Multimolecule ALMA observations toward the Seyfert 1 galaxy NGC 1097

¹ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Dom. Univ., 38406 St Martin d’Hères, France
e-mail: smartin@iram.fr
² Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, 181-0015 Tokyo, Japan
³ Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo, 113-0033 Tokyo, Japan
⁴ New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM, USA
⁵ National Radio Astronomy Observatory, Pete V. Domenici Array Science Center, PO Box O, Socorro, NM, 87801, USA
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago 19, Chile
⁷ Academia Sinica, Institute of Astronomy & Astrophysics, PO Box 23-141, 10617 Taipei, Taiwan
⁸ Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, 384-1305 Nagano, Japan
⁹ Department of Astronomical Science, The Graduate University for Advanced Studies (Sokendai), Nobeyama, Minamimaki, Minamisaku, 384-1305 Nagano, Japan
¹⁰ Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
¹¹ Joint ALMA Observatory (JAO), 3107 Alonso de Córdova, Vitacura, Santiago, Chile
¹² National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, 181-8588, Tokyo, Japan
¹³ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁴ The Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601 Aichi, Japan
¹⁵ Department of Physics, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Ehime, Japan
¹⁶ Stockholm Observatory, Department of Astronomy, Stockholm University, AlbaNova Centre, 106 91 Stockholm, Sweden
¹⁷ Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, 106 91 Stockholm, Sweden
¹⁸ Institute of Astronomy, National Central University, No. 300, Jhongda Road, Jhongli City, 32001 Taoyuan County, Taiwan
¹⁹ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA
²⁰ Division of Physics, Faculty of Pure and Applied Science, University of Tsukuba, Tsukuba, 305-8571 Ibaraki, Japan
²¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA

Received: 3 October 2014
Accepted: 10 October 2014

Abstract

Context. The nearby Sy 1 galaxy NGC 1097 represents an ideal laboratory for exploring the molecular chemistry in the surroundings of an active galactic nucleus (AGN).

Aims. Exploring the distribution of different molecular species allows us to understand the physical processes affecting the interstellar medium both in the AGN vicinity and in the outer star forming molecular ring.

Methods. We carried out 3 mm ALMA observations that include seven different molecular species, namely HCN, HCO⁺, CCH, CS, HNCO, SiO, HC₃N, and SO, as well as the ¹³C isotopologues of the first two. Spectra were extracted from selected positions and all species were imaged over the central 2 kpc (~30′′) of the galaxy at a resolution of ~2.2′′ × 1.5′′ (150 pc × 100 pc).

Results. HCO⁺ and CS appear to be slightly enhanced in the star forming ring. CCH shows the largest variations across NGC 1097 and is suggested to be a good tracer of both obscured and early stage star formation. HNCO, SiO, and HC₃N are significantly enhanced in the inner circumnuclear disk surrounding the AGN.

Conclusions. Differences in the molecular abundances are observed between the star forming ring and the inner circumnuclear disk. We conclude that the HCN/HCO⁺ and HCN/CS differences observed between AGN-dominated and starburst (SB) galaxies are not due to a HCN enhancement due to X-rays, but rather this enhancement is produced by shocked material at distances of 200 pc from the AGN. Additionally, we claim that lower HCN/CS is a combination of a small underabundance of CS in AGNs, together with excitation effects, where a high density gas component (~10⁶ cm^-3) may be more prominent in SB galaxies. However, the most promising are the differences found among the dense gas tracers that, at our modest spatial resolution, seem to outline the physical structure of the molecular disk around the AGN. In this picture, HNCO probes the well-shielded gas in the disk, surrounding the dense material moderately exposed to the X-ray radiation traced by HC₃N. Finally SiO might be the innermost molecule in the disk structure.