Relying on the deepest visible-light images ever taken in space,
astronomers using NASA’s Hubble Space Telescope (HST) have reliably
measured the age of the spherical halo of stars surrounding the
neighboring Andromeda galaxy (M31).

To their surprise, they have discovered that approximately one-third of
the stars in Andromeda’s halo formed only 6 to 8 billion years ago.
That’s a far cry from the 11-to-13-billion-year age of the stars in the
Milky Way’s halo.

Why the difference in halo ages? You might call it a tale of rich
galaxy/poor galaxy. Apparently, M31 must have gone through a major
“corporate merger” with another large galaxy, or a series of mergers
with smaller galaxies, billions of years ago. Astronomers cannot yet
tell whether this was one tumultuous event or a more continual
acquisition of smaller galaxies.

The newly discovered younger stars in Andromeda’s halo are richer in
heavier elements than the stars in our Milky Way’s halo, or in most of
the small dwarf galaxies that surround the Milky Way. Indeed the level
of chemical enrichment seen in these younger stars is characteristic of
relatively massive galaxies, containing at least a billion stars.

This suggests three possibilities: (1) collisions destroyed the young
disk of M31 and dispersed many of its stars into the halo; (2) a single
collision destroyed a relatively massive invading galaxy and dispersed
its stars and some of Andromeda’s disk stars into the halo; and/or (3)
many stars formed during the collision itself.

Astronomers say it will take more detailed observations to unravel the
acquisition history of these early cataclysmic events. Located only 2.5
million light-years away, the magnificent Andromeda galaxy, visible as a
naked-eye spindle of light in the autumn sky, has long been considered a
near twin to our Milky Way in terms of size, shape, and age. This new
finding promises to offer new clues about how giant galaxies like M31
and our Milky Way formed by gravitationally shredding galaxies, like a
cosmic Cuisinart, and then devouring them.

Dr. Tom Brown of the Space Telescope Science Institute (STScI) is
reporting the findings today in Baltimore at the STScI May Symposium,
“The Local Group as an Astrophysical Laboratory.” His team used Hubble’s
Advanced Camera for Surveys (ACS) to peer into a small sample of the
Andromeda halo for 120 Hubble orbits. This allowed for a study of the
entire demographics of the halo population, down to its extremely faint
stars.

Previously, telescopes could only see the bright giant stars in the halo
population, but the population of “normal” stars like our own Sun was
beyond our grasp, because such stars in M31 are so faint. The ACS is the
first astronomical camera to combine ultra-sharp vision and sensitivity
to ferret out M31’s faint halo population.

An estimated 300,000 of these never-before-seen halo stars can be
resolved, peppering Hubble’s narrow sample of the halo population.
Looking far beyond the halo stars, Hubble reveals thousands of
background galaxies (down to 31st magnitude) billions of light-years
away.

A large fraction of the background galaxies in the image also have
peculiar shapes due to collisions. This reinforces the fact: we live in
a vibrant and dynamic universe undergoing constant change.

Electronic image files and additional information are available at
http://hubblesite.org/news/2003/15

The Association of Universities for Research in Astronomy, Inc. (AURA)
operates the Space Telescope Science Institute (STScI) for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble
Space Telescope is a project of international cooperation between NASA
and the European Space Agency (ESA).