Case Western Reserve University Astronomers Discover New Galaxy Orbiting Andromeda

CLEVELAND–Case Western Reserve University astronomers have
announced the discovery of a new galaxy, termed Andromeda VIII. The new
galaxy is so widespread and transparent that astronomers did not suspect
its existence until they mapped the velocity of stars thought to belong to
the well-known and nearby large Andromeda spiral galaxy and found them to
move independently of Andromeda.

Heather Morrison, Paul Harding and Denise Hurley-Keller of Case’s
department of astronomy and George Jacoby of the WIYN Observatory, will
report their discovery in an upcoming article in Astrophysical Journal
Letters.

“This is particularly exciting because it allows us to watch the
ongoing growth of the nearby Andromeda galaxy from smaller galaxies,” says
Morrison.

The astronomers used Case’s Burrell Schmidt telescope and the 3.5m WIYN
telescope to identify the galaxy. Both telescopes are located at Kitt
Peak National Observatory near Tucson, Ariz. NOAO is operated by the
Association of Universities for Research in Astronomy (AURA) Inc., under
a cooperative agreement with the National Science Foundation.

The newly found galaxy is being torn apart into streams of stars,
which leaves a trail of stars that are strung out along the new galaxy’s
orbit around the Andromeda galaxy in the way a jet’s contrail shows its
route. Andromeda is the nearest large spiral galaxy to our own Milky Way
galaxy two million light years away. It is visible as a hazy glowing object
to the naked eye in a dark sky in the northern hemisphere and is found in
the constellation of Andromeda.

Discovered over 1,000 years ago by the Persian astronomer Azophi Al-Sufi,
Andromeda is a member of the Local Group of approximately 30 galaxies in
the Milky Way’s celestial backyard.

In early August, Morrison finished analyzing the data of these
stars from the Andromeda celestial neighborhood. “I was amazed to find a
new dwarf galaxy orbiting Andromeda. It is a ‘see-thru’ galaxy, which was
only discovered once we obtained velocity measurements for some of its
stars, said Morrison.

She adds that the reason Andromeda VIII escaped detection was the
fact that it is located in front of the bright regions of Andromeda’s
galaxy disk.

Andromeda VIII’s total brightness is comparable to that of
Andromeda’s well-known companion M32, a small nearby galaxy, but Andromeda
VIII is spread over an area of the sky as much as ten times or more larger
than M32. Its elongated shape is caused by Andromeda’s gravitational pull,
which has stretched it out due to the stronger gravity on the side nearest
Andromeda.

Morrison and her collaborators also suggested that a very faint
stream of stars, detected near the large Andromeda galaxy in 2001 by the
Italian Astronmer R. A. Ibata and colleagues, was pulled off Andromeda VIII
in an earlier passage around the parent galaxy. “Future research in this
area should provide rich and fruitful results,” stated Morrison.

Theory has predicted for decades that galaxies are assembled in a
“bottom-up” process, forming first as small galaxies that later merge to
form large ones.

“Since 1994, when Ibata and colleagues announced the discovery of a new
satellite in the process of being swallowed by the Milky Way, we have
been able to see the process taking place in our own galaxy,” stated
Morrison. “Now we find the same process in our nearest large neighbor.”

She adds that now it looks like Andromeda is even more inundated by small
galaxies than the Milky Way. Ibata and colleagues have taken deep images
of Andromeda which show a rich collection of star streams wreathed
about the galaxy. Morrison and her colleagues have now identified the
source of one of these star streams. They plan future observations to
connect the different star streams with their progenitors, and thus learn
more about the properties of the companion galaxy, the Andromeda galaxy and
its elusive dark matter halo, the unseen matter that is suspected to be
present in the universe.

The galaxy research was supported by a five-year National Science
Foundation Early Career Development Award.

The Burrell Schmidt telescope is part of Case’s Warner and Swasey
Observatory. The WIYN 3.5-meter telescope is a partnership of the
University of Wisconsin, Indiana University, Yale University and the
National Optical Astronomy Observatory (NOAO). NOAO is operated by the
Association of Universities for Research in Astronomy (AURA) Inc., under a
cooperative agreement with the National Science Foundation.

— An image of Andromeda with the position of Andromeda VIII marked in
graphics by Case student Nancy Lin over a photo by Robert Gensler is
available at http://smaug.cwru.edu/heather/stars.jpg.

— A movie which shows a large galaxy tearing apart a smaller one in
the way that the Andromeda spiral is tearing apart Andromeda VIII is
available at http://www.astro.wesleyan.edu/~kvj/mw.html (Movie credit:
Kathryn Johnson).