Newly Discovered Olympian Galaxy Will Provide Fresh Insights into Galactic Formation
- From: baalke@xxxxxxxxxxxxx
- Date: 29 May 2007 11:39:07 -0700
Caltech News Release
For Immediate Release
May 29, 2007
Newly Discovered Olympian Galaxy Will Provide
Fresh Insights into Galactic Formation
PASADENA, Calif.--A newly discovered dwarf galaxy
in our local group has been found to have formed
in a region of space far from our own and is
falling into our system for the first time in its
history.
The dwarf is formally known as Andromeda XII
because it is the 12th dwarf galaxy associated
with Andromeda, our nearest galactic neighbor.
The discoverers have nicknamed it the Olympian
Galaxy after the 12 Olympian gods in the Greek
pantheon. The discovery was made possible with
data obtained at the W. M. Keck Observatory atop
Mauna Kea, Hawaii.
According to Andrew Blain, an astronomer at the
California Institute of Technology and a member
of the discovery team, the Olympian Galaxy marks
the best piece of evidence that at least some
small galaxies are just now arriving in our local
group, which primarily includes the Milky Way,
Andromeda, and various dwarf galaxies in the
vicinity of both. The finding provides an
important test for simulations of galaxy
formation.
Dwarf galaxies and streams of stellar material
mark the visible remnants of galactic merging
events from which large galaxies are made.
Cosmology models predict that small galaxies form
along a web of filamentary structures in the
universe and then gradually fall into dense
groups and cluster environments. Small galaxies
should still be falling into the local group, yet
none have been found--until now.
"Other local group dwarf galaxies are thought to
have extreme orbits, including Leo I, Andromeda
XIV and Andromeda XI, but the Olympian Galaxy
really stands out as a contender for a new
entrant into the local group," says the lead
author of the study, Scott C. Chapman of the
University of Cambridge Institute of Astronomy.
"The others have likely already been seriously
harassed by Andromeda and the Milky Way."
The Olympian Galaxy was first discovered in
October 2006 during a wide-field survey taken
with the Canada-France-Hawaii Telescope's MegaCam
instrument. It is the faintest dwarf galaxy ever
discovered near Andromeda (also known as M31),
and may have the lowest mass ever measured. Dwarf
galaxies are the smallest stellar systems showing
evidence for a substantial amount of dark matter.
Chapman's observations confirmed that the
Olympian Galaxy is distinct from all other
satellite galaxies in the local group. It is a
fast-moving galaxy on a highly eccentric orbit,
located at a great distance-about 115 kiloparsecs
(375,000 light-years)-from the center of M31.
Importantly, the Olympian Galaxy lies
significantly behind M31 as viewed from the Milky
Way, which indicates that it is almost certainly
falling in for the first time. Because the dwarf
galaxy has lived its life in a very different
environment from that of the local group, it
gives astronomers a pristine object for studying
star-formation histories, dark-matter
distribution, and other parameters that would be
influenced by the local-group gravity that has
affected other dwarf galaxies.
"The Olympian Galaxy may be the first galaxy of
the local group ever observed that has not yet
been disrupted by the strong gravity of the local
group," says Jorge Penarrubia of the University
of Victoria, a coauthor of the study.
The Deep Extragalactic Imaging Multi-Object
Spectrograph (DEIMOS) at Keck II, one of two
10-meter telescopes the W. M. Keck Observatory
operates on the summit of Mauna Kea, was key in
making the discovery. It was used to observe 49
stars in the region of the Olympian Galaxy, and
confirmed that eight were members of the new
dwarf galaxy. Follow-up observations were also
conducted at the Green Bank Telescope in West
Virginia to measure the amount of interstellar
gas in the galaxy, and the Subaru telescope in
Hawaii helped determine a more precise distance.
"Without the spectra we obtained with DEIMOS, it
would have been impossible to make any useful
claims about the orbit of the Olympian Galaxy,
its evolution, its speed, or its dark-matter
content," adds Chapman.
The Olympian Galaxy is falling very quickly
through the local group from behind Andromeda,
and is the only one of Andromeda's satellites
that exceeds the apparent escape velocity for
Andromeda. It is possible that the Olympian
Galaxy may be just a short-term visitor. It is
such a low-mass galaxy that it may not slow down
much as it passes through the local group.
"It is a pleasure to see the speed of this new,
fascinating member of the local group clocked
using Keck II and DEIMOS," adds W. M. Keck
Observatory director Taft Armandroff. "The
powerful combination of Keck and DEIMOS has added
many contributions to our understanding of
local-group galaxies."
The age of the universe is not old enough for the
Olympian Galaxy to have started in the dense
local group and be on its second trip through our
system. The dwarf galaxy probably formed in a
dense filament structure, toward the general
direction of the M81 group. However, the distance
to that group is about three times too far for
the galaxy to have actually come from there. A
likely scenario is that the Olympian Galaxy
formed in a filamentary region of space that
connects the local group to the M81 group.
"The high speed really surprised me; I wasn't
expecting to see any of our newly discovered
dwarfs moving so fast. We will likely have to
revise our mass estimates of Andromeda upward as
a result," adds Rodrigo Ibata, another author of
the study.
A paper reporting the discovery, "Strangers in
the Night: The Discovery of a Dwarf Spheroidal
Galaxy on its First Local Group Infall," will
appear in an upcoming issue of the Astrophysical
Journal. Funding was provided by a fellowship
from the Canadian Space Agency and the NaturalSciences and Engineering Research Council of
Canada. Additional support was provided by Adrian
Jenkins, who made available important computer
simulations.
The study was coauthored by Jorge Penarrubia,
Alan McConnachie, and Aaron Ludlow of the
University of Victoria; Rodrigo Ibata,
Observatoire de Strasbourg; Nicolas F. Martin,
Max-Planck-Institut für Astronomie; Andrew Blain
and Bruno Letarte, California Institute of
Technology; Michael Irwin, University of
Cambridge Institute of Astronomy; Geraint Lewis,
University of Sydney Institute of Astronomy; Fred
Lo and Karen O'Neil, NRAO Green Bank Telescope.
The W. M. Keck Observatory is operated by the
California Association for Research in Astronomy
(CARA), a nonprofit 501 (c) (3) corporation whose
governing board includes directors from the
California Institute of Technology and the
University of California. In addition, the
National Aeronautics and Space Administration and
the W. M. Keck Foundation each have liaisons to
the board. Construction of the twin Keck
telescopes and domes was made possible with
generous grants totaling more than $140 million
from the W. M. Keck Foundation in Los Angeles.
Contact: Robert Tindol
(626) 395-3631
tindol@xxxxxxxxxxx
.
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