January 20, 1997

Keck Telescope spies the likely building blocks of modern galaxies

Acting as the world's most powerful telescopic tandem, the Hubble Space Telescope and the Keck Telescope are starting to unravel the evolutionary histories of galaxies dating back to when the universe was just 10 percent of its current age.

A year ago, Hubble's most penetrating view into the cosmos--the Hubble Deep Field--gave researchers a bonanza of opportunities to explore how galaxies in the preadolescent universe matured into today's hulking goliaths, including our Milky Way. Now, astronomers at UC Observatories/Lick Observatory, UCSC, have used the Keck Telescope to analyze 24 of those faint blobby galaxies in detail.

Their most noteworthy conclusions include the following:

• So many distant objects freckle the Hubble Deep Field that they probably could not all have grown uniformly into future massive galaxies. Rather, the variety of shapes and sizes suggests that many of them merged, to evolve later into the smaller number of "grand design" galaxies we now see. Others may have faded into the unobtrusive dwarf galaxies scattered throughout today's universe.

• The distant galaxies were at least as luminous as the Milky Way but were about one-tenth as large. They created new stars at rates comparable to the pace of starbirth today, but they were much less prolific than galaxies that unleashed tremendous bursts of fresh stars billions of years later. This is among the strongest evidence to date that rates of star formation have risen and fallen markedly as the universe has aged.

• Close scrutiny of the starlight from these objects exposes evidence of carbon, oxygen, silicon, and various metals in the stars' atmospheres. This means that generations of stellar birth and death already had occurred, even at the universe's tender age of one to two billion years.

"We don't know the masses of these objects, but we suspect some may be low in mass and bright with starbursts, and that they form the building blocks of more massive galaxies to follow," says postdoctoral researcher James Lowenthal, lead author of the study. "But if these galaxies didn't merge together, they would build up into far too many bright galaxies as the universe evolved--many more galaxies than we see around the Milky Way today."

Lowenthal, a former Hubble Fellow, presented his group's research on January 16 at the American Astronomical Society meeting. His coworkers included several astronomers from UCO/Lick and UCSC: David Koo, Rafael Guzman, Jesus Gallego, Andrew Phillips, Sandra Faber, Nicole Vogt, Garth Illingworth, and Caryl Gronwall.

For its study, the team used the Low-Resolution Imaging Spectrograph (LRIS) on the Keck I Telescope for three nights in April 1996. Exposure times for each of the 24 galaxies varied from two to four hours. The results were detailed spectra--light spread out across its component wavelengths. The team used the spectra to determine the distances to about half of the galaxies. Their redshifts, a measure of how quickly the galaxies are receding from earth, ranged from z = 2.2 to 3.4. (A galaxy with a redshift of z = 3 existed at a time about 90 percent of the way back to the Big Bang.)

"We pushed the limit to among the faintest galaxies for which anyone has attempted to obtain spectra," says Koo. "A lot of other information naturally falls out after you get distances to the galaxies, including their sizes and their intrinsic luminosities."
Images of several of the galaxies

The team used an innovative technique based on the precise colors of the objects to identify dozens of other galaxies in the Hubble Deep Field that also may lie at similar distances. However, the spectra for some were too complex or too noisy to determine their redshifts easily, and the team has not yet observed the rest of the galaxies with Keck. If many of them turn out to have redshifts of about z = 3, then astronomers would need to raise their current estimates of the population density of galaxies in that epoch by a factor of 3 or 4.

Lowenthal and Koo note that the Hubble Deep Field certainly captured only the most intense bursts of star formation during that slice of the universe's history. Indeed, Hubble's image may depict what the researchers call the "Christmas tree effect," where bright individual star-forming blobs come and go within a much bigger--and largely invisible--structure. Astronomers will need further spectral signatures to tease out the masses of these remote objects and their internal dynamics.

"The interpretation of the fates of these galaxies is still very much open, because this field is so new," Koo says. "Determining their masses will be critical to knowing how they relate to nearby modern galaxies."

The UCSC team is part of the Deep Extragalactic Evolutionary Probe (DEEP), a collaboration of UC, Johns Hopkins, Caltech, and the University of Chicago. DEEP's goal is to use the light-gathering power of Keck II and a sophisticated new spectrograph, now being built at UCSC, to obtain the redshifts of about 15,000 galaxies as a probe of the distant universe.

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