by Playfuls Staff |
3rd March 2007
There are many galaxies of different shapes and sizes around
us today. Roughly half are gas-poor elliptical-shaped galaxies with little new
star formation activity, and half are gas-rich spiral and irregular [more] galaxies
with high star formation activity. Observations have shown that gas-poor
galaxies are most often found near the centre of crowded galaxy clusters,
whereas spirals spend most of their lifetime in solitude.
The mystery, gleaned from deep observations of the Universe,
is that when the Universe was half its present age only one in five galaxies
was a gas-poor galaxy. So, where do all of today’s gas-poor galaxies come from?
Scientists suspect that some kind of transforming process must have taken
place, but because galaxy evolution occurs over billions of years, scientists
have so far not been able to see the transformation at work.
New observations with Hubble by an international team led by
Luca Cortese of Cardiff University,
United Kingdom,
provide one of the best examples to date of this metamorphosis. While looking
at the galaxy cluster Abell 2667, astronomers found an odd-looking spiral
galaxy (shown here in the upper left hand corner of the image) that ploughs
through the cluster after being accelerated to at least 3.5 million km/h by the
enormous combined gravity of the cluster’s dark matter, hot gas and hundreds of
galaxies. "By combining Hubble observations with various ground- and
space-based telescopes, we have been able to shed some light on the
evolutionary history of galaxies", said Cortese.
As the galaxy speeds through, its gas and stars are being
stripped away by the tidal forces exerted by the cluster – just as the tidal forces
exerted by the moon and Sun push and pull the Earth’s oceans. Also contributing
to this destructive process is the pressure of the cluster’s hot gas plasma
reaching temperatures as high as 10-100 million degrees.
Both processes – the tidal forces and the aptly named
"ram pressure stripping" resulting from the action of the hot cluster
gas – resemble those affecting comets in our Solar System. For this reason,
scientists have nicknamed this peculiar spiral with its tail the "Comet
Galaxy".
"This unique galaxy, situated 3.2 billion light-years
from Earth, has an extended stream of bright blue knots and diffuse wisps of
young stars driven away by the tidal forces and the ‘ram pressure stripping’ of
the hot dense gas," said Jean-Paul Kneib, a study collaborator from the
Laboratoire d'Astrophysique de Marseille.
"Millions of now homeless stars have been snatched away
from their mother galaxy, which will lead the galaxy to age prematurely,"
said co-investigator Giovanni Covone of Osservatorio Astronomico di
Capodimonte.
Even though its mass is slightly larger than that of the
Milky Way, the spiral will inevitably lose all its gas and dust as well as its
chance of generating new stars later, and become a gas-poor galaxy with an old
population of red stars. "However, in the midst of all this destruction,
the cluster’s strong forces have triggered a baby-boom of star formation,"
adds Covone.
Scientists estimate that the total duration of the
transformation process is close to one billion years. What is seen now in the
Hubble image is roughly 200 million years into the process.
The strong gravitational pull exerted by the galaxy
cluster’s collective mass has bent the light of other, more distant galaxies
and distorted their shapes – an effect called gravitational lensing. The giant
bright banana-shaped arc seen just to the right of the centre in the photo
corresponds to the magnified and distorted image of a distant galaxy that lies
behind the cluster’s core.
At the cluster’s centre another rare feature can be seen:
the vivid blue light from millions of stars created in a so-called cooling
flow. Some of the hot cluster gas is cooling in a filamentary structure as it
falls into the cluster’s core, setting off the birth of lots of bright blue
stars outshining their environment. This may be the clearest picture of this
phenomenon yet.
The Hubble image was taken by Hubble’s Wide Field and
Planetary Camera 2 in October 2001 and is a composite of three observations
through a blue filter (F450W, 12,000 seconds), a green filter (F606W, 4,000
seconds) and a near-infrared filter (F814W, 4,000 seconds). ESO’s Very Large
Telescope and the twin Keck Telescopes were used for optical spectroscopy and
near-infrared photometry, which helped determine the age of the star-forming
region. NASA’s Spitzer Space Telescope and Chandra X-Ray Observatory were used
jointly to confirm that the activity in the "Comet Galaxy" was due to
vigorous star-formation and not a super-massive black hole.
