}$ relative to molecular hydrogen, using the rotation diagram method and non-LTE radiative transfer models. This CH${}_2$NH 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.

arXiv comment17 pages, 14 figures, submitted to Astronomy and Astrophysics

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