Date: 2 January 2003                     Ref. PN 03/01

*EMBARGOED* For release 6 January 2003 at 9.30 a.m. PST (4.30 p.m. GMT)

Issued by: RAS Press Officer

Dr Jacqueline Mitton
Phone: +44 ((0)1223) 564914
Fax: +44 ((0)1223) 572892

RAS web:


A giant stellar structure surrounding the Milky Way

A vast, but previously unknown structure has been discovered around our own
Milky Way galaxy by an international team of astronomers. The announcement
is being made at the American Astronomical Society's meeting in Seattle,
Washington, on behalf of Drs Annette Ferguson, Rodrigo Ibata, Mike Irwin,
Geraint Lewis and Nial Tanvir. Their observations suggest that there is a
giant ring of several hundred million stars surrounding the main disk of the
Milky Way. Despite its size, the ring has not been clearly seen before since
the stars are spread around the whole sky, and are far fewer in number than
the tens of billions of stars making up the rest of the Galaxy.

What has made the new discovery possible are the first large area surveys of
the sky with sensitive CCD cameras. "Until now we haven't been able to see
the wood for the trees", said Rodrigo Ibata one of the team members. "Large
numbers of intervening stars, not to mention clouds of dust, makes it hard
to probe these regions." By comparing two major surveys covering different
regions of the sky, the team realised that they had evidence for what looks
like a complete ring of distant stars surrounding the outer disk of our

Although known to be warped, probably from encounters with its orbiting
satellite galaxies, the disk of the Milky Way was otherwise thought to be a
relatively simple structure. The disk is roughly 100,000 light years across,
with the Sun embedded in it and offset some 30,000 light years from the
centre. From this vantage point, the nearest edge of the ring is about
30,000 light years away, in the direction of the constellation Monoceros,
opposite the centre of the Galaxy. This region of sky is where traces of the
ring were first discovered.

Further detailed surveys in the constellation Andromeda showed that stars
belonging to the ring are visible 100 degrees away from the original
discovery site and that these stars closely mimic the vertical distribution
of the Milky-Way's so-called thick disk. Additional survey areas also
serendipitously yielded evidence of the ring's presence, allowing the
astronomers to get the first hints of the immense size of the structure.

The newly discovered ring seems roughly to encircle the disk, but is
considerably thicker, probably shaped like a giant doughnut. "We can't yet
be sure where it's come from", commented team member Geraint Lewis, "but the
stars themselves are clearly very old and one possible explanation is that
this is the debris of a small satellite galaxy of the Milky Way which has
been torn apart by tidal forces." "Another possibility is that the stars
originally came from the disk of our galaxy and their orbits have been
warped or spread over time so that they now wander far from the disk plane,"
added Annette Ferguson.

Ultimately, detailed studies of this kind of the structure of the Milky Way
and other galaxies, reveal how they came into being and have evolved over
the lifetime of the universe. If the old stars in this ring-like structure
are inherently part of the outer disk, they pose an interesting challenge
for galaxy formation models; alternatively if they are the remnants of a
disrupted satellite, they will provide a first-hand opportunity to study the
effects of massive accretions on the disks of large galaxies.


Dr Annette Ferguson, University of Groningen
+31 50 363 8324

Dr Rodrigo Ibata, Observatoire de Strasbourg
+33 3 90 24 23 91
+33 3 88 45 76 94 (home)

Dr Mike Irwin, University of Cambridge
+44 1223 337524

Dr Geraint Lewis, University of Sydney
+61 2 9351 5184

Dr Nial Tanvir, University of Hertfordshire
+44 1707 286299
+44 1763 241841 (home)


1. The time and date of release of this announcement (9.30 a.m. PST on 6
January 2003) are to coincide with a press conference on related work by
Newberg et al. at the American Astronomical Society meeting in Seattle,

2. A graphic showing the ring schematically in relation to the Milky Way
galaxy, and the text of this press notice, is available at

2. The research team acknowledges that its discoveries would not have been
possible without access to the Wide Field Camera on the 2.5-m Isaac Newton
Telescope on La Palma, in the Canary Islands and the foresight of the UK and
Dutch astronomical communities in promoting large scale surveys with this