X-Ray Observatory
reveals details of complex hot gas clouds
A NASA Chandra X-ray Observatory image has revealed a complex of
several intergalactic hot gas clouds in the process of merging. The
superb Chandra spatial resolution made it possible to distinguish
individual galaxies from the massive clouds of hot gas. One of the
clouds, which envelopes hundreds of galaxies, has an
extraordinarily low concentration of iron atoms, indicating that it
is in the very early stages of cluster evolution.
"We may be seeing hot intergalactic gas in a relatively pristine
state before it has been polluted by gas from galaxies," said Q.
Daniel Wang of the University of Massachusetts in Amherst, and lead
author on an upcoming Astrophysical Journal article describing the
study. "This discovery should provide valuable insight into how the
most massive structures in the universe are assembled."
The complex, known as Abell 2125, is about 3 billion light years
from Earth and is seen at a time about 11 billion years after the
Big Bang. This is a period when astronomers believe many galaxy
clusters are formed. Chandra's Abell 2125 image shows several huge
elongated clouds of multimillion-degree-gas coming together from
different directions. These hot gas clouds, each of which contains
hundreds of galaxies, appear to be in the process of merging to
form a single massive galaxy cluster.
Chandra, Hubble Space Telescope, and Very Large Array radio
telescope data show that several galaxies in the Abell 2125 core
cluster are being stripped of their gas as they fall through
surrounding high-pressure hot gas. This stripping process has
enriched the core cluster's gas in heavy elements such as iron.
The gas in the pristine cloud, which is still several million
light years away from the core cluster, is conspicuous for its lack
of iron atoms. This anemic cloud must be in a very early
evolutionary stage. The iron atoms produced by supernovas in the
embedded galaxies must still be contained in and around the
galaxies, perhaps in grains of dust not well mixed with the
observed X-ray-emitting gas. Over time, as the cluster merges with
the other clusters and the hot gas pressure increases, the dust
grains will be driven from the galaxies, mixed with the hot gas,
and destroyed, liberating the iron atoms.
Building a massive
galaxy cluster is a step-by-step enterprise that takes billions of
years. Exactly how long it takes for such a cluster to form
depends on many factors, such as the density of subclusters in the
vicinity, the rate of the expansion of the universe, and the
relative amounts of dark energy and dark matter.
Cluster formation also involves complex interactions between the
galaxies and the hot gas that may determine how large the galaxies
in the cluster can ultimately become. These interactions determine
how the galaxies maintain their gas content, the fuel for star
formation. The observations of Abell 2125 provide a rare glimpse
into the early steps in this process.
Frazer Owen (National Radio Astronomy Observatory) and Michael
Ledlow (Gemini Observatory) are co-authors on the upcoming
Astrophysical Journal paper. Chandra observed Abell 2125 with
its Advanced CCD Imaging Spectrometer on August 24, 2001, for
approximately 22 hours.