Universität zu Köln

Gratier, P., Braine, J., Schuster, K., Rosolowsky, E., Boquien, M., Calzetti, D., Combes, F., Kramer, C., Henkel, C., Herpin, F., Israel, F., Koribalski, B. S., Mookerjea, B., Tabatabaei, F. S., Roellig, M., van der Tak, F. F. S., van der Werf, P. and Wiedner, M. (2017). The molecular gas mass of M 33. Astron. Astrophys., 600. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Do some environments favor efficient conversion of molecular gas into stars? To answer this, we need to be able to estimate the H-2 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 H-2. Previous attempts to derive gas masses from dust emission suffered from biases. Generally, dust surface densities, HI column densities, and CO intensities are used to derive a gas-to-dust ratio (GDR) and the local CO intensity to H-2 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) similar to 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 HI 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, HI, and dust emission, we find that (i) allowing for CO dark gas (K-dark) 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 H-2 where CO is photodissociated. The total amount of H-2, including dark gas, yields an average XCO of twice the galactic value of 2 x 10(20) cm(-2)/Kkm 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 H-2 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.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Gratier, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Braine, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Schuster, K. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rosolowsky, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Boquien, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Calzetti, D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Combes, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Kramer, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Henkel, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Herpin, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Israel, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Koribalski, B. S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Mookerjea, B. UNSPECIFIED UNSPECIFIED UNSPECIFIED Tabatabaei, F. S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Roellig, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED van der Tak, F. F. S. UNSPECIFIED UNSPECIFIED UNSPECIFIED van der Werf, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Wiedner, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-234657
DOI:	10.1051/0004-6361/201629300
Journal or Publication Title:	Astron. Astrophys.
Volume:	600
Date:	2017
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language CO-TO-H-2 CONVERSION FACTOR; STAR-FORMATION; SUBMILLIMETER EXCESS; DUST EMISSION; HERSCHEL; M33; CLOUDS; POPULATION; RESERVOIR; GALAXIES Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/23465