Volume 600, April 2017
|Number of page(s)||14|
|Published online||23 March 2017|
The molecular gas mass of M 33
1 Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
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
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 (XCO), sometimes allowing for an additional CO-dark gas component (Kdark). 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, XCO, and Kdark 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 (Kdark) significantly improves fits; (ii) Kdark 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 XCO 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 XCO, possibly linked to the absence of a radial decrease in CO line ratios.
Key words: ISM: general / galaxies: individual: M 33 / submillimeter: ISM / radio lines: ISM / Local Group / ISM: structure
© ESO, 2017
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.