Volume 584, December 2015
|Number of page(s)||12|
|Published online||18 November 2015|
Probing highly obscured, self-absorbed galaxy nuclei with vibrationally excited HCN⋆
Department of Earth and Space SciencesChalmers University of Technology,
2 Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406St. Martin d’Hères, France
3 Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía, s/n, (IAA-CSIC), 18008 Granada, Spain
4 Universidad de Alcalá de Henares,Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
5 Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 10617 Taipei, Taiwan
6 Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
7 Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
8 Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
9 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
10 Observatoire de Paris, LERMA (CNRS:UMR8112), 61 Av. de l’Observatoire, 75014 Paris, France
11 Department of Physics and Astronomy, UCL, Gower St., London, WC1E 6BT, UK
12 Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
13 Infrared Processing and Analysis Center, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
14 NRAO, 520 Edgemont Road, Charlottesville, VA 22903, USA
15 University of Virginia, Charlottesville, VA 22904, USA
16 Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
17 Group of Molecular Astrophysics, ICMM, CSIC, C/Sor Juana Ines de La Cruz N3, 28049 Madrid, Spain
18 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
19 Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
Received: 25 April 2015
Accepted: 27 July 2015
We present high resolution (0.̋4) IRAM PdBI and ALMA mm and submm observations of the (ultra) luminous infrared galaxies ((U)LIRGs) IRAS 17208-0014, Arp220, IC 860 and Zw049.057 that reveal intense line emission from vibrationally excited (ν2 = 1) J = 3–2 and 4–3 HCN. The emission is emerging from buried, compact (r< 17–70 pc) nuclei that have very high implied mid-infrared surface brightness > 5 × 1013 L⊙ kpc-2. These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200 K) extreme starbursts. Vibrational, ν2 = 1, lines of HCN are excited by intense 14 μm mid-infrared emission and are excellent probes of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H2 column densities exceed 1024 cm-2. It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, we show strong evidence that the ground vibrational state (ν = 0), J = 3–2and 4–3 rotational lines of HCN and HCO+ fail to probe the highly enshrouded, compact nuclear regions owing to strong self- and continuum absorption. The HCN and HCO+ line profiles are double-peaked because of the absorption and show evidence of non-circular motions – possibly in the form of in- or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early-type spiral LIRGs, and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.
Key words: galaxies: evolution / galaxies: active / galaxies: nuclei / galaxies: ISM / ISM: molecules
Based on observations carried out with the IRAM Plateau de Bure and ALMA Interferometers. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). 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, 2015
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