Astronomers find nearest galaxy to the Milky Way (Forwarded)

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Dr Rodrigo Ibata
Strasbourg Observatory, France
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Nicolas Martin
Strasbourg Observatory, France
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Dr Geraint Lewis
School of Physics, University of Sydney, Australia
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Dr Mike Irwin
Institute of Astronomy, University of Cambridge, UK
Phone: +44 (0)1223 337524
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Date: 4 November 2003

PN03-39

ASTRONOMERS FIND NEAREST GALAXY TO THE MILKY WAY

An international team of astronomers from France, Italy, the UK and Australia
has found a previously unknown galaxy colliding with our own Milky Way. This
newly-discovered galaxy takes the record for the nearest galaxy to the centre of
the Milky Way. Called the Canis Major dwarf galaxy after the constellation in
which it lies, it is about 25,000 light years away from the solar system and
42,000 light years from the centre of the Milky Way. This is closer than the
Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the
Milky Way. The discovery shows that the Milky Way is building up its own disk by
absorbing small satellite galaxies. The research is to be published in the
Monthly Notices of the Royal Astronomical Society within the next few weeks.

The discovery of the Canis Major dwarf was made possible by a recent survey of
the sky in infrared light (the Two-Micron All Sky Survey or "2MASS"), which has
allowed astronomers to look beyond the clouds of dust in the disk of the Milky
Way. Until now, the dwarf galaxy lay undetected behind the dense disk. "It's
like putting on infrared night-vision goggles," says team-member Dr Rodrigo
Ibata of Strasbourg Observatory. "We are now able to study a part of the Milky
Way that has been previously out of sight".

The new dwarf galaxy was detected by its M-giant stars -- cool, red stars that
shine especially brightly in infrared light. "We have used these rare M-giant
stars as beacons to trace out the shape and location of the new galaxy because
its numerous other stars are too faint for us to see," explains Nicolas Martin,
also of Strasbourg Observatory. "They are particularly useful stars as we can
measure their distances, and so map out the three-dimensional structure of
distant regions of the Milky Way disk." In this way, the astronomers found the
main dismembered corpse of the dwarf galaxy in Canis Major and long trails of
stars leading back to it. It seems that streams of stars pulled out of the
cannibalised Canis Major galaxy not only contribute to the outer reaches of the
Milky Way's disk, but may also pass close to the Sun.

Astronomers currently believe that large galaxies like the Milky Way grew to
their present majestic proportions by consuming their smaller galactic
neighbours. They have found that cannibalised galaxies add stars to the vast
haloes around large galaxies. However, until now, they did not appreciate that
even the disks of galaxies can grow in this fashion. Computer simulations show
that the Milky Way has been taking stars from the Canis Major dwarf and adding
them to its own disk -- and will continue to do so.

"On galactic scales, the Canis Major dwarf galaxy is a lightweight of about only
one billion Suns," said Dr. Michele Bellazzini of Bologna Observatory. "This
small galaxy is unlikely to hold together much longer. It is being pushed and
pulled by the colossal gravity of our Milky Way, which has been progressively
stealing its stars and pulling it apart." Some remnants of the Canis Major dwarf
form a ring around the disk of the Milky Way.

"The Canis Major dwarf galaxy may have added up to 1% more mass to our Galaxy,"
said Dr Geraint Lewis of the University of Sydney. "This is also an important
discovery because it highlights that the Milky Way is not in its middle age --
it is still forming." "Past interactions of the sort we are seeing here could be
responsible for some of the exquisite detail we see today in the structure of
the Galaxy," says Dr Michael Irwin of the University of Cambridge.

ILLUSTRATIONS

Graphics and animations may be found at
http://astro.u-strasbg.fr/images_ri/canm-e.html

NOTES

1. Other team members:
Dr Michele Bellazzini, Osservatorio Astronomico di Bolgna, Italy
Dr Walter Dehnen, Department for Physics and Astronomy, University of Leicester, UK

2. Publication
The paper reporting this work has been accepted by the Monthly Notices of the
Royal Astronomical Society. It is expected to be published within about 6 weeks
but at this time it has not been assigned to a particular issue of the journal.