Weekly Science Update

Wednesday, April 30, 2008
Cold but Luminous
Over the past two decades astronomers have come to realize that many galaxies are fabulously luminous -- some are more than a thousand times brighter than our Milky Way. Yet the most powerful of these galaxies are practically invisible at optical wavelengths because this and other short wavelength light is absorbed by the galaxies' dust and re-emitted in the infrared by the dust. Astronomers are quite sure that the energy to heat the dust comes either from processes around a massive black hole at the galaxy's nucleus, or bursts of star formation that are hidden from optical view by the dust itself, but they do not know whether these galaxies are unusual because of their size, or composition, or evolutionary stage. Perhaps many galaxies, including our own Milky Way, pass through similar phases of dramatic infrared luminosity.

The Spitzer Space Telescope studied a small region of the sky and discovered within it forty-three bright infrared galaxies whose light has been traveling for between one and eight billion years. The light travel times mean that we are seeing these galaxies in a key cosmic transition period, at a time after the formative epoch when galaxies were most actively making new stars, but before today's age of more quiescent galaxy behavior when luminous infrared galaxies are rare.

SAO astronomers Steve Willner, Jiasheng Huang, and Giovanni Fazio, together with three colleagues, used the Spitzer infrared measurements of these galaxies -- both the intensity of the radiation and its dependence on wavelength -- to model the character of the dust and the nature of the star-forming activity that in these objects appears able to heat it. One distinguishing feature of this study is the extreme faintness of most of the galaxies that, although intrinsically luminous, are very distant and thus hard to detect. The scientists conclude that these distant objects are not like local, bright galaxies. Instead, these remote objects appear to have much more very cold dust, colder than about 30 kelvin. Although the reasons for this difference are still uncertain, these cold but luminous galaxies may represent a previously suspected but until now undetected phase in the evolution of galaxies. The results help to fill in the range of possible galaxy types, confirm some previous theoretical predictions, and provide key insights into the cosmic epoch of galaxy development that immediately preceded our own era.

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