Volume 627, July 2019
|Number of page(s)||13|
|Published online||15 July 2019|
The hidden heart of the luminous infrared galaxy IC 860
I. A molecular inflow feeding opaque, extreme nuclear activity⋆
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Observatory, 439 92 Onsala, Sweden
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
Accepted: 14 May 2019
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 TB > 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 Td ∼ 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 TB 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 νin ≲ 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.
Key words: galaxies: evolution / galaxies: individual: IC 860 / galaxies: active / galaxies: ISM / ISM: jets and outflows / ISM: molecules
Based on observations carried out with the ALMA Interferometer. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.
© ESO 2019
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