Arp 302 , a luminous infrared source ( L _ { IR } = 4.2 \times 10 ^ { 11 } ~ { } \hbox { L$ { } _ { \odot } $ } ) , consisting of two spiral galaxies ( VV340A and VV340B ) with nuclear separation of \sim 40 ^ { \prime \prime } , has the highest CO luminosity known .
Observations with the BIMA array at 5 \arcsec \times 7 \arcsec resolution reveal that the CO emission is extended over 23.0 kpc in the edge-on spiral galaxy , VV340A , corresponding to 6.7 \times 10 ^ { 10 } ~ { } \hbox { M$ { } _ { \odot } $ } of H _ { 2 } .
In the companion face-on galaxy , VV340B , the CO emission is extended over \sim 10.0 kpc , with 1.1 \times 10 ^ { 10 } ~ { } \hbox { M$ { } _ { \odot } $ } of H _ { 2 } .
The large CO extent is in strong contrast to starburst systems , such as Arp 220 , in which the CO extent is typically \leq 1 kpc .
Furthermore , L _ { IR } / M ( H _ { 2 } ) is found to be \buildrel < \over { \sim } 6.0 L _ { \odot } /M _ { \odot } throughout both galaxies .
Thus the high IR luminosity of Arp 302 is apparently not due to starbursts in the nuclear regions , but is due to its unusually large amount of molecular gas forming stars at a rate similar to giant molecular clouds in the Milky Way disk .
Arp 302 consists of a pair of very gas-rich spiral galaxies that may be interacting and in a phase before a likely onset of starbursts .