By Marlene Cimons, National Science Foundation
Historically, astronomers seeking to discover new galaxies have looked for their stars. More recently, however, they have begun to detect galaxies by looking for something else, hydrogen gas, which scientists believe is the primordial element from which the Universe evolved.
“In the end, some galaxies have gas, some don’t, but this is a different way of looking for them,” says Jessica Rosenberg, assistant professor of physics and astronomy at George Mason University. “It’s one piece of the puzzle.”
Scientists believe that almost everything began as hydrogen atoms at the time of the Big Bang, the prevailing theory of the creation of the Universe, and that, over billions of years, these gases generated galaxies composed of stars, gases, and dust.
Astronomers want to know more about this evolution so they can better understand the Universe in its entirety, and how it works. The current thinking is that they can do this best by searching beyond only the stars, and examining the stars and gas together.
“People are fascinated by the nature of the Universe, how it came to be,” Rosenberg says. “We, on Earth, are just this tiny speck. What we are trying to understand is how some of these biggest things in the Universe have changed over time from the Big Bang until now.”
Rosenberg is collaborating in an ambitious study that combines data from two major surveys, the Arecibo Legacy Fast ALFA (ALFALFA) radio survey, which detects galaxies by looking for gas using a radio telescope, and the Sloan Digital Sky Survey, which uses an optical telescope to find galaxies through their collections of stars.
They study the “baryonic” components of galaxies, which are “the atoms, molecules, and normal matter in the Universe,” she says. “In astronomy, we talk about baryons and non- baryons, which is all the weird stuff we don’t understand, like dark matter. I am looking at the normal stuff in the universe that we are all familiar with, the stars and the hydrogen gas.”
Rosenberg, who has been involved with the ALFALFA survey since it began in 2005, predicts that it will find some 30,000 new galaxies by the end of this year. Sloan, which began in 2000, has detected about 700,000 thus far. Rosenberg is studying the galaxies in a single area of the sky covered by both surveys. They are discovering new galaxies all the time.
“Part of Sloan and part of Alfalfa are in the same region of the sky,” she says. “What we are doing is looking in that region of the sky at the galaxies detected by Alfalfa and by Sloan, and comparing them. Some are the same, and some are different.”
Rosenberg expects there will be about 40,000 galaxies in the overlap region.
By combining the optical and radio data, astronomers will measure the fraction of mass in the two major components of galaxies, which is the key to understanding how galaxies form and evolve. “We can measure the stars and the gas in galaxies; which have stars? Which have gas? Which have both? What’s the ratio between them?” she says. “That actually tells you something about the nature of galaxies.”
She then plans to compare the information to existing models about how galaxies form and evolve. “Does what we see in terms of baryonic content agree with our expectations from the current best models?” she says. To be sure, “we are only looking at them in one snapshot in time, but things don’t change in the Universe that fast,” she adds.
In addition to studying the baryons, the researchers also are using the combination of these surveys for other related projects. One of them involves studying the gas and stars in pairs of galaxies that are “interacting,” that is, in the process of colliding.
“This is the part where we probably have the most results thus far,” she says. “We have found that pairs of galaxies that are running into each other produce more new stars if they have more gas. Collisions of galaxies that have gas help to build up the stellar portion of galaxies.”