January 19, 2007
David Fisher, an astronomy graduate student at The University of Texas at Austin, is making important contributions to the future understanding of galaxy evolution by studying the different types of bulges at the hearts of nearby spiral galaxies. This work was presented at the 207th meeting of the American Astronomical Society in Seattle, Washington.
A bulge is a concentration of stars in the center of a spiral galaxy. In recent years, evidence has shown that there are two types of bulges — the so-called "classical bulges" and "pseudobulges."
Studying these bulges and finding out how many nearby galaxies have the different types is important, Fisher says, because "we believe that the formation of these two types of bulges is dramatically different." Such findings could be important to theories of galaxy formation.
A classical bulge is a mostly featureless, round ball of stars, he says. A pseudobulge, on the other hand, "looks very much like the [galaxy's] outer disk, with a spiral structure. It can have a bar, and can be very flat, instead of round."
Fisher and his colleague Niv Drory (Max-Planck-Institut fur extraterrestrische Physik) studied archival images of 40 galaxies within about 150 million light-years from both Hubble Space Telescope and the Sloan Digital Sky Survey. The archival Hubble images were of the very heart — the bulge — of these galaxies. The Sloan images provided a look at the same galaxies in their entirety — providing information on the context for the bulge.
Fisher and Drory find that the global properties of galaxies are tightly coupled to the type of bulge a galaxy contains, even when the bulge accounts for only a few percent of the galaxy's mass.
This work is the basis for Fisher's doctoral dissertation. His goal is to find a much more quantitative way to distinguish between the two bulge types. His work presented at this conference shows that he is well on his way. Once achieved, this would enable astronomers "to count how many of each bulge type there are in the local universe," he says.
He explains that current theories of galaxy evolution focus on mergers, and typically argue that the vast majority of nearby galaxies were built up through mergers with other galaxies over time. Further, most galaxies are predicted to have experienced a major merger in the past billion years.
Classical bulges are the result of mergers, Fisher says. "We feel comfortable that they form through some kind of merger. You take two sets of stars, you mix them up, and you're left with a ball that's essentially featureless."
But pseudobulges form differently, through so-called "secular evolution." Essentially, this means that the galaxy evolves on its own, without any mergers. "In secular evolution," Fisher says, "the galaxy will start to re-arrange itself, do it in an ordered way, rotating very fast. This causes the spiral structure and nuclear bars in the center" that are seen in pseudobulges. Pseudobulges are signposts of a history (since the formation of the disk) that is free of mergers. Current theories of galaxy formation do not predict this to be a common process. "We are trying to find out how right or wrong these theories are," Fisher says.
An efficient method for counting the number of classical bulges and pseudobulges in local galaxies will reveal which type is more prominent. If there are a lot of pseudobulges, then there were not as many galactic mergers in the past as astronomers think.
"The hints are there" that this is the case, Fisher says. If this turns out to be correct, then "our galaxy formation models need more work."