Release 920 of 1,082

Astronomers Rule Out Starburst Galaxies as Contributing to the Far-Ultraviolet Background

Release date: Jun 13, 1995 2:00 PM (EDT)

Astronomers using the Hopkins Ultraviolet Telescope (HUT), flown aboard the Shuttle ASTRO-2 mission, have been able to exclude one explanation for the mysterious far ultraviolet background radiation that existed when the universe was young. They find that starburst galaxies -- galaxies forming new stars at an extremely high rate -- were largely opaque to the UV radiation from hot newborn stars embedded within them. Contrary to earlier ideas, this means that starburst galaxies did not contribute significantly to heating, or ionizing, the early universe.

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Release date: Jun 13, 1995
Astronomers Rule Out Starburst Galaxies as Contributing to the Far-Ultraviolet Background

Astronomers using the Hopkins Ultraviolet Telescope (HUT), flown aboard the Shuttle ASTRO-2 mission, have been able to exclude one explanation for the mysterious far ultraviolet background radiation that existed when the universe was young. They find that starburst galaxies - galaxies forming new stars at an extremely high rate - were largely opaque to the UV radiation from hot newborn stars embedded within them. Contrary to earlier ideas, this means that starburst galaxies did not contribute significantly to heating, or ionizing, the early universe.

"Our results are an important first step toward understanding whether or not young star forming galaxies might be responsible for ionizing the early universe," says Claus Leitherer of the Space Telescope Science Institute in Baltimore, Maryland.

Previously, astronomers had proposed that UV energy from quasars - extremely bright active nuclei of young galaxies - played a key role in heating the early universe. However, estimates of the early quasar population have always fallen short of fully explaining how the early universe could be so ionized. So, researchers have looked to other inhabitants of the early universe. Starburst galaxies appeared to be natural candidates for the generation of the ultraviolet background radiation. Although each starburst galaxy is by far less bright than a quasar, there are many more starburst galaxies in the universe than quasars. Therefore all starburst galaxies together might produce as much ultraviolet light as quasars.

HUT was ideally suited to make the critical observation because it required looking at distant, early galaxies. The UV light from these galaxies is redshifted, by the expanding universe to the region of the spectrum (950 angstroms), accessible only to HUT. This wavelength is also a convenient "transparent" window to peer out of our Milky Way galaxy, which absorbs shorter wavelength UV light from young stars within our own Galaxy.

The HUT research team making the observation are Claus Leitherer and Henry Ferguson (STScI); Timothy Heckman (the Johns Hopkins University); and James Lowenthal (University of California, Santa Crud). They are presenting their results at the 186th meeting of the American Astronomical Society in Pittsburgh, PA.

The astronomers made spectroscopic observations of the galaxies Markarian 1267, Markarian 66, NGC 6090 and IRAS 08339+6517, all estimated to have conditions very similar to galaxies which existed when the universe was a few percent of its present age. They found that in all four galaxies, only a few percent of the UV radiation leaked out, as a result of absorption by their own interstellar gas. About 50% of the radiation would need to have escaped the candidate galaxies to contribute to the ultraviolet background.