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
25000 light years away from thesolar system and 42000 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.


Graphics and animations may be found at


Dr Rodrigo Ibata – Strasbourg Observatory, France.
Phone. (+33) (0)3 90 24 23 91 (office) or (+33) (0) 3 88 45 76 94

Nicolas Martin Strasbourg Observatory. France.
Phone (+33). (0)3 90 24 24 88

Dr Geraint Lewis, School of Physics, University of Sydney, Australia.
Phone: (+61) 9351 5184

Dr Mike Irwin, Institute of Astronomy, University of Cambridge, UK.
Phone: +44 (0)1223 337524


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.