Herschel spectroscopic observations of the compact obscured nucleus in Zw 049.057^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: niklas.falstad@chalmers.se
² Universidad de Alcalá de Henares, Departamento de Física, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington, DC 20375, USA
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 17 March 2015
Accepted: 21 May 2015

Abstract

Context. The luminous infrared galaxy Zw 049.057 contains a compact obscured nucleus where a considerable amount of the galaxy’s luminosity is generated. This nucleus contains a dusty environment that is rich in molecular gas. One approach to probing this kind of environment and to revealing what is hidden behind the dust is to study the rotational lines of molecules that couple well with the infrared radiation emitted by the dust.

Aims. We probe the physical conditions in the core of Zw 049.057 and establish the nature of its nuclear power source (starburst or active galactic nucleus).

Methods. We observed Zw 049.057 with the Photodetector Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE) onboard the Herschel Space Observatory in rotational lines of H₂O, H₂¹⁸O, OH, ¹⁸OH, and [O I]. We modeled the unresolved core of the galaxy using a spherically symmetric radiative transfer code. To account for the different excitation requirements of the various molecular transitions, we use multiple components and different physical conditions.

Results. We present the full high-resolution SPIRE FTS spectrum of Zw 049.057, along with relevant spectral scans in the PACS range. We find that a minimum of two different components (nuclear and extended) are required in order to account for the rich molecular line spectrum of Zw 049.057. The nuclear component has a radius of 10−30 pc, a very high infrared surface brightness (~10¹⁴L_⊙kpc^-2), warm dust (T_d > 100 K), and a very large H₂ column density (N_H2 = 10²⁴−10²⁵ cm^-2). The modeling also indicates high nuclear H₂O (~5 × 10^-6) and OH (~4 × 10^-6) abundances relative to H₂ as well as a low ¹⁶O/¹⁸O-ratio of 50−100. We also find a prominent infall signature in the [O I] line. We tentatively detect a 500 km s^-1 outflow in the H₂O 3₁₃ → 2₀₂ line.

Conclusions. The high surface brightness of the core indicates the presence of either a buried active galactic nucleus or a very dense nuclear starburst. The estimated column density towards the core of Zw 049.057 indicates that it is Compton-thick, making a buried X-ray source difficult to detect even in hard X-rays. We discuss the elevated H₂O abundance in the nucleus in the context of warm grain and gas-phase chemistry. The H₂O abundance is comparable to that of other compact (ultra-)luminous infrared galaxies such as NGC 4418 and Arp 220 – and also to hot cores in the Milky Way. The enhancement of ¹⁸O is a possible indicator that the nucleus of Zw 049.057 is in a similar evolutionary stage as the nuclei of Arp 220 – and more advanced than NGC 4418. We discuss the origin of the extreme nuclear gas concentration and note that the infalling gas detected in [O I] implies that the gas reservoir in the central region of Zw 049.057 is being replenished. If confirmed, the H₂O outflow suggests that the nucleus is in a stage of rapid evolution.