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SALT LAKE CITY — What happens to a galaxy when it gets older? Much like the rest of us, it gets a little fat around the belly. That's what University of Utah astronomers have been studying by looking far into the universe's past in order to see the future of many galaxies.
According to Dr. Adam Bolton, lead author on a new study, most recent research suggests that old and massive elliptical galaxies grow by snacking on many smaller galaxies, that is, absorbing their mass in collisions.
Bolton has a different idea, based on some previous research.
"We're suggesting that major collisions between massive galaxies are just as important as those many small snacks," he said. In other words, eating big meals matters as much as snacking on smaller galaxies.
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Utah-made satellite has discovered a "bonanza of black holes" and has found 1,000 examples of a rare and violent phenomenon that produces the brightest objects in the universe."I feel like if we can't explain why these galaxies look the way they do, we don't have any hope with other types of galaxies," Bolton said.
When two galaxies of roughly equal size collide, they tend to combine more completely, penetrating deeper and eventually concentrating mass closer to the center of the combined galaxy. When smaller collisions happen, they tend to glance off, leaving mass closer to the outskirts of the galaxies.
Basically, if you know how big a massive elliptical galaxy's belly is, you know a lot more about its history and formation.
Gravity lenses
In order to study that possibility, Bolton made use of data from another project — to map the sky using the Sloan Digital Sky Survey-III. That project intends to put together a 3D map of the universe, which would be useful for any number of reasons, including getting a better idea of early history close to the Big Bang and understanding the properties of dark energy, the mysterious stuff that makes the universe accelerate and makes up roughly 72 percent of the mass in the universe.
A side benefit of mapping as many galaxies as you can is that it's possible to find galaxies that show gravitational lensing, a very rare phenomenon showing up in perhaps 1 in 100 galaxies, according to Bolton.
It also allows you to measure mass.
When two galaxies are exactly superimposed from the view of Earth, with one much farther away, the gravity of the closer galaxy will bend the light from the distant one, making it look like a ring from our point of view. The more bent the light is, the bigger the ring is and the more massive the close galaxy is.
Once the team found a galaxy that showed lensing, they could point the Hubble Telescope at the galaxy and take accurate readings of the mass of the galaxy and approximate where the mass is concentrated, accurate to about 2 percent, according to Bolton.
"There's really no other way other than gravitational lensing to make precise measurement of galaxies this far away," he said.
They found that the density of older massive elliptical galaxies tends to get more concentrated at the center, meaning that bigger meals — that is, bigger collisions — make an important mark on their history and development.
The study was performed with post doctorate researcher Joel Brownstein, graduate student Yiping Shu and undergraduate Ryan Arneson, as well as several members of the the Sloan Digital Sky Survey - III, and has been published in The Astrophysical Journal. The research was made possible by the generous support given to the U. by the Willard L. Eccles Foundation.
