Galaxy Clusters, Near and Far, Have A Lot in Common

Using two orbiting X-ray telescopes, a team of international astronomers
has examined distant galaxy clusters in order to compare them with their
counterparts that are relatively close by.

Speaking today at the RAS National Astronomy Meeting in Birmingham, Dr.
Ben Maughan (Harvard-Smithsonian Center for Astrophysics), presented the
results of this new analysis.

The observations indicate that, despite the great expansion that the
Universe has undergone since the Big Bang, galaxy clusters both local and
distant have a great deal in common.

This discovery could eventually lead to a better understanding of how to
“weigh” these enormous structures, and, in so doing, answer important
questions about the nature and structure of the Universe.

Clusters of galaxies, the largest known gravitationally-bound objects, are
the knots in the cosmic web of structure that permeates the Universe.
Theoretical models make predictions about the number, distribution and
properties of these clusters.

Scientists can test and improve models of the Universe by comparing these
predictions with observations. The most powerful way of doing this is to
measure the masses of galaxy clusters, particularly those in the distant
Universe. However, weighing galaxy clusters is extremely difficult.

One relatively easy way to weigh a galaxy cluster is to use simple laws
(“scaling relations”) to estimate its weight from properties that are easy
to observe, like its luminosity (brightness) or temperature. This is like
estimating someone’s weight from their height if you didn’t have any
scales.

Over the last 3 years, a team of researchers, led by Ben Maughan, has
observed 11 distant galaxy clusters with ESA’s XMM-Newton and NASA’s
Chandra X-ray Observatory. The clusters have redshifts of z = 0.6-1.0,
which corresponds to distances of 6 to 8 billion light years. This means
that we see them as they were when the Universe was half its present age.

The survey included two unusual systems, one in which two massive clusters
are merging and another extremely massive cluster which appears very
“relaxed” and undisturbed.

The X-ray data allowed the scientists to measure the temperatures and
luminosities of the gas in the clusters. They were then able to infer
their total masses, which varied between 200 and 1,100 times the mass of
our Milky Way galaxy.

These measurements were then used to test whether galaxy clusters of
different sizes and located at different distances from us are simply
scaled versions of each other — a condition known as being
“self-similar.” This is an important characteristic for astronomers to
identify if they hope to get the true weights of galaxy clusters.

“For example, chocolate bars are strongly self-similar,” said Maughan. “If
you shrank a king-size bar to a fun-size bar, they would be identical
versions of each other but just different sizes.”

“However, if you shrank a castle to the size of a bungalow, they would be
very different structures, despite being the same size. This means that
they are not strongly self-similar objects.”

Another possible type of relationship between clusters is what scientists
call “weakly self-similar.” In this case, galaxy clusters in the distant
universe and those nearby are almost identical to each other, but not
exactly the same. (The only differences between them can be accounted for
by the expansion of the Universe since the Big Bang.)

Although astronomers have known for some time that galaxy clusters are not
strongly self-similar, the question of whether or not they are weakly
self-similar has remained open.

The new results show that as long as astronomers take into account the
continuous expansion of the Universe, then galaxy clusters are, in fact,
weakly self-similar. This means that the same scaling relations used to
weigh nearby galaxy clusters hold true for these very distant clusters.

“Our results mean that weighing distant galaxy clusters could become as
easy as converting from Fahrenheit to Celsius,” said Maughan. “This will
help to answer important questions about the nature and structure of the
Universe.”

The other members of the team were: Laurence Jones (University of
Birmingham, UK)
Harald Ebeling (Institute for Astronomy, HI, USA), and Caleb Scharf
(Columbia Astrophysics Laboratory, NY, USA).

The observations were made with the European Photon Imaging Camera (EPIC)
on XMM and the Advanced Camera for Imaging and Spectroscopy (ACIS) on
Chandra. They were part of the WARPS survey of distant galaxy clusters
detected by chance in observations made with the UK-US-Dutch ROSAT X-ray
satellite.

CONTACT:

From Wednesday 6 April to Friday 8 April, Dr. Maughan can be contacted via
the NAM press office (see above).

Normal contact details:

Dr. Ben Maughan
High Energy Astrophysics Division
Harvard-Smithsonian CfA
Cambridge, MA 02138
USA
Tel: +1 617-496-1887
Fax: +1 617-496-7969
E-mail: bmaughan@cfa.harvard.edu

NOTES FOR EDITORS
The 2005 RAS National Astronomy Meeting is hosted by the University of
Birmingham, and sponsored by the Royal Astronomical and the UK Particle
Physics and Astronomy Research Council (PPARC).

LINKS AND AN IMAGE CAN BE FOUND ON THE WEB AT:
http://hea-www.harvard.edu/~bmaughan/scaling/press.html