MOLECULAR GAS IN THE GALACTIC-CENTER REGION - II - GAS MASS AND N-H2 I-12(CO) CONVERSION BASED ON A (CO)-O-18(J=1-]0) SURVEY/

The large scale structure and physics of molecular gas in the Galactic center region is discussed based on the detailed analysis of a 9' res olution survey of the Galactic center region in the J = 1 --> 0 line o f (CO)-O-18. Emphasis is placed on the comparison with (CO)-C-12(1-0) data. The line shapes of (CO)-O-18(1-0) and (CO)-C-12(1-0) differ sign ificantly. The ratio of the intensities of the two isotopomers in the Galactic center region is generally higher than the value of similar t o 15 expected from the ''Standard Conversion Factor'' (SCF) of (CO)-C- 12 integrated line intensity to H-2 column density. In the 9'-beam, th is ratio is in the range from 30 to 200, mostly similar to 60 to 80. F rom LVG calculations, we estimate that the large scale (CO)-C-12(1-0) emission in the Galactic center region is of moderate (tau greater tha n or similar to 1) or low optical depth (tau < 1). Higher optical dept hs (tau greater than or equal to 10) are restricted to very limited re gions such as SgrB2. In addition, we estimate H-2 densities and kineti c temperatures for different ranges of intensity ratios. A considerabl e amount of molecular mass is in a widespread molecular gas component with low densities and high kinetic temperatures. From our (CO)-O-18 m easurements and from results based on dust measurements, the total mol ecular mass is found to be (3(-1)(+2)) . 10(7) M.. We show that the SC F is not valid toward the Galactic bulge. It overestimates the H-2 col umn density by an order of magnitude because the assumptions required for this factor of optically thick (CO)-C-12 emission and virializatio n of the molecular clouds are not fulfilled for a significant fraction of the molecular gas. Therefore, also one cannot apply a modified con version factor to the Galactic center region since the N-H2/I-12CO rat io is highly variable and cannot be represented by a universal constan t. Results from external galaxies indicate that the (CO)-C-12 emission is generally not a suitable tracer of H-2 masses in galactic bulges.