The molecular gas mass of M 33

¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
e-mail: pierre.gratier@u-bordeaux.fr
² Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’ Hères, France
³ Department of Physics, 4-181 CCIS, University of Alberta, Edmonton, AB T6G 2E1, Canada
⁴ Unidad de Astronomía, Fac. Cs. Básicas, Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile
⁵ Department of Astronomy, University of Massachusetts – Amherst, Amherst, MA 01003, USA
⁶ Observatoire de Paris, LERMA (CNRS: UMR 8112), 61 Av. de l’Observatoire, 75014 Paris, France
⁷ Instituto de Radioastronoma Milimtrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
¹¹ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
¹² Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
¹³ Instituto de Astrofísica de Canarias, via Láctea S/N, 38205 La Laguna, Spain
¹⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
¹⁵ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
¹⁶ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
¹⁷ Kapteyn Astronomical Institute, University of Groningen, The Netherlands

Received: 13 July 2016
Accepted: 25 August 2016

Abstract

Do some environments favor efficient conversion of molecular gas into stars? To answer this, we need to be able to estimate the H₂ mass. Traditionally, this is done using CO observations and a few assumptions but the Herschel observations which cover the far-IR dust spectrum make it possible to estimate the molecular gas mass independently of CO and thus to investigate whether and how the CO traces H₂. Previous attempts to derive gas masses from dust emission suffered from biases. Generally, dust surface densities, H i column densities, and CO intensities are used to derive a gas-to-dust ratio (GDR) and the local CO intensity to H₂ column density ratio (X_CO), sometimes allowing for an additional CO-dark gas component (K_dark). We tested earlier methods, revealing degeneracies among the parameters, and then used a sophisticated Bayesian formalism to derive the most likely values for each of the parameters mentioned above as a function of position in the nearby prototypical low metallicity (12 + log (O/H) ~ 8.4) spiral galaxy M 33. The data are from the IRAM Large Program mapping in the CO(2–1) line along with high-resolution H i and Herschel dust continuum observations. Solving for GDR, X_CO, and K_dark in macropixels 500 pc in size, each containing many individual measurements of the CO, H i, and dust emission, we find that (i) allowing for CO dark gas (K_dark) significantly improves fits; (ii) K_dark decreases with galactocentric distance; (iii) GDR is slightly higher than initially expected and increases with galactocentric distance; (iv) the total amount of dark gas closely follows the radially decreasing CO emission, as might be expected if the dark gas is H₂ where CO is photodissociated. The total amount of H₂, including dark gas, yields an average X_CO of twice the galactic value of 2 × 10²⁰ cm^-2/ K km s^-1, with about 55% of this traced directly through CO. The rather constant fraction of dark gas suggests that there is no large population of diffuse H₂ clouds (unrelated to GMCs) without CO emission. Unlike in large spirals, we detect no systematic radial trend in X_CO, possibly linked to the absence of a radial decrease in CO line ratios.