Donald Savage<br />Headquarters, Washington May 7, 2003<br />(Phone: 202/358-1547)<br /><br />Ray Villard<br />Space Telescope Science Institute, Baltimore<br />(Phone: 410/338-4514)<br /><br />RELEASE: 03-160<br /><br />DEEPEST VIEW OF SPACE YIELDS YOUNG STARS IN ANDROMEDA HALO<br /><br /> Relying on the deepest visible-light images ever taken<br />in space, astronomers using NASA's Hubble Space Telescope<br />(HST) have reliably measured the age of the spherical halo of<br />stars surrounding the neighboring Andromeda galaxy (M31).<br /><br />To their surprise, they have discovered that approximately<br />one-third of the stars in Andromeda's halo formed only 6 to 8<br />billion years ago. That's a far cry from the 11-to-13<br />billion-year age of the stars in the Milky Way's halo.<br /><br />Why the difference in halo ages? You might call it a tale of<br />rich galaxy/poor galaxy. Apparently, M31 must have gone<br />through a major "corporate merger" with another large galaxy,<br />or a series of mergers with smaller galaxies, billions of<br />years ago. Astronomers cannot yet tell whether this was one<br />tumultuous event or a more continual acquisition of smaller<br />galaxies.<br /><br />The newly discovered younger stars in Andromeda's halo are<br />richer in heavier elements than the stars in our Milky Way's<br />halo, or in most of the small dwarf galaxies that surround<br />the Milky Way. Indeed the level of chemical enrichment seen<br />in these younger stars is characteristic of relatively<br />massive galaxies, containing at least a billion stars.<br /><br />This suggests three possibilities: (1) collisions destroyed<br />the young disk of M31 and dispersed many of its stars into<br />the halo; (2) a single collision destroyed a relatively<br />massive invading galaxy and dispersed its stars and some of<br />Andromeda's disk stars into the halo; and/or (3) many stars<br />formed during the collision itself.<br /><br />Astronomers say it will take more detailed observations to<br />unravel the "acquisition history" of these early cataclysmic<br />events. Located only 2.5 million light-years away, the<br />magnificent Andromeda galaxy, visible to the naked eye as a<br />spindle of light in the autumn sky, has long been considered<br />a near twin to our Milky Way in terms of size, shape and age.<br />This new finding promises to offer new clues about how giant<br />galaxies, like M31 and our Milky Way, formed by<br />gravitationally shredding galaxies, like a cosmic Cuisinart,<br />and then devouring them.<br /><br />Dr. Tom Brown of the Space Telescope Science Institute<br />(STScI) is reporting the findings today in Baltimore at the<br />STScI May Symposium, "The Local Group as an Astrophysical<br />Laboratory." His team used Hubble's Advanced Camera for<br />Surveys (ACS) to peer into a small sample of the Andromeda<br />halo for 120 Hubble orbits. This allowed for a study of the<br />entire demographics of the halo population, down to its<br />extremely faint stars.<br /><br />Previously, telescopes could only see the bright giant stars<br />in the halo population, but the population of "normal" stars<br />like our own sun was beyond our grasp, because such stars in<br />M31 are so faint. The ACS is the first astronomical camera to<br />combine ultra-sharp vision and sensitivity to ferret out<br />M31's faint halo population.<br /><br />An estimated 300,000 of these never-before-seen halo stars<br />can be resolved, peppering Hubble's narrow sample of the halo<br />population. Looking far beyond the halo stars, Hubble reveals<br />thousands of background galaxies (down to 31st magnitude)<br />billions of light-years away.<br /><br />A large fraction of the background galaxies in the image also<br />have peculiar shapes due to collisions. This reinforces the<br />fact: we live in a vibrant and dynamic universe undergoing<br />constant change.<br /><br />Electronic image files and additional information are<br />available on the Internet at:<br /><br />http://hubblesite.org/news/2003/15<br /><br />The Association of Universities for Research in Astronomy,<br />Inc. operates the STScI for NASA, under contract with the<br />Goddard Space Flight Center, Greenbelt, Md. The HST is a<br />project of international cooperation between NASA and the<br />European Space Agency.<br /><br />For information about NASA and space science on the Internet,<br />visit:<br /><br />http://www.nasa.gov