By Rachel Stern
In Outer Space, there’s a lot of room for new discoveries.
The latest-that there’s an enormous halo of stars bound to the Andromeda Galaxy, revealing it to be five times larger than previously thought-can be attributed to a group that includes three UC Santa Cruz astronomers. Monday, Jan. 8, they presented their findings at an American Astronomical Society (AAS) meeting in Seattle.
"It was quite literally like finding a needle in a haystack," said Puragra Guhathakurta, a professor of astronomy and astrophysics at UCSC, of the galaxy that had eluded astronomers for over twenty years.
Guhathakurta and his collaborators at UCSC, UCLA and the University of Virginia are conducting a continual study of Andromeda’s halo from the Kitt Peak National Laboratory in Arizona and the W.M. Keck Observatory in Hawaii.
Andromeda is part of a "local group" that includes the Milky Way and about 30 smaller galaxies. As a spiral galaxy similar to the Milky Way, it is located about 2.5 million light years away from the earth. Unlike the Milky Way, Andromeda (also known as M31) is close enough to allow astronomers to observe individual stars within it.
There are three main components of spiral galaxies: a flattened disk, a luminous central bulge with a dense concentration of stars, and an extended halo composed of sparsely distributed stars. Before this month, Andromeda was only known to fit into the first two groupings.
The reason that the halo was not discovered before, said Kalarai, was due to other astronomers "mistakenly identifying the outer parts of the Andromeda bulge as its halo."
The halo was detected by a technique to distinguish halo stars in Andromeda from the more numerous foreground stars in the Milky Way. Foreground stars with low luminosity and luminous stars further away can be hard to tell apart because they appear to be equally vibrant to the human eye.
Jason Kalarai, a team member and post-doctoral fellow at UCSC, made an analogy between these foreground and more distant stars.
"It’s like taking a firefly in front of one’s face and a beacon of light in the distance," Kalarai said. "We found a way to limit the ambiguity."
The technique for weeding out the fireflies from the beacons was developed by Karoline Gilbert, a UCSC graduate student. She separated the two populations of stars through a diagnosis that utilized photometry, or brightness measurements, and spectroscopy, or the placement of starlight into a spectrum of varying wavelengths.
"We focused on red giant stars in the halo because they’re bright enough for us to obtain spectra," Gilbert said.
The halo stars contain lesser amounts of heavier elements, making them "metal poor," according to Kalarai. This also makes them consistent with "theoretical models of galaxy formation," he said, which tell us that halos are the oldest components of the galaxy.
The continual investigation of Andromeda’s halo, Kalarai continued, will shed new light on how similar large galaxies are formed.