ARP 302: Nonstarburst Luminous Infrared Galaxies

Arp 302, a luminous infrared source (L_IR = 4.2 × 10¹¹ L_solar) consisting of two spiral galaxies (VV 340A and 340B) with nuclear separation of ~40", has the highest CO luminosity known. Observations with the Berkeley-Illinois-Maryland Association array at 5" × 7" resolution reveal that the CO emission is extended over 23.0 kpc in the edge-on spiral galaxy, VV 340A, corresponding to 6.7 × 10¹⁰ M_solar of H₂. In the companion face-on galaxy, VV 340B, the CO emission is extended over ~10.0 kpc, with 1.1 × 10¹⁰ M_solar of H₂. The large CO extent is in strong contrast to starburst systems, such as Arp 220, in which the CO extent is typically <=1 kpc. Furthermore, L_IR/M(H₂) is found to be <~6.0 L_solar/M_solar throughout both galaxies. Thus the high IR luminosity of Arp 302 is apparently not due to starbursts in the nuclear regions but to its unusually large amount of molecular gas, forming stars at a rate similar to giant molecular clouds in the Milky Way's 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.