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Formation of present-day spiral galaxies observed

Posted: December 29, 2000

An international team of astronomers has made a significant advance in the study of galaxy formation by linking a particular class of distant galaxies with present-day spiral galaxies.

The team of French, American, and Chilean astronomers believes that luminous compact galaxies, a type of galaxy common seven to ten billion years ago but rare today, are the direct predecessors of spiral galaxies such as the Milky Way. Their results, publicized this month, will be published in an upcoming issue of The Astrophysical Journal.

Illustration shows a sketchy sequence of compact galaxy morphologies on the right. From the top picture to the bottom closely interaction are shown, and then formation of stars in bulges and finally in the disk.
Spectra of some luminous compact galaxies, taken with one of the 8-meter mirrors of the European Southern Observatory's Very Large Telescope in Chile, revealed absorption lines from iron and other metals. The spectra also revealed large quantities of dust surrounding them; that dust is believed to be associated with regions of intense star formation within the galaxies.

The spectral signatures of the luminous compact galaxies closely match the spectra from the bulges of spiral galaxies. That coincidence, along with other observations of the shapes of such galaxies, led astronomers to conclude that luminous compact galaxies are the progenitors of current spiral galaxies, dating back to a time before the spiral arms formed. The observations also rule out a competing explanation that luminous compact galaxies are related to modern-day dwarf galaxies

"Our work has shown that luminous compact galaxies have old stars with a well evolved metal content, such as what is found in the core of massive galaxies, and conversely to what is found in dwarf galaxies," explained Francois Hammer, an astronomer at the Observatoire de Meudon and one of the members of the team that studied the galaxies. This work is the first to conclusively tie luminous compact galaxies to spiral galaxies, he said.

Hammer said that this work should prove useful in studies of the formation and evolution of galaxies. "It provides an observational confirmation of the hierarchical scenario of galaxy formation, for which a massive galaxy is formed after a series of mergers of smaller units," he noted. "Second, it shows that an important part of the 'heavy' elements (carbon, oxygen, nitrogen, etc.) in the present-day Universe were not formed at the earliest epochs of the Universe, but more likely during a period when the Universe have a third of its present age."

Additional work is still needed to better understand galactic evolution, though, but data coming from a new generation of telescopes and instruments should be a boon for astronomers. "We could now, or soon in the next decade, study physics -- dynamics, chemistry, stellar properties -- in distant galaxies as it has been done for nearby ones during the last 20 years," said Hammer. "This means that we would be able to directly observe the galaxy formation. A general modeling of these observations will certainly provide a solid theory of galaxy formation."