The Lives of Galaxy Clusters: A Summary
Some of the big questions pondered by cosmologists today are about how the
universe formed: "If everything started with the Big Bang, when things
hot and very smooth, how did things become clumpy?" "How did stars start to
form?" "How did Galaxies start to form?" These questions are some of the most
basic, but also some of the hardest to answer. It has been billions of years since
the Big Bang, and even since the first galaxies formed. How can we look at what
we can see now and guess at what came before?
One place we look to for answers to these questions is clusters of galaxies.
Galaxy clusters are huge: some may be more than ten million light years across
and contain thousands of galaxies. In many cosmological models, these massive
clusters form from smaller, more common clusters of galaxies. If this is true then
galaxy clusters from long ago should be smaller on average than younger
clusters. The " mass function"
(which describes the number density of clusters as a function of
change in a particular way over time. But how can you look at clusters from
"long ago?" The answer is that because light travels at a finite speed, the light
from some things that we see now left them millions of years ago. And because
the Universe is expanding, light such as this is redshifted; the more redshift, the
longer the light has traveled to reach us. By observing very high redshift objects,
we can look back in time.
Of course, one problem is that it is not very easy to look at objects at very high
redshifts (they tend to be very dim). This is especially true if you need a large
statistical sample, such as one to make a general statement about a trend in the size
of galaxy clusters over long times. Such a "trend" is only believable if you
observe it for lots and lots of clusters.
Tackling the Question
Our research group uses X-ray observations to develop surveys of clusters of
galaxies and their total X-ray luminosity function (XLF). The total X-ray
luminosity function of a cluster is much easier to measure than its MF, but
it's a lot more
complicated to interpret. To measure the XLF of clusters, the Wide Angle
ROSAT Pointed Survey (WARPS (http://lheawww.gsfc.nasa.gov/~horner/warps/)) was begun at Goddard Space Flight Center.
WARPS covers a small area of the sky (and hence fewer rich clusters), but maps
down to very low luminosities. To compile this survey, we use data from the
What Do Cluster Surveys Show?
Another cluster XLF survey, the Einstein Extended Medium Sensitivity Survey
(EMSS), seems to show that there was almost no difference in the XLFs of
clusters with z > 0.3 and those with z < 0.3. At the high end of the
luminosity function, there was the hint of a trend towards fewer high-luminosity
(more massive) clusters at redshifts greater than 0.3. This is the opposite of what
we expected based on the simplest cosmological models, which predict that
there should be more high luminosity clusters at earlier times in the
What does the future hold? WARPS is an ongoing project and is only partially
completed. When finished, we can use it to quantify the evolution or lack thereof
in the XLF for low-luminosity clusters.
WARPS and surveys like it also provide a sample of clusters to use as input for
future studies by X-ray missions like Chandra X-ray Observatory and XMM. These mission will be
able to measure the X-ray temperatures of clusters even at high redshift.
Measuring the temperature of clusters will allow determination of the temperature
function (TF) of clusters. Temperature is much more simply related to mass than
luminosity and avoids many complications like energy injection by early
Comparing evolution of the high and low redshift TFs will allow for a
much better constrains on cosmological models.
Thank you to Don Horner (http://www.astro.umass.edu/~horner/) for contributing to this article.