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Astron. Astrophys. 331, 959-976 (1998)


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Molecular gas in the Galactic center region

II. Gas mass and [FORMULA]H2 / I12CO conversion based on a C18 O([FORMULA]) survey

G. Dahmen 1, 3, S. Hüttemeister 1, 2, 4, T.L. Wilson 1, 5 and R. Mauersberger 1, 6

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Physics Department, Queen Mary & Westfield College, University of London, Mile End Road, London E1 4NS, UK
4 Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
5 Submillimeter Telescope Observatory, The University of Arizona, Tucson, AZ 85721, USA
6 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA

Received 8 July 1997 / Accepted 7 November 1997

Abstract

The large scale structure and physics of molecular gas in the Galactic center region is discussed based on the detailed analysis of a [FORMULA] resolution survey of the Galactic center region in the [FORMULA] line of C18 O. Emphasis is placed on the comparison with 12 CO(1-0) data. The line shapes of C18 O(1-0) and 12 CO(1-0) differ significantly. The ratio of the intensities of the two isotopomers in the Galactic center region is generally higher than the value of [FORMULA] 15 expected from the "Standard Conversion Factor" (SCF) of 12 CO integrated line intensity to H2 column density. In the [FORMULA] -beam, this ratio is in the range from 30 to 200, mostly [FORMULA] 60 to 80. From LVG calculations, we estimate that the large scale 12 CO(1-0) emission in the Galactic center region is of moderate ([FORMULA] [FORMULA] 1) or low optical depth ([FORMULA]). Higher optical depths ([FORMULA]) are restricted to very limited regions such as Sgr B2. In addition, we estimate H2 densities and kinetic temperatures for different ranges of intensity ratios. A considerable amount of molecular mass is in a widespread molecular gas component with low densities and high kinetic temperatures. From our C18 O measurements and from results based on dust measurements, the total molecular mass is found to be [FORMULA]. We show that the SCF is not valid toward the Galactic bulge. It overestimates the H2 column density by an order of magnitude because the assumptions required for this factor of optically thick 12 CO emission and virialization of the molecular clouds are not fulfilled for a significant fraction of the molecular gas. Therefore, also one cannot apply a modified conversion factor to the Galactic center region since the [FORMULA] ratio is highly variable and cannot be represented by a universal constant. Results from external galaxies indicate that the 12 CO emission is generally not a suitable tracer of H2 masses in galactic bulges.

Key words: Galaxy: center – radiative transfer – ISM: molecules – ISM: structure – galaxies: nuclei – radio lines: ISM

Send offprint requests to: S. Hüttemeister, Radioastronomisches Institut, Universität Bonn

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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