Gas and dust cooling along the major axis of M 33 (HerM33es)

ISO/LWS [C ii] observations^⋆,⋆⋆

¹ Instituto Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
e-mail: kramer@iram.es
² Observatorio Astronómico Nacional (OAN) − Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
³ Departamento de Fisica Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁴ Department of Radio and Space Science, Onsala Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
⁵ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁶ Laboratoire d’Astrophysique de Bordeaux, Observatoire de Bordeaux, 33270 Floirac, France
⁷ IRAM, 300 rue de la Piscine, 38406 Saint-Martin d’Hères, France
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
¹⁰ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹¹ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, 15236 Athens, Greece

Received: 1 November 2012
Accepted: 29 March 2013

Abstract

Aims. We aim to better understand the heating of gas by observing the prominent gas cooling line [C ii] at 158 μm in the low-metallicity environment of the Local Group spiral galaxy M 33 on scales of 280 pc. In particular, we describe the variation of the photoelectric heating efficiency with the galactic environment.

Methods. In this study, we present [C ii] observations along the major axis of M 33 using the Infrared Space Observatory in combination with Herschel continuum maps, IRAM 30 m CO 2−1, and VLA H i data to study the variation in velocity integrated intensities. The ratio of [C ii] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, far-ultraviolet radiation fields, and average column densities of the molecular clouds.

Results. The heating efficiency stays constant at 0.8% in the inner 4.5 kpc radius of the galaxy, where it increases to reach values of ~3% in the outskirts at about a 6 kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields for optical extinctions of only a few magnitudes at constant metallicity. For the significant fraction of H i emission stemming from PDRs and for typical pressures found in the Galactic cold neutral medium (CNM) traced by H i emission, the CNM contributes ~15% to the observed [C ii] emission in the inner 2 kpc radius of M 33. The CNM contribution remains largely undetermined in the south, while positions between radial distances of 2 and 7.3 kpc in the north of M 33 show a contribution of ~40% ± 20%.