1998A&A...331..959D

The large scale structure and physics of molecular gas in the Galactic center region is discussed based on the detailed 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 the 9'-beam, 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 (τ>1) or low optical depth (τ<1). Higher optical depths (τ≥10) 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 C¹⁸O measurements and from results based on dust measurements, the total molecular mass is found to be (3⁺²_–1)x10⁷M_☉. 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 ¹²CO 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 N_H2/I12^CO_ ratio is highly variable and cannot be represented by a universal constant.
Result. from external galaxies indicate that the ¹²CO emission is generally not a suitable tracer of H₂ masses in galactic bulges.