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Giving Birth to Galactic Beauties

NGC 3949

A Hubble Space Telescope view of NGC 3949, one of Weinzirl’s target galaxies. It is about 50 million light-years away. [NASA/ESA/Hubble Heritage Team (STScI/AURA)]

In any galactic beauty contest, the clear winners are the spirals. Their delicate arms, laced with jewel-like stellar nurseries, are among the most impressive sights in the universe.

University of Texas at Austin graduate student Tim Weinzirl is helping explain the processes that give birth to these galactic beauties with VIRUS-P and the Harlan J. Smith Telescope at McDonald Observatory. His observations are providing important data that will help astronomers select from several models of spiral-galaxy formation.

In one model, galaxies form from the merger of two large, near-equal mass clouds of gas, dust, and mysterious dark matter. In this process, called a major merger, the gas and dust cool and fall toward the center of the giant “haloes” of cold dark matter. (Dark matter produces no detectable energy, but reveals its presence through its gravitational pull on the visible matter around it.) As the clouds of gas and dust collapse, some of the gas forms stars in rapid bursts of star formation. Most of the stars and gas congregate in a central “bulge,” with the remainder forming a small, flat disk around the bulge.

“For a long time it’s been thought that major mergers have been very important in the development of galaxies,” Weinzirl says. “We’re pretty sure that this is true for elliptical galaxies,” which are shaped like fuzzy footballs, although it is less clear that major mergers have played a recent role in the formation of spiral galaxies.

NGC 4314

A Hubble view of the core of NGC 4314, another target galaxy. [G. Benedict et.al./NASA/STScI]

Astronomers have studied many spiral galaxies to help them determine if this model is correct. Weinzirl, for example, studied the invisible infrared glow of about 150 galaxies. He measured the brightness of each galaxy’s central bulge and the surrounding disk. The distribution of the light within the bulge offers constraints on how the bulge could have formed.

Weinzirl says that he and his collaborators determined that in about 70 percent of the surveyed galaxies, the bulge made up less than 20 percent of the total stellar mass, suggesting that most massive spirals have not undergone a recent major merger. “This suggests that major mergers, at least for the spiral galaxies that we’re looking at, apparently did not have a very big influence in most cases for a substantial portion of the age of universe,” he says.

Instead, the spiral galaxies probably formed in another way.

One possibility is “minor” mergers, in which one cloud of material is roughly 4 to 10 times more massive than the other. And another is that spirals form from the evolution of the galaxy itself, known as secular evolution. “Secular in this case means slow paced,” Weinzirl explains. “There are mechanisms — bars for instance — that can drive gas inward from the disk of these galaxies to the central region. This gas will pile up and it will form stars.”

“The stars in the bulge that we see from most of galaxies from this study were apparently either formed from some combination of minor mergers or secular processes,” he says.

While his earlier study measure the intensity of the light in his target galaxies, the VIRUS-P study will provide far more detail about several of those galaxies. VIRUS-P’s individual spectrographs will allow Weinzirl to look at perhaps 200 or more small regions within each galaxy and obtain information about the stars in each region.

“The spectrum that we get in each part of the galaxy will be determined by, among other things, what kind of stellar population is present at that particular spot in the galaxy,” Weinzirl says. “We can make a two-dimensional map of these galaxies. We can look at the spectra in different portions — the bulge, disk, and bar — and we can use models to get a pretty good idea of what kind of stellar populations are present in those regions. ...  Based on what kind of stars are present, this has constraints on the process by which the galaxy formed and also the timescale in which the galaxy formed.”

The VIRUS-P study began in early 2009, with data collection and analysis continuing through 2010.

NGC 3227

NGC 3227 (bottom) is interacting with a smaller companion galaxy in this view from the Sloan Digital Sky Survey. [SDSS]