1997/11/11 by G. Dahmen, Dahmen, G., S. Huettemeister +5
Chemistry · Engineering · Physics and Astronomy · #Astrophysics (astro-ph) #Calibration and Measurement Techniques #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Spectroscopy and Laser Applications #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9711117
20 pages, 8 postscript figures; accepted for publication in A&A Main Journal; uses laa.sty and psfig_laa.sty
arxiv created 1997/11/11 · openalex publication_date 1997/11/11 · arxiv updated 2009/12/01 · openalex created_date 2024/04/11 · openalex updated_date 2026/07/28
The large scale structure and physics of molecular gas in the Galactic center region is discussed based on the analysis of a 9' resolution survey of the Galactic center region in the J=1->0 line of C18O. Emphasis is placed on the comparison with 12CO(1--0) data. The line shapes of C18O(1--0) and 12CO(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 ~15 expected from the ``Standard Conversion Factor'' (SCF) of 12CO integrated line intensity to H2 column density. In our data, this ratio is in the range from 30 to 200, mostly ~60 to 80. From LVG calculations, we estimate that the large scale 12CO(1--0) emission in the Galactic center region is of moderate or low optical depth. Higher optical depths are restricted to very limited regions. 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. The total molecular mass is found to be (3+2-1) 107 Msun. 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 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 N(H2)/I(12CO) ratio is highly variable and cannot be represented by a universal constant. Results from external galaxies indicate that the 12CO emission is generally not a suitable tracer of H2 masses in galactic bulges.