The early universe, astronomers tell us, was a very smooth place, its matter and energy spread out uniformly like a pea-soup fog. Nowadays, it contains both airy voids and huge, dense structures: galaxies, and clusters of galaxies, and clusters upon clusters ad infinitum.
How did all this variety take shape?
There's a standard response. As the universe expands, it cools, and once it has cooled enough a couple of protons here and there begin to cling together. Each little pair picks up another proton, then another, and pretty soon gravity takes over. As these tiny objects gain mass they gain gravity, pulling in more and more particles, each time compounding their advantage to grow still bigger. Or, as Jane Charlton, Penn State assistant professor of astronomy and astrophysics, puts it, "The rich get richer." The process repeats itself at increasingly larger scales, and before you know it you've got clusters of galaxies, a "clumpy" universe. Astronomers call it the hierarchical theory of structural formation.
But do all galaxies form this way? And what other factors account for the several sorts of galaxies that now exist — their different sizes, shapes, and brightnesses? Environment is one factor, says Charlton. In dense regions of the universe, for example, there are more elliptical galaxies; in less dense areas, more spirals. But why are some galaxies so much brighter than others? And what about dwarfs?
Most of the galaxies in the universe happen to be dwarfs, smaller and less visible than their "giant" counterparts, and therefore less well understood. Are dwarfs merely giants that haven't merged yet, as the hieracrhical theory would suggest? Or are they formed in other ways?
Dwarfs have properties that distinguish them from giants, notes Charlton. Less compact cores, for instance, and lower surface brightness. There's another type of evidence, too, to suggest a different mechanism for their formation, Charlton says. When two giant galaxies collide, she explains, "It takes a while — maybe 500 million years — for them to slow down enough to merge." In the meantime, they revolve around each other in a furious orbit, a cosmic cat-fight. These perturbations, appropriately, leave tails: sweeping appendages of tidal debris. And in these tails can clearly be seen dwarf galaxies.
Is collision in fact a mechanism for galactic formation? Or is it just coincidence that the dwarfs appear where they do?
To find the answer, Charlton, Penn State graduate student Sally Hunsberger, and Dennis Zaritsky of the University of California at Santa Cruz undertook a survey. Using the 1.5 meter telescope on Palomar mountain in California, they looked at a compact group of galaxies called Stephan's quintet, a setting particularly rich in tidal tails.
Counting the dwarfs present in these debris, they factored in the relatively short life of a tail before its parts are either scattered across the heavens or re-consumed in the gravity of its parents. Then, comparing their estimate of the number of tidal dwarfs that should be present to the total number of dwarfs actually seen, they concluded that as many as half of all dwarfs in compact groups may be formed when giant galaxies collide. The inescapable allusion is to birth: the parent galaxies have spawned these tiny offspring.
The Palomar survey is a preliminary step, Charlton stresses. "There's only so much we can tell from the ground." To get a closer look, the trio have applied for time on the Hubble space telescope. By analyzing the Stephan quintet's tidal tails from the proximity of space, they hope to learn much more about the age and composition of the dwarfs the tails contain, and the physics of their stars.
But if their early estimate is verified, and a collision-based mechanism is found to be a significant factor in dwarf formation, Charlton writes, "a long-sought missing link between the populations of giant and dwarf galaxies has been found."
She is confident that such is indeed the case.
"I'm never saying that all dwarf galaxies are made this way," she says. "But it isn't just a fluke."
Jane C. Charlton, Ph.D., is assistant professor of astronomy and astrophysics, 515B Davey Laboratory, University Park, PA 16802; 814-863-6040. Sally D. Hunsberger is a Ph.D. student in the department. Dennis Zaritsky, Ph.D., is assistant professor of astronomy and astrophysics at the University of California, Santa Cruz. Funding for the study reported was provided by Penn State's Eberly College of Science and by the California Space Institute.