July 2010 - Swinburne University of Technology

swinburne JULY 2010
ASTRONOMY
Our cannibal galaxy
In a fascinating new piece of ‘galactic archaeology’
astronomers have found that up to one-quarter of the Milky Way’s
galactic clusters are intruders BY JULIAN CRIBB
Key points
18 TO SEE ITS STARFIELDS hung in radiant group of star clusters, averaging about elements, pointing to formation processes
Astronomers have long
suspected that galaxies
comprise remnants of other,
earlier galaxies.
New research has found
that a quarter of the Milky
Way’s galactic clusters
were ‘born’ elsewhere and
at a different time from the
majority of other clusters.
Some clusters are as young
as two billion years – less
than half the age of the
Earth.
splendour across the night sky, you would
scarcely suspect the Milky Way of being a
cannibal, a gigantic buzz-saw of a galaxy
that has chopped its neighbours into bits and
ingested their fragments into its own clouds
of stars.
But that is what a fascinating new piece
of ‘galactic archaeology’ by Swinburne
astrophysicist Professor Duncan Forbes and
his colleague Dr Terry Bridges, of Queen’s
University, Canada, has revealed.*
In a painstaking analysis of the age and
metallic composition of almost 100 galactic
clusters – groups of one million or so stars
– out of the 160 clusters that comprise our
galaxy, the researchers conclude that up to a
quarter are aliens … born elsewhere and at
a different time to the majority of clusters in
our own Milky Way.
Astronomers have long suspected that
the galaxies we see today are accretions
consisting of the remnants of other galaxies,
but the true extent of this intergalactic
churning and star-exchange has never before
been quite so evident. In fact, say Professor
Forbes and Dr Bridges, about a quarter of
the galactic clusters in the Milky Way today
may themselves be the remnants of between
six and eight smaller galaxies which it has
chewed up and partially absorbed over time.
Investigating the age and iron abundance
of each cluster, the team detected two
distinctive signatures – that of the main
12.8 billion years of age that make up the
bulk of the Milky Way, and of a second
group, which are significantly younger.
Notable among the youngsters are clusters
with a strong family resemblance to the
remnants of the dwarf galaxies Sagittarius
and Canis Major, both of which appear to
have been torn apart by the vast tidal forces
of gravity in the past. It now seems they
passed close enough to the Milky Way, the
dominant local galaxy, for it to rob them of
large clusters of their stars, some of which
are only half the general age of the clusters
in our galaxy.
“A great circle in the sky connects
the Fornax, Leo (I and II) and Sculptor
galaxies,” Professor Forbes says. “One
possibility is that some clusters were tidally
stripped from the Fornax galaxy as it
crossed the orbit of the Milky Way. Another
possibility is that these clusters came from
the remains of a completely disrupted dwarf
galaxy that was torn to pieces.”
Another set of star clusters in our Milky
Way exhibit a contrary motion to most of the
others, making it quite likely they have been
drawn in from outside.
The cluster known as Omega Centaurus
and another called M54, may indeed be the
remnant nuclei of ruined dwarf galaxies
which our own has devoured.
Yet another group of star clusters has
signatures unusually rich in helium and other
somewhat different to those of clusters in our
galaxy as a whole.
The team found some galactic clusters
as young as a mere two billion years – less
than half the age of the Earth – suggesting
that the process of disruption and accretion is
proceeding more or less continuously, as star
clusters are born from gas clouds in dwarf
galaxies past which the Milky Way hurtles,
absorbing some of them on its journey.
Similar star-stripping it appears is now
starting to befall the Magellanic Clouds,
large and small, which have approached
close enough to the Milky Way to be feeling
the power of its gravitational hunger,
Professor Forbes says. And a vast event, the
collision of the Milky Way with the giant
spiral galaxy Andromeda, is due to take
place in five billion years from now.
“The universe seems in some ways
to be a very violent place, with all these
interactions, mergers and collisions taking
place, as the giant galaxies cannibalise the
smaller ones. But on the other hand, so
vast are the distances that even when two
galaxies collide the stars do not come into
contact with one another, although they
are subject to each other’s gravitational
influence.”
Instead, he says, astronomers speculate
these mergers may bring about the change
from the classic spiral-shaped galaxy to the
larger and more chaotic elliptical form – and