Image: Ultra-luminous infrared galaxies
STScI / NASA via Caltech
This selection of ultra-luminous infrared galaxies was documented by the Hubble Space Telescope. The infrared brilliance of these extreme objects, which are galaxy mergers in progress, results from intense concentrations of hot gas at their centers.
updated 6/12/2006 3:31:34 PM ET 2006-06-12T19:31:34

Researchers have found a way to probe the secrets of remote, extreme galaxies by looking at their cosmic cousins closer by.

Intriguing, highly active galaxies detected at distances of 8 billion to 10 billion light-years are too far and faint to study in sufficient detail with current instruments. But astronomers can get a good look at similar galaxies that lie within our own cosmic neighborhood.

Amid the hosts of different galaxy types, luminous infrared galaxies (LIRGs) and their much bigger brothers, ultraluminous infrared galaxies (ULIRGs) shine powerfully in infrared wavelengths of light. The infrared brilliance of these extreme objects, which are galaxy mergers in progress, results from intense concentrations of hot gas at their centers.

The hot gas powers furious star-forming activity or the growth of a giant black hole, making such galaxies more than 10 times as luminous as the Milky Way in the far-infrared.

Study results presented here last week at the 208th meeting of the American Astronomical Society showed that even within the "local population" of galaxies, extreme galaxies are full of surprises.

Astronomer Christine Wilson and her colleagues from the Smithsonian Astrophysical Observatory used the Submillimeter Array on Mauna Kea, Hawaii, to examine a representative sample of 12 LIRGs and ULIRGs located less than 600 million light-years away.

"This gives us snapshots of galaxies at several different stages in their evolution. From this, we hope to get a better understanding of star formation and the gas physics involved, to determine how the gas properties change as the galactic mergers progress," Wilson told "This in turn will help us to understand better the cosmologically significant population of very luminous dusty galaxies which carry a lot of the star formation" at great distances.

To the surprise of Wilson and her team, the LIRGs and ULIRGs they observed turned out to contain about 100 times more gas than dust, similar to what is normally seen in mature and less active galaxies like the Milky Way.

"This suggests that these galaxies are more like our own than we might have guessed," Wilson said.

With such a goldmine of dusty luminous galaxies relatively close at hand, Wilson now looks forward to observing extreme galaxies with the coming generation of more powerful instruments like the Atacama Large Millimeter Array (ALMA), currently under construction in Chile.

"With ALMA's much higher resolution and sensitivity," Wilson said, "we'll be able to zoom right in and look at individual clusters and clumps of dust and gas in the star-forming regions, or get a detailed picture of the kinematics around a central black hole. This gives us a really bright future for studying extreme regions of star formation in very luminous galaxies."

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