Astronomers discover an enormous halo of red giant stars around Andromeda (Forwarded)

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University of California-Santa Cruz

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Tim Stephens (831) 459-2495

January 7, 2007

Astronomers discover an enormous halo of red giant stars around Andromeda

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
at the American Astronomical Society meeting in Seattle, suggests that
Andromeda is as much as five times larger than astronomers had previously
thought.

"I am absolutely astounded by how big this halo is. As we looked farther and
farther out, we kept finding stars that look like halo stars," said Puragra
(Raja) Guhathakurta, professor of astronomy and astrophysics at the
University of California, Santa Cruz, who will present the findings at the
meeting.

Guhathakurta and his collaborators at UCSC, UCLA, and the University of
Virginia are conducting an ongoing study of Andromeda's stellar halo, 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
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 probably represent Andromeda's
stellar halo, a distinct structural component of the galaxy that has eluded
astronomers for over 20 years, Guhathakurta said.

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, said Jasonjot Kalirai, a
postdoctoral fellow at UCSC. 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, Kalirai said.

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 30 smaller galaxies.

"The physical size of this galaxy is really striking," said coauthor R.
Michael Rich of UCLA. "The suburbs of M31 and the Milky Way are so extended
that they nearly overlap in space, despite the great distance between these
two galaxies. If the whole of M31 were bright enough to be visible to the
naked eye, it would appear to be huge, larger in apparent size than the Big
Dipper."

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, according to Guhathakurta.

"We now believe that previous groups have been mistakenly identifying the
outer parts of the Andromeda bulge as its halo," he said.

Guhathakurta's group was able to detect the halo by developing a
sophisticated technique for clearly distinguishing halo stars in Andromeda
from the more numerous foreground stars in the Milky Way. A foreground star
with low luminosity and a luminous star that is much farther away can be
hard to tell apart because they appear to be equally bright from our
perspective, Guhathakurta said.

"A firefly 10 feet away and a powerful beacon in the distance can have the
same apparent brightness. In this case, the fireflies are dwarf stars in our
own galaxy and the beacons are red giant stars in Andromeda," he said.

Karoline Gilbert, a UCSC graduate student, developed the technique for
separating the fireflies from the beacons. Her technique provided a clear
separation between the two populations of stars by combining five diagnostic
criteria based on photometry (brightness measurements) and spectroscopy
(which separates starlight into a spectrum of different wavelengths). The
diagnostic criteria include radial velocity and parameters based on
differences in surface gravity between red giants and dwarf stars.

"We focused on detecting red giant stars in the halo because they are bright
enough for us to obtain spectra," Gilbert said. "There are assuredly other
kinds of stars in Andromeda's halo, but they are just too faint for us to
get spectra of them."

In addition to Gilbert, Guhathakurta, Kalirai, and Rich, the other
collaborators include Steven Majewski, James Ostheimer, and Richard
Patterson at the University of Virginia and David Reitzel at UCLA.

The group's ongoing investigation of Andromeda's halo promises to shed new
light on the question of how large galaxies formed, Guhathakurta said.

"Galaxy formation theories tell us that halos are pristine -- the oldest
component of the galaxy -- but this is based almost entirely on studies of
our own galaxy. A detailed study of this newly discovered Andromeda halo
will allow us to test whether these theories apply more generally to
galaxies other than the Milky Way," he said.

Note to reporters: You may contact Guhathakurta at (831) 459-5169; Kalirai
at (831) 459-3804; and Gilbert at (831) 459-3259.