Populating an astronomical void.

Populating an astronomical void

One of the surprises of the last decade was the discovery of an enormous void -- a hole in the sky -- in the direction of the constellation Bootes. Although that region of the sky appears well populated with bright galaxies, careful distance measurements show that virtually all those bright objects are in either the foreground or the background, leaving a vast, apparently unoccupied region at intermediate distances (approximately 500 million light-years from Earth). More recent surveys of fainter sources in that direction now show the presence of a new dim galaxies within the void. Those surveys also provide a tantalizing hint that the distribution of the different types of galaxies found in the void may be somewhate different from the distribution found elsewhere.

"We know for sure that the Bootes void is not filled with big, bright galaxies like ours," says Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "We're now investigating the possibility there might be a somewhate different population of galaxies in the void. It's possible that somehow the density [of galaxies] over a large region affects the kind of galaxies there are. That has interesting implications for the distribution of galaxies in the universe." Kirshner was a member of the team that first identified the Bootes void in 1981.

Data collected by the Infrared Astronomical Satellite (IRAS) have turned out to be useful for garnering information about occupants of the Bootes void. The satellite's instruments were particularly sensitive to infrared light, meaning they could detect emissions from galaxies obscured by dust. In such galaxies, dust tends to absorb visible light, subsequently reradiating the energy as infrared radiation.

In the latest analysis of IRAS data, presented at this week's meeting in Boston of the American Astronomical Society, Gregory D. Bothun and Gregory S. Aldering of the University of Michigan in Ann Arbor identify several more galaxies that lie within the void. Because the void is evidently region of space with a below-average density of galaxies, and because earlier surveys have shown that the number of "IRAS" galaxies closely tracks the overall density of bright, ordinary galaxies, astronomers would have expected to find noticeable deficiency in the number of IRAS galaxies. However, the new findings suggest there are as many such galaxies at the void's location as in randomly selected regions of the sky.

The results hint that low-density regions like the Bootes void may have a higher proportion of galaxies that have not yet converted all of their dust and gas into stars, indicating regions where star formation is still going on. Such galaxies generally tend to be dim, small in size and low in mass.

"On the other hand, we don't know those numbers very well," Kirshner says. "We don't really know what fraction of the galaxies we're seeing in these surveys." In other words, what's detected may be biased by the methods used to make the observations. "I think the question [of whether the galaxy types are different in voids] is still open," he says. "We need more data to get a better handle on it."

Observational studies of the galaxy population in low-density regions, or voids, provide useful data for astro-physicists trying to construct theoretical models of galaxy formation and the distribution of mass in the universe. The Bootes void is by far the largest of all known low-density regions. "It's the largest scale on which we know the structure," Kirshner says. "Everybody believes that if you look on the largest scale, the structure [of the universe] will even out and be homogeneous. But on the largest scale where we've actually done the work, we see striking inhomegeneities." Furthermore, he says, none of the present galaxy-formation models produces density contrasts as extensive or deep as what's observed.