Ultraluminous infrared galaxies have luminosities that exceed a trillion suns. (For comparison, the Milky Way's luminosity is only that of about ten billion suns.) Extreme infrared activity is known to be associated with interacting galaxies, and optical imaging indeed shows that many ultraluminous systems are in collision. The physical mechanism(s) that actually power the luminosity, however, are still not understood. Might the same process(es) be underway at a low level in our galaxy?
One of the primary sources of global energy production in galaxies is star formation, and ultraluminous galaxies show all the diagnostic signs of having vigorous star formation. In a new paper by CfA astronomer Desika Narayanan and six colleagues, the case is made that this activity is the result of a higher proportion of dense clouds of gas in these objects, and that these clumps are probably the result of the collision. The conclusion counters earlier arguments that X-rays from the nuclear black holes are responsible by chemically enhancing the gas with molecules that facilitate star formation.
The astronomers reached their conclusions by analyzing a set of thirty-four nearby, infrared luminous galaxies in the emitted light of three key molecules: CO, ionized HCO, and HCN. These species are sensitive probes of total gas densities ranging from about one thousand molecules per cubic centimeter to nearly one hundred million per cubic centimeter. The team compared the brightness of the molecular emission from each species to the overall galaxy luminosity, and found a strong correlation in the sense that the brighter the lines, the higher the luminosity. This result had been well known before, and seemed sensible since new stars form out of the gas. New in the study is the authors' finding that denser gas makes stars at a faster rate: the three species in this study, for example, sample gas that spans a factor of about one million in stellar production rates. The new research convincingly shows that other suggested mechanisms, for example enhanced chemical abundances, are less important. In addition, the paper provides a welcome, relatively comprehensive study of gas densities in luminous galaxies.