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A very old galaxy that may be very young.

A very old galaxy that may be very young

Back toward the beginning of time liespresumably the epoch when the basic structural elements of the universe began to form. Looking far out into the universe, and therefore far back in time, astronomers hope to see galaxies as they looked when they were just beginning to form. This week in Pasadena, Calif., at the meeting of the American Astronomical Society, a group reported the finding of a galaxy two-thirds of the way back to time zero that has the appearance of a galaxy that is just beginning to form.

Astronomers know of a few other galaxiesjust as far back or slightly farther back that look much more mature than this one. So if this one, known as 3C326.1, is truly a protogalaxy--a galaxy at its very beginning--it may change some astronomers' minds about how and when galaxies formed.

The object 3C326.1 is one of the 300bright radio sources listed in the Third Cambridge Catalogue. For quite a few years Hyron Spinrad, an astronomy professor at the University of California at Berkeley, and a group of associates and students have been surveying the items in this catalog looking for optical counterparts, hoping to find examples of galaxies at an early stage of their development. This one appears to be the most primitive and is certainly quite different from others of the same age and distance. Associated in the discovery are Patrick McCarthy, Michael Strauss and Wil Van Breugel of Berkeley; S. George Djorgovski of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and James Liebert of the University of Arizona in Tucson.

They think 3C326.1 is a protogalaxy becauseit contains a large amount of ionized hydrogen and comparatively few stars. As McCarthy observes, a mature galaxy exhibits a great deal of starlight and very little gas. The light from 3C326.1 is mostly the emission of ionized hydrogen called Lyman alpha. At rest in the laboratory, Lyman alpha appears in the ultraviolet at 1,216 angstroms. Here, it is redshifted to the edge of the visible violet (3,400 angstroms). On that basis the object has a redshift of 1.825, placing it two-thirds of the way back to the Big Bang.

To put it another way, we are seeing it asit looked 12 billion years ago.

Lots of gas and few stars are what astronomerswould expect a protogalaxy to have. The object fulfills another protogalaxy criterion in that it appears to be forming new stars at a very high rate, between 1,000 and 5,000 per year (about 1,000 times the rate for a mature galaxy such as our own). It is about 100 kiloparsecs, or 300,000 light-years, across and has a mass equal to about 100 billion suns. It is very luminous--100 billion times the sun's luminosity.

If 3C326.1 is a protogalaxy, it poses theoreticalquestions of galaxy formation. As Djorgovski points out, astronomers have tended to believe that most of the galaxies formed in a fairly short period, about 100 million years or so, somewhere near the beginning of the universe. That 3C326.1 could be so primitive in comparison with other known galaxies at the same epoch would mean that the period during which galaxies could have formed is much longer than theorists had thought, perhaps even a few billion years long.

The current work began in the springof 1986 with high-resolution maps of the radio source made with the Very Large Array of radiotelescopes near Socorro, N.M. This shows a double source, two close centers of radio emission. In June 1986 an optical image of the same part of the sky made with the 120-inch telescope at the Lick Observatory on Mt. Hamilton, Calif., using a special ultraviolet sensor, showed that the radio source lies within the area where the ionized hydrogen is concentrated. McCarthy says the optical work could not have been done two years ago; an ultraviolet sensor of the requisite sensitivity did not exist. Djorgovski compares the observation of this 24th-magnitude object to seeing a 100-watt light bulb 100 million kilometers away. It took a 4 1/2-hour exposure to build up the image with the 120-inch telescope; Spinrad estimates it would take 100 hours with the Space Telescope.

Although doubts have been expressed,3C326.1 seems in most respects to look as a protogalaxy should. The radio source is the prime exception; theorists would not have expected it in a protogalaxy. But Spinrad says the observers are willing to let natural objects be what they are: "We are not disposed to believe everything we are told.'
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Title Annotation:3C326.1 protogalaxy
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Jan 10, 1987
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