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Making big galaxies by merging smaller ones.

Astronomers have gathered fresh evidence that millions of mini-galaxies merged to form today's collection of spiral and elliptical galaxies -- including, perhaps, our own Milky Way.

The first hints of this cosmological drama came late last year, when astronomer Lennox L. Cowie and his colleagues at the University of Hawaii in Honolulu reported a baffling result: New infrared maps indicated that the vast majority of galaxies that existed when the universe was half its current age were small, amorphous blobs (SN: 11/16/91, p.312). Those galaxies lacked the familiar spiral or elliptical shape of galaxies common in the present-day universe.

Few of the small galaxies appear to reside within 60 million light-years of the Milky Way. Thus, Cowie speculated that clusters of the small galaxies may have served as seeds for today's elliptical and spiral galaxies, merging in the recent past to form these bigger galaxies. Alternatively, he noted, the tiny galaxies may have simply faded from view or even self-destructed as the universe grew older.

New observations by Cowie's team, made in the last six months with the Canada-France-Hawaii telescope atop Mauna Kea, strongly support the merger scenario, he says. Cowie reported the results this week in Paris at an astronomy workshop.

The findings show that as an observer views regions of space more than 2 billion light-years from Earth -- the same as looking back in time -- the number of small galaxies increases. Moreover, the number of spirals and ellipticals declines correspondingly. Peering into the past, it appears that "the population of large galaxies is replaced by the smaller ones," says Cowie. If a movie of the history of the universe were run in reverse, "it's clear that the large, present-day galaxies break up into smaller galaxies," he adds.

Cowie says it would take 10 to 100 of the small galaxies to form a single spiral. That may pose a problem, he notes, since current models indicate that the merger of so many mini-galaxies would form a fatter disk than spiral galaxies, such as the Milky Way, actually have. He suggests that theorists need to develop more detailed simulations of the motion of stars and gas in interacting galaxies.

"It's a tricky business once [a galaxy] is absorbed and gets mixed into the rest of a group of galaxies," Cowie notes.

Preliminary observations with the Hubble Space Telescope seem to support Cowie's merger notion. Richard E. Griffiths and Kavan U. Ratnatunga of the Space Telescope Science Institute in Baltimore and their colleagues used Hubble to resolve the shapes of a small sample of galaxies believed to lie between 3 billion and 10 billion light-years from Earth. The team found that more than half the roughly 300 galaxies in their survey were small and amorphous, and some appeared to be merging.

Though a definitive interpretation will require many more years of observations, Ratnatunga says the early data hint that small galaxies were the building blocks for today's galaxies. Griffiths reported the results last week at a Hubble workshop in Sardinia, Italy.
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Title Annotation:research from observation of space regions that are 2 billion light-years from Earth
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Jul 11, 1992
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