The Milky Way's centre is the busy core of a metropolis, crowded
with huge populations of stars frantically dancing to the rhythm of
gravitation. These stars are precious for astronomers: they hold many
clues to unveil the past and future history of our galaxy. But the
galactic centre has remained a fairly unexplored place so far, due to
the thick dust covering it. The European Space Agency's infrared space
telescope, ISO, has crossed that dusty barrier and has observed the
stellar populations at the galactic centre with a very high resolution
during more than 255 hours. The results already show 100 000 stars
never seen before. Further analysis of the data could confirm that the
Milky Way swallowed neighbouring galaxies in the past.
The Milky Way is a large spiral galaxy 130 000 light-years
across, which began to form about 10 000 or 15 000 million years
ago - shortly after the origin of the Universe. It is structured
in a thin disk with spiral arms and a great bulge in the centre,
which as seen from the Earth lies towards the constellation of
Sagittarius. Our Solar System is in the edge of one of the arms,
about 25 000 light-years from the centre: a very quiet area
compared to the inner central bulge.
"The inner bulge of the Milky Way is like the core of a very
busy metropolis. The density of stars is 500 times larger than
elsewhere in the galaxy - stars can even bump into each other!.
These populations of stars give us a lot of information about the
whole galaxy. For example, their relative motions might reveal
traces of other galaxies devoured by our own in the past", says
Alain Omont, at the Institut d'Astrophysique de Paris.
Despite its interest, current knowledge about the centre of the
Milky Way is far from complete because the dust enshrouding it has
blocked the view of most telescopes so far. Only ESA's ISO, the
first space observatory working at infrared wavelengths - and
hence able to see through the dust - has performed a very deep
exploration of its stellar populations. One of ISO's longest
observing programme, ISOGAL, has devoted 255 hours to this aim,
focusing especially on the inner central bulge. The first results
from this programme, a joint effort by astronomers from France,
the UK, Holland, Italy, Germany, Spain, Sweden, India, South
Africa, Chile and the US are already being published in the
100 000 red giants newly identified
In a region of the galactic centre that as seen from Earth is
only about four times the angular size of the full moon, ISO has
identified a population of more than 100 000 stars of the 'red
giant' type. Most of them are the so-called AGB (Asymptotic Giant
Branch) stars, which for astronomers adds value to the finding.
AGB stars are very evolved stars that provide key clues to
unveil the star-formation history of the Milky Way, because their
masses vary according to their age; it is therefore easy to
determine how long ago a certain population of AGB stars was born.
Also, AGB stars are special because they are one of the main
dust-producing factories of the galaxy. They expel huge amounts of
dust to the environment during a brief stage of their lives - dust
in which many chemical elements, including those essential for
life - are present.
"These stars enrich the galaxy with the chemical compounds
present in their gas and dust. Even some of the carbon out of
which we are made comes from them", Omont says. "And they give us
clues about the star-formation history of our galaxy, because we
know their age. For instance, knowing that some of them formed a
few thousand million years ago will allow us to infer the
star-formation efficiency at that time".
The coordinates of the newly identified stars will be published
soon in a new catalogue.
To Martin Kessler, ISO project scientist, "these results are an
excellent example of ISO's pioneering role in unveiling a Universe
that is hidden by dust and can only be seen in infrared light.
ISO's continuing discoveries are laying the foundations on which
all future infrared space missions will build."
Traces of 'galactic cannibalism'
Studying the stars in the galactic centre gives also
information about how the galaxy formed. Astronomers would like to
know, for example, the origin of the extremely dense group of
stars in the central inner bulge: is it a remnant of the original
core around which the galaxy grew, or is it the result of a past
collision with other small galaxies? The way to find out is by
measuring the stellar motions, which are especially frantic in the
centre of the galaxy due to the strong pull of the gravitation. If
a star comes from a galaxy devoured by our own in the past, it
will probably move differently than most of its companions.
As Omont explains, "We still don't know the details about the
formation of the Milky Way. For instance we cannot yet fully
confirm that our galaxy grew by 'swallowing' dwarf galaxies
attracted by its gravitational force. We expect that stars from
these smaller galaxies can still be found as they will move in
different directions from the others".
Galactic 'cannibalism' and collisions have been identified as
one of the most common processes driving galaxy evolution.
Observations during the past years have in fact shown that the
Milky Way is at present strongly attracting smaller neighbour
galaxies, such as the Magellanic Clouds, and that it could collide
with the closest large galaxy, Andromeda,in a time as short as 3
ISOGAL observations were complemented with those made within
the international DENIS project, a near infrared survey of the
Southern sky using a 1-m telescope at La Silla (Chile), of the
European Southern Observatory.
Footnote on ISO
The European Space Agency's infrared space telescope, ISO,
operated from November 1995 till May 1998, almost a year longer
than expected. As an unprecedented observatory for infrared
astronomy, able to examine cool and hidden places in the Universe,
ISO successfully made nearly 30 000 scientific observations. /td>