Mar. 4, 1999 ATHENS, Ohio -- An Ohio University astronomer and an international team of collaborators have obtained the clearest view yet of the center of the Andromeda galaxy, the nearest large galactic neighbor to our Milky Way. What they've learned may confirm a theory for the mystery surrounding the heart of this massive collection of stars, gas and dust.
From the first observations in the early 1970s and later views with the Hubble Space Telescope in the 1990s, astronomers have noted that Andromeda has a double nucleus -- or two points of brightness -- unlike the single nuclei observed in almost all other galaxies. It is a matter that has confounded astronomers for years. But Thomas Statler, Ohio University assistant professor of physics and astronomy, and his colleagues have studied new Hubble images of the center of Andromeda that may offer a simple explanation for the double cores.
Most galaxies, including the Milky Way, are believed to have a core that contains a massive black hole. Black holes have gravitational fields so strong that light cannot escape them. Even though a black hole is invisible, astronomers can detect black holes in the centers of galaxies from their gravitational effect on the millions of stars that orbit closely around them. If they are orbiting in a circular disk, as astronomers presume, the stars often look like one point of bright light.
But as past observations have suggested, Andromeda has two points of brightness at its center. Statler and his colleagues used the Hubble's Faint Object Camera to record the spectrum of the two nuclei. A spectrum, unlike a direct picture, splits the light into its component colors. Using this information, the astronomers were able to map the orbital motions of the stars around the center.
Their findings, published in the February issue of The Astronomical Journal, support a model that suggests stars in Andromeda are orbiting the galaxy's black hole in a lopsided path and are piling up -- sort of a cosmic traffic jam -- at the section of the orbit that is farthest away from the black hole.
"When stars swing closer to the center, they go faster. When they move away from it, they go slower. It's almost like you're getting a traffic jam at the slow section of the orbit," Statler says. "One of the bright spots in the nucleus would be the area where the stars are piling up, and the other marks where they rush through on their closest approach to the black hole."
The theory for Andromeda's center first was offered in 1995 by Scott Tremaine of the Canadian Institute of Theoretical Astrophysics, now at Princeton University.
Statler isn't sure how this arrangement of orbits around Andromeda's black hole could have developed. One possibility, he says, is that the powerful gravity from the black hole disrupted a passing cluster of stars. However, this concept conflicts with other aspects of his data, he says. New spectra taken by Hubble of the center of Andromeda later this year might clear up some of the inconsistencies, Statler says.
While researchers aren't clear why Andromeda is such an anomaly, this close look at its nucleus tells astronomers more about the nature of galaxies, Statler says, including our own. Even though Earth is part of the Milky Way galaxy, astronomers can't examine the center of the Milky Way as clearly as that of Andromeda's.
"In our galaxy, we're not able to see the forest for the trees," Statler says. "There's too much stuff in the way for us to see the nucleus."
Although Andromeda, known as M31, is the closest major galaxy nearest ours, it is so distant that it takes about 2 million years for light from Andromeda to reach us. It is faintly visible to the naked eye from Earth and is the largest member of the Local Group, a loose cluster of about 30 galaxies that includes the Milky Way. Like ours, it is a spiral galaxy, but it contains more stars, estimated in the hundreds of billions.
Astronomers first detected something strange in Andromeda's core in 1971 from images taken by a balloon-borne telescope. The double structure of the nucleus was established by Hubble in 1993, but it wasn't until December 1995 that the observations with Hubble's spectrograph, first planned by Ivan King of the University of California at Berkeley, were done. Statler led the project to analyze the spectrum shots.
Statler and his colleagues' research was supported by grants from NASA and the National Science Foundation. His research was co-authored by King, Philippe Crane of the European Southern Observatory and Dartmouth College and Robert I. Jedrzejewski of the Space Telescope Science Institute. Statler holds an appointment in the College of Arts and Sciences.
Attention reporters and editors: The journal article on which this story is based is available by calling Melissa Rake at 740-593-1891 or Kim Walker at 740-593-1043.
A photo of Andromeda is available courtesy of the National Optical Astronomy Observatories at http://www.noao.edu/image_gallery/html/im0424.html.
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