Probing highly obscured, self-absorbed galaxy nuclei with vibrationally excited HCN^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
e-mail: saalto@chalmers.se
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

Abstract

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.