Ultraluminous infrared galaxies (ULIRGs), powered by starburst activity and often with supermassive black holes accreting material at their nuclei, contain large reservoirs of molecular gas. This is to be expected: Molecular gas is the raw material for new stars and moreover the presence of the infrared luminous warm dust implies an abundance of molecular gas. Galaxy collisions often trigger star formation activity and simulations reveal that as the two galaxies merge their gas tends to fall towards the nuclear region where it develops into a disk with a radius of roughly 1500 light-years. Many such galaxies are observed to have strong circumnuclear starbursts, apparently as a result. Observations of the carbon monoxide gas (CO) in ULIRGs, an abundant but low density molecular species, have indeed found evidence for circumnuclear disks in the broad range of velocities the gas displays, characteristic of rotating disks. However, astronomers know that star formation requires the presence of gas that is 10-100 times denser than that traced by CO; they are unsure about the distribution of denser material, and also the role that the active nucleus might play in shaping the disk.
