The hidden heart of the luminous infrared galaxy IC 860

I. A molecular inflow feeding opaque, extreme nuclear activity^⋆

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Observatory, 439 92 Onsala, Sweden
e-mail: saalto@chalmers.se
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
³ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141 10617 Taipei, Taiwan
⁴ Department of Astronomy, University of Florida, PO Box 112055, USA
⁵ Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
⁶ Observatoire de Paris, LERMA (CNRS:UMR8112), 61 Av. de l’Observatoire, 75014 Paris, France
⁷ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
⁸ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁹ Department of Physics and Astronomy, UCL, Gower St., London WC1E 6BT, UK
¹⁰ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
¹¹ University of Virginia, Charlottesville, VA 22904, USA
¹² NRAO, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹³ Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ Institute of Astronomy, The University of Tokyo, Osawa, Mitaka, Tokyo 181-0015, Japan
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁸ Astron. Dept. King Abdulaziz University, PO Box 80203 Heddah 21589, Saudi Arabia

Received: 16 March 2019
Accepted: 14 May 2019

Abstract

High-resolution (0.​​″03–0.​​″09 (9–26 pc)) ALMA (100–350 GHz (λ3 to 0.8 mm)) and (0.​​″04 (11 pc)) VLA 45 GHz measurements have been used to image continuum and spectral line emission from the inner (100 pc) region of the nearby infrared luminous galaxy IC 860. We detect compact (r ∼ 10 pc), luminous, 3 to 0.8 mm continuum emission in the core of IC 860, with brightness temperatures T_B >  160 K. The 45 GHz continuum is equally compact but significantly fainter in flux. We suggest that the 3 to 0.8 mm continuum emerges from hot dust with radius r ∼ 8 pc and temperature T_d ∼ 280 K, and that it is opaque at millimetre wavelengths, implying a very large H₂ column density N(H₂)≳10²⁶ cm⁻². Vibrationally excited lines of HCN ν₂ = 1f J = 4 − 3 and 3–2 (HCN-VIB) are seen in emission and spatially resolved on scales of 40–50 pc. The line-to-continuum ratio drops towards the inner r = 4 pc, resulting in a ring-like morphology. This may be due to high opacities and matching HCN-VIB excitation- and continuum temperatures. The HCN-VIB emission reveals a north–south nuclear velocity gradient with projected rotation velocities of ν = 100 km s⁻¹ at r = 10 pc. The brightest emission is oriented perpendicular to the velocity gradient, with a peak HCN-VIB 3–2 T_B of 115 K (above the continuum). Vibrational ground-state lines of HCN 3–2 and 4–3, HC¹⁵N 4–3, HCO⁺ 3–2 and 4–3, and CS 7–6 show complex line absorption and emission features towards the dusty nucleus. Redshifted, reversed P-Cygni profiles are seen for HCN and HCO⁺ consistent with gas inflow with ν_in ≲ 50 km s⁻¹. Foreground absorption structures outline the flow, and can be traced from the north-east into the nucleus. In contrast, CS 7–6 has blueshifted line profiles with line wings extending out to −180 km s⁻¹. We suggest that a dense and slow outflow is hidden behind a foreground layer of obscuring, inflowing gas. The centre of IC 860 is in a phase of rapid evolution where an inflow is building up a massive nuclear column density of gas and dust that feeds star formation and/or AGN activity. The slow, dense outflow may be signaling the onset of feedback. The inner, r = 10 pc, IR luminosity may be powered by an AGN or a compact starburst, which then would likely require a top-heavy initial mass function.