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

Herschel/PACS [C II] and [O I] observations^⋆,⋆⋆

¹ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
e-mail: kramer@iram.fr
² IRAM, Av. Divina Pastora 7, 18012 Granada, Spain
³ Cornell University, Ithaca, NY 14852, USA
⁴ Frankfurter Allgemeine Zeitung, Hellerhofstraße 2-4, 60327 Frankfurt am Main, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Unidad de Astronomía, Universidad de Antofagasta, Av. Angamos 601, Antofagasta 1270300, Chile
⁸ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Georoy Saint-Hilaire, 33615 Pessac, France
⁹ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹⁰ Observatoire de Paris, LERMA, College de France, CNRS, PSL Univ., Sorbonne University, UPMC, Paris, France
¹¹ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹² Department of Astronomy, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
¹³ Instituto de Astrofísica de Andalucía (IAA-CSIC), CAHA, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Dept. Física Teórica y del Cosmos, Universidad de Granada, 18012 Granada, Spain
¹⁶ Institute for Research in Fundamental Sciences-IPM, Larak Garden, 19395-5531 Tehran, Iran
¹⁷ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁸ Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
¹⁹ Instituto de Astrofísica de Canarias, Vía L’actea S/N, 38205 La Laguna, Spain
²⁰ Instituto Universitario Carlos I de Física Teórica y Computacional, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
²¹ Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, P. Penteli, 15236 Athens, Greece

Received: 22 September 2019
Accepted: 5 May 2020

Abstract

Context. M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular clouds.

Aims. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum.

Methods. We mapped the emission of gas and dust in M 33 using the far-infrared lines of [C II] and [O I](63 μm) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the Herschel Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H II regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24 μm from Spitzer/MIPS, and at 70 μm, 100 μm, and 160 μm from Herschel/PACS were combined to obtain a map of the TIR.

Results. TIR and [C II] intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G_0, obs ∼ 2 to 200 in units of the average Galactic radiation field. The binned [C II]/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the [C II] emission, as estimated from VLA H I data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the [O I] data before comparing it with models of photon dominated regions. Most of the ratios of [C II]/[O I] and ([C II]+[O I])/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n ∼ 2 × 10² cm⁻³, G₀ ∼ 60, and a second low-FUV solution at n ∼ 10⁴ cm⁻³, G₀ ∼ 1.5.

Conclusions. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.