Colliding Galaxies Create Cosmic "Cold Front"
Chandra X-ray Observatory Maps Pressure Fronts
When you think of a cold front, you probably imagine a temperature
drop of five or ten degrees. Astronomers and astrophysicists deal with a very different scale. Fifty million degrees may not seem cold to you. But compared to 70 or 100 million degrees, it's quite chilly. Scientist recently mapped a region of those relatively cool temperatures buried inside a large region of
colliding galaxies and 70 to 100 million degree gases. It's the first time the
pressure fronts in the system have been mapped in detail.
Chandra X-ray Observatory image of Abell 2142. The image shows a bright, but relatively cool 50 million degree
central region (white) embedded in large elongated cloud of 70 million
degree gas (magenta), all of which is roiling in a faint atmosphere of
100 million degree gas (faint magenta and dark blue). The bright source
in the upper left is an active galaxy in the cluster.
The gas clouds are in the core of a galaxy cluster known
as Abell 2142. The cluster is six million light years across. It contains
hundreds of galaxies and enough gas to make a thousand more. It is one
of the most massive objects in the universe. Galaxy clusters grow to vast
sizes as smaller clusters are pulled inward under the influence of gravity.
They collide and merge over the course of billions of years, releasing
tremendous amounts of energy that heats the cluster gas to 100 million
degrees. The elongated shape of the bright cloud suggests that two clouds
were in the process of merging into one.
Data from the Chandra X-ray Observatory provide the first
detailed look at the late stages of this merger process. Previously, scientists
had used the German-US Roentgen satellite to produce a less detailed image
of the cluster. Chandra is currently the world's most powerful x-ray telescope.
It's about one billion time more powerful than the first x-ray telescope,
created about 30 years ago. Chandra's resolving power is equivalent to
the ability to read letters of a stop sign from a distance of 19 kilometers.
The observatory is able to measure variations of temperature, density
and pressure with unprecedented resolution.
"Now we can begin to understand the physics of these mergers,
which are among the most energetic events in the universe," said Maxim
Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge
Massachusetts. Mr. Markevitch is the leader of the international team
involved in the analysis of the observations. "The pressure and density
maps of the cluster show a sharp boundary that can only exist in the moving
environment of a merger."
Scientists can compare these images with computer simulations
of cosmic mergers. Early results of the comparison show that this merger
has progressed to an advanced stage. Strong shock waves predicted by the
theory for the initial collision of clusters are not observed. It appears
likely that these sub-clusters have collided two or three times in a billion
years or more, and have nearly completed their merger.
An artist rendering of the Chandra X-ray Observatory in orbit
The observations were made on August 20, 1999 using the Advanced CCD
Imaging Spectrometer (ACIS) instrument aboard the Chandra X-ray Observatory.
The team involved scientists from Harvard-Smithsonian; the Massachusetts
Institute of Technology, Cambridge, Mass.; NASA's Marshall Space Flight
Center, Huntsville, Ala.; the University of Hawaii, Honolulu; the University
of Birmingham, U.K.; the University of Wollongong, Australia; the Space
Research Organization Netherlands; the University of Rome, Italy; and
the Russian Academy of Sciences.