Grappling with Galaxy Formation

Science News, Sept 25, 1999 by Ron Cowen

Connecting the dots between galaxies near and far

One little, two little, three little I galaxies. Four little, five little, six little galaxies....

Astronomers peering at some of the earliest, most distant gatherings of stars in the universe are trying to figure out exactly what these dim objects were and how they relate to the billions of galaxies that fill the sky today. Some of these bodies are so remote that the light they emitted several billion years ago is only now reaching Earth. Telescope images therefore provide snapshots of what these galaxies looked like when they were very young.

Just 4 years ago, researchers knew of perhaps 70 of these baby galaxies, which date from a time when the universe was no more than 3 billion years old. Since 1996, that number has jumped to more than 850 (SN: 5/2/98, p. 280).

The study of distant galaxies, however, is no longer just a numbers game. Astronomers have begun to closely scrutinize distant galaxies in infrared light, a range of wavelengths that promises to reveal the mass of these faraway bodies. As they determine which are heavy and which are light, researchers may be able to choose among competing theories of galaxy assembly--and find out whether the universe will end in a Big Crunch or expand forever.

Scientists are also now collecting sharper images in visible light, enabling them to discern subtle color variations within individual galaxies and gather new clues about galaxy formation.

"We're trying to piece together the story of how the big, regular galaxies that we see around us today formed," says Richard S. Ellis of the University of Cambridge in England.

"It's a new era in the study of distant galaxies," says theorist Matthias Steinmetz of the University of Arizona in Tucson.

Two sets of exquisitely sharp images recorded by the Hubble Space Telescope are making a special contribution to the study of galaxy formation. In late 1995, Hubble stared at a small patch of sky in the northern celestial hemisphere for 2 weeks, and in 1998 it did the same in the southern hemisphere. Observations of the Hubble Deep Field North and South, as the patches of sky are now called, have produced the sharpest images of galaxies ever made.

Many astronomers have used these color portraits to pick out the galaxies likely to lie farthest from Earth and therefore be the youngest. One successful strategy is to search for galaxies that glow brightly at red wavelengths but are invisible in the near ultraviolet (SN: 2/24/96, p. 120).

Ellis, Roberto Abraham of the University of Cambridge, and their colleagues are studying the Hubble images in a different way. Rather than examining the overall hue of a galaxy, they are scrutinizing the variations of color within it. Although the most remote galaxies in the Hubble fields are too faint to clearly show a color variation, the astronomers have discerned differences in color within galaxies that lie as far as halfway to the edge of the visible universe.

"For a couple of years now, we've been thinking that the Hubble data has not been fully explored," says Ellis. "People have really just been studying the overall properties of the galaxies--getting their shapes--but they haven't been actually using the colors of the individual subcomponents within each galaxy."

Mark Dickinson of the Space Telescope Science Institute in Baltimore notes, "You see [distant] galaxies that have blue arms or blue knots and red centers--just as you see color variations in nearby galaxies."

Observed in detail, most galaxies resemble a Seurat painting, with each picture element, or pixel, a differently colored dot. Gradations in color from one pixel to another can suggest which parts of a galaxy formed first and which came last.

Massive, short-lived stars tend to emit most of their light at shorter, bluer wavelengths. Lightweight, long-lived stars glow more brightly at longer, redder wavelengths. Thus, a reddish tinge typically indicates a galactic region that contains stars that formed earlier than those in bluer regions.

By fitting the color of each pixel to models for age and star-formation history, "we try to work out how different components of the galaxy form," Abraham notes. "Most people now view galaxy formation as a process, not an event, and the key notion is that different parts of a galaxy probably form at different times," he says. When astronomers assign a single color to a galaxy as a whole, they "effectively assign a single formation history to the [entire] galaxy," he says.

Many remote galaxies, observed as they appeared when the cosmos was perhaps 25 percent of its current age, look like nothing more than compact blobs. These could be the building blocks of the more mature galaxies that lie nearer Earth.

Modern-day galaxies come in three types: spirals with swirling, starlit arms; football-shape ellipticals; and irregular galaxies, which have no organized structure. This set of galaxy types is known as the Hubble sequence.

"The real challenge for those of us studying the origin of galaxies is to try to understand the steps by which the Hubble sequence came into being," says Ellis.