Astronomers Discover Halo of Red Giant Stars Around Andromeda

Jan. 9, 2007 -- U.Va. astronomers have found an enormous halo of stars bound to the Andromeda galaxy and extending far beyond the swirling disk seen in images of the famous galaxy, our nearest large galactic neighbor. The discovery, reported Jan. 7 at the semi-annual American Astronomical Society meeting in Seattle, suggests that Andromeda is as much as five times larger than astronomers had previously thought.

“It’s truly remarkable how big a stellar structure this Andromeda halo is,” said Steven Majewski, professor of astronomy at the University of Virginia. “What’s even more extraordinary is that as we keep looking farther and farther out from the Andromeda center, we keep identifying stars that belong to it. It’s not clear that we have even reached the outer limits of the Andromeda halo yet.”

Majewski and U.Va. senior scientist Richard Patterson, graduate student (now Ph.D.) James Ostheimer, and undergraduate student Rachael Beaton, and collaborators at the University of California Santa Cruz and UCLA, are conducting an ongoing study of Andromeda's outer structure, using observations at the Kitt Peak National Observatory in Arizona and the W. M. Keck Observatory in Hawaii. Their new findings are based on data gathered over the past decade using the 4-meter Mayall Telescope at Kitt Peak and the DEIMOS spectrograph on the 10-meter Keck II Telescope in Hawaii.

The researchers detected a sparse population of red giant stars—bright, bloated stars in a late stage of stellar evolution—that appear to be smoothly distributed around the galaxy out to a distance of at least 500,000 light-years from the center. Even at that great distance, the stars are bound to the galaxy by gravity. These stars represent Andromeda's stellar halo, a distinct structural component of the galaxy that has eluded astronomers for decades.

Following up on their discovery of Andromeda's halo, the researchers have found evidence that stars in the halo are chemically anemic compared with stars in the inner parts of the galaxy. The halo stars are "metal-poor," meaning they contain smaller amounts of the heavier elements, a finding that is consistent with theoretical models of galaxy formation.

Andromeda (also known as M31) is a large spiral galaxy very similar to our own Milky Way. While it is difficult for astronomers to study the overall structure of the Milky Way from Earth's vantage point within it, Andromeda offers a global view of a classic spiral galaxy that is close enough for astronomers to observe individual stars within it. Andromeda is about 2.5 million light-years from Earth and is the largest galaxy in the "Local Group," which also includes the Milky Way and about 40 smaller known galaxies.

“The physical size of M31 is remarkable,” Patterson noted. “This new result suggests that the outer parts of Andromeda are so extended, that they nearly overlap the outer parts of our own Milky Way, despite the enormous distance separating the two galaxies. To give some sense of the angular size of this newly found structure on the sky, imagine a circular structure large enough on the sky to cover the Big Dipper, that is, a circle with a diameter of 46 full moons placed side-by-side.”

Spiral galaxies typically have three main components: a flattened disk, a bright central bulge with a dense concentration of stars, and an extended spherical halo of sparsely distributed stars. The concentration of stars in the central bulge decreases exponentially with increasing distance from the center, whereas the density of the halo stars falls off more gradually (as an inverse power of the radius).

In Andromeda, the disk has a radius of about 100,000 light-years. Outside the plane of the disk, stars plausibly belonging to the central bulge can be found as far out as 100,000 light-years from the center of the galaxy, while the halo extends five times farther than that. One important clarification that the new work brings is that previous groups working closer to the M31 center have been mistakenly identifying the outer parts of the Andromeda galaxy’s central bulge as its halo.  This mistake has left them puzzling over why this previously suspected “halo” is so unlike that of our own Milky Way. The new research makes evident that Andromeda, in fact, has a true halo that is very similar to our own galaxy’s halo after all.

The researchers were  first able to detect the halo using a sophisticated technique developed at U.Va. for clearly distinguishing halo stars in Andromeda from the more numerous foreground stars in the Milky Way. A low luminosity, red dwarf star from our own galaxy and a high luminosity, red giant star much farther away in the Andromeda galaxy can be difficult to tell apart because they appear to be equally bright from our perspective. To cope with this problem, special light filters were used to image the outer parts of Andromeda at very specific colors of light that can give information on the true brightnesses, and therefore distances, of the stars. 

The inspiration to look for an extended population of M31 stars actually came from a U.Va. undergraduate researcher working in the Majewski-Patterson group.  Majewski recalls, “while we were developing our special filtering technique for other purposes, Jamie [Ostheimer] came to me with this bold, exciting idea to use the technique to probe parts of the Andromeda galaxy well beyond where people had studied before.  Apparently the National Optical Astronomy Observatories liked the idea too, because they granted us time on their telescopes in 1998 to make the initial probes.”  The experiment, which subsequently became Ostheimer’s Ph.D. dissertation project, completed in 2002 at U.Va. yielded strong evidence that extremely extended populations of Andromeda halo stars were there. 

From there, the U.Va. group teamed up with astronomers at UCSC and UCLA to use the powerful Keck telescope in Hawaii to get spectroscopic proof. The positive results of that study were announced at the meeting on Sunday by study coauthors Puragra (Raja) Guhathakurta of UCSC and R. Michael Rich of UCLA. Other study participants include Jasonjot Kalirai and Karoline Gilbert of UCSC and David Reitzel at UCLA.

The group's ongoing investigation of Andromeda's halo paints a new picture of our nearest large galactic neighbor and promises to shed new light on the question of how large galaxies formed, because galaxy halos generally represent the oldest, most pristine parts of galaxies. Paul Hodge from the University of Washington, an expert on the Andromeda galaxy who was not involved with the study, reported his impressions of the work on “It’s a new galaxy.  The outer parts of [M31] are finally being revealed and it’s turning out to be much more interesting and beautiful than we could have imagined.”