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false-color X-ray image of the galaxy cluster Abell 1033. The white
contours help identify the X-ray flux levels, and the red contours trace
the radio emission. The elongated red structure in the lower center is
a radio phoenix: fossil gas that has been reheated by shocks from a
nearby galaxy merger (obscured in this view). Credit: Chandra, VLA
1033 is a cluster of over 350 galaxies located about 1.7 billion
light-years away. Collisions between galaxies in clusters are common
events, and each merger heats and shocks the nearby gas. The rapidly
moving, ionized gas then radiates intensely at radio wavelengths. There
are three types of radio sources found in these clusters. The first,
called radio relics, are found in the outskirts of galaxies and have
radiation signatures characteristic of shocked material over large
scales. The second type, called radio haloes, are centrally located in
the cluster and are probably the result of large turbulent motions set
up during collisions.
A radio phoenix is the third type of cluster radio source, and is
much less well studied. After the initial effects of a collision have
died down and the gas has cooled, the radio emission subsides. But a
subsequent merger nearby can produce a strong shock wave, and if that
passes through the fossil material it can compress and re-energize it to
emit in the radio again.
CfA astronomers Georgiana Ogrean and Reinout van Weeren, with five
colleagues, used data from the Chandra X-ray Observatory, the Westerbork
Synthesis Radio Telescope, the Very Large Array ad the optical Sloan
Digital Sky Survey to study the Abell 1033 cluster and its family of
galaxies. They discovered two subclusters in the source that seem to
have recently collided; they were spotted from their X-ray emission.
Close to this region, and to a galactic nucleus, the team spotted a
radio source with the emission and charged particle characteristics of a
radio phoenix. The scientists conclude that shocks from the recent
merger have propagated into old gas, reinvigorating this fossil remnant
to new life.
"Abell 1033: Birth of a Radio Phoenix," F. de Gasperin, G. A. Ogrean, R.
J. van Weeren, W. A. Dawson, M. Bruggen, A. Bonafede1 and A.
Simionescu,MNRAS448, 2197, 2015.