Ultra-Luminous Infrared Galaxies

Ultraluminous infrared galaxies ("ULIRGs") shine with the luminosity of one hundred or more Milky Way galaxies. Their most striking feature, however, is not their tremendous energy output but the fact that nearly all of their radiation is invisible, lying at infrared wavelengths. The source of this energy is intense star formation, and because that activity takes place within dust-filled clouds, the ultraviolet and visible light generated is absorbed by the dust grains and remitted in the infrared.

ULIRGs are thought to result from collisions between galaxies, and since collisions are common, ULIRGs and the slightly dimmer "luminous infrared galaxies" may represent a period of stellar growth and enrichment that many galaxies (perhaps even our own galaxy) briefly experience, especially at early times in the age of the universe when collisions were common. The connections between galaxy interactions and star formation, however, are poorly understood, in part because the obscuring dust makes it difficult to probe the small nuclei of the two merging galaxies.

SAO astronomers Christine Wilson, Glen Petitpas, Daisuke Iono, Alison Peck, Melanie Krips, and Tom Cox, together with ten colleagues, have just published the first in a landmark series of papers on luminous and ultraluminous galaxies using data obtained with SAO's Submillimeter Array (SMA). The SMA allowed the astronomers to measure for the first time the spatial distribution of the warm gas around luminous galaxies' nuclei.

The scientists reached two surprising conclusions. First, they found that the ratio of the amount of gas to dust is about the same as it is in the Milky Way despite the very different level of star formation activity. Second, they discovered that it is peak gas density, not the peak gas mass, that correlates with the region of maximum brightness. This suggests that the increased rate of star birth is the result of increased availability of molecular gas as the fuel, a conclusion that is in direct contrast to the conventional wisdom that abundant gas increases the efficiency but not the rate of star birth. The new paper, the first in a set that will analyze fourteen nearby galaxies, pioneers new, high spatial resolution diagnostics to provide new insights into these powerful cosmic beacons.