High resolution observations of ^ { 12 } CO ( 2. ^ { \prime \prime } 3 ) , ^ { 13 } CO ( 3. ^ { \prime \prime } 9 ) , and HCN ( 5. ^ { \prime \prime } 4 ) J =1–0 in the galaxy merger Arp 299 ( IC 694 and NGC 3690 ) show the line ratios vary dramatically across the system .
The ^ { 12 } CO/ ^ { 13 } CO ratio is unusually large , 60 \pm 15 , at the IC 694 nucleus , where ^ { 12 } CO emission is very strong , and much smaller , 10 \pm 3 , in the southern extended disk of that galaxy .
Elsewhere , the ^ { 12 } CO/ ^ { 13 } CO 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 \pm 3 in the east to 20 \pm 4 in the west .

The ^ { 12 } CO/HCN line ratio also varies across Arp 299 , although to a lesser degree .
HCN emission is bright towards 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 nuclei of IC 694 and NGC 3690 the ratios are 9 \pm 1 and 14 \pm 3 respectively .
In the western part of the overlap region it is 11 \pm 3 .

The large ^ { 12 } CO/ ^ { 13 } CO 1–0 intensity ratio at the nucleus of IC 694 can primarily be attributed to a low to moderate optical depth ( \tau { { } _ { < } \atop { { } ^ { \sim } } } 1 ) in the ^ { 12 } CO 1–0 line .
These data support the hypothesis that unusually high ^ { 12 } CO/ ^ { 13 } CO line ratios ( > 20 ) are associated with extremely compact molecular distributions in the nuclei of merging galaxies .
Relative to ^ { 12 } CO , ^ { 13 } CO 1–0 is brightest in quiescent regions of low ^ { 12 } CO surface brightness and weakest in starburst regions and the galactic nuclei .
A medium consisting of dense ( n = 10 ^ { 4 } -10 ^ { 5 } { cm ^ { -3 } } ) and warm ( T _ { k } > 50 K ) gas will reproduce the extreme line ratios observed in the nucleus of IC 694 , where the area filling factor must be at least 20 % .