VARIATION OF MOLECULAR LINE RATIOS AND CLOUD PROPERTIES IN THE ARP-299 GALAXY MERGER

High-resolution observations of (CO)-C-12 (2 ''.3), (CO)-C-13 (3 ''.9) , and HCN (5 ''.4)J = 1-0 in the galaxy merger Arp 299 (IC 694 and NGC 3690) show that the line ratios vary dramatically across the system. The (CO)-C-12/(CO)-C-13 ratio is unusually large, 60 +/- 15, at the IC 694 nucleus, where (CO)-C-12 emission is very strong, and much smalle r, 10 +/- 3, in the southern extended disk of that galaxy. Elsewhere, the (CO)-C-12/(CO)-C-13 line ratio is 5-20, typical of spiral galaxies . The line ratio variation in the overlap between the two galaxies is smaller, ranging from 10 +/- 3 in the east to 20 +/- 4 in the west. Th e (CO)-C-12/HCN line ratio also varies across Arp 299, although to a l esser degree. HCN emission is bright toward each galaxy nucleus and in the extranuclear region of active star formation; it was not detected in the IC 694 disk or the eastern part of the overlap region, leading to lower limits of 25 and 20, respectively. By contrast, at the nucle i of IC 694 and NGC 3690 the ratios are 9 +/- 1 and 14 +/- 3, respecti vely. In the western part of the overlap region it is 11 +/- 3. The la rge (CO)-C-12/(CO)-C-13 1-0 intensity ratio at the nucleus of IC 694 c an primarily be attributed to a low-to-moderate optical depth (T less than or similar to 1) in the (CO)-C-12 1-0 line. These data support th e hypothesis that unusually high (CO)-C-12/(CO)-C-13 line ratios (> 20 ) are associated with extremely compact molecular distributions in the nuclei of merging galaxies. Relative to (CO)-C-12, (CO)-C-13 1-0 is b rightest in quiescent regions of low (CO)-C-12 surface brightness and weakest in starburst regions and the galactic nuclei. A medium consist ing of dense (n = 10(4)-10(5) cm(-3)) and warm (T-k > 50 K) gas will r eproduce the extreme line ratios observed in the nucleus of IC 694, wh ere the area filling factor must be at least 20%.