Ancestors of Milky Way-Type Galaxies Found, Analyzed

John Roach
for National Geographic News

January 9, 2008

Scientists have peered back into the earliest epochs of the universe and discovered the types of galaxies that form modern galaxies, including spiral types like our Milky Way.

The newfound galaxies represent some of the first to form in the universe—their light has traveled between 9 and 12 billion years to reach Earth, the bulk of the universe's 13.7-billion-year existence.

The discoveries help unravel the mysterious origins of the modern universe, sort of like finding a key fossil in the path of human evolution, according to one of the researchers.

The discoveries were described to reporters Tuesday at the American Astronomical Society annual meeting in Austin, Texas.

Milky Way Ancestors

Eric Gawiser is an astrophysicist at Rutgers University in New Brunswick, New Jersey. He said researchers have now directly observed Lyman alpha galaxies, the building blocks of spiral galaxies.

These ancient galaxies are more efficient than others at star formation, producing numerous hot and bright stars that ionize the hydrogen atoms around them. This causes the galaxies to emit a telltale band of ultraviolet light called Lyman alpha.

Previously, researchers had only been able to observe the light emissions and had to extrapolate that they came from galaxies. (Related: "Faint 'Teenage' Galaxies Found in Early Universe" [November 29, 2007].)

The newfound galaxies are tiny, only about 10 percent of the size and 5 percent of the total mass of the Milky Way. They also have only about a fortieth as many stars as in present-day spiral galaxies.

The Milky Way is thought to have at least 200 billion stars.

"We're seeing a very different kind of object in the early universe," Gawiser said at the briefing.

"Several of these objects will have to merge together and additionally add mass accreted from the cosmos in order to form a single, modern-day spiral galaxy."

Gawiser and his colleagues ran computer simulations and statistical analyses of how galaxies cluster together to determine that only Lyman alpha galaxies could merge into spirals like the Milky Way.

Other types of early galaxies would become too large after mergers to form a typical spiral. They form so-called elliptical galaxies instead.

"This determines that the typical present-day descendant of the galaxies we've identified is indeed a typical-mass galaxy like the Milky Way, which would typically be a spiral," Gawiser said.

Massive Relics

Some of these other types of galaxies in the very early universe were anything but tiny, pointed out Elizabeth McGrath, an astrophysicist at the University of California, Santa Cruz.

She presented research on massive disk-shaped galaxies spied by the Hubble Space Telescope.

These galaxies are more massive than the Milky Way, yet they formed when the universe was only a fifth its current age. (Related: "Giant 'Blob' Is Largest Thing in Universe [July 31, 2006].)

Surprisingly, McGrath noted, several of the galaxies were flat, pancake-like disks—not the football-shaped elliptical galaxies that form when galaxies merge. Such elliptical galaxies are common in the local universe.

"These could not have been formed through galaxy mergers," she said. "More likely they were formed from the rapid collapse of a massive gas cloud."

This contrasts greatly with the commonly held belief that all massive galaxies formed by a slow, gradual coalescence of smaller galaxies, McGrath noted.

McGrath and her colleagues believe the disk-shaped galaxies eventually collide with other old galaxies and then reform into the elliptical-shaped galaxies present in the local universe.

"These massive galaxies are very different than the galaxies we see in the local universe," she noted.

In a telephone interview after the briefing, Gawiser noted that these disk-shaped galaxies are "about a hundred times more massive than the Lyman alpha emitters and much rarer" in the early universe.

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