When the titans of space—galaxy clusters—collide, extraordinary things can
happen. A new study using NASA's Chandra X-ray Observatory examines the
repercussions after two galaxy clusters clashed.
Galaxy clusters are the largest structures in the Universe held together by
gravity, containing hundreds or even thousands of individual galaxies
immersed in giant oceans of superheated gas. In galaxy clusters, the normal
matter—like the atoms that make up the stars, planets, and everything on
Earth—is primarily in the form of hot gas and stars. The mass of the hot gas
between the galaxies is far greater than the mass of the stars in all of the
galaxies. This normal matter is bound in the cluster by the gravity of an
even greater mass of dark matter.
Because of the huge masses and speeds involved, collisions and mergers
between galaxy clusters are among the most energetic events in the Universe.
In a new study of the galaxy cluster Abell 1775, located about 960 million
light years from Earth, a team of astronomers led by Andrea Botteon from
Leiden University in the Netherlands announced that they found a
spiral-shaped pattern in Chandra's X-ray data. These results imply a
turbulent past for the cluster.
When two galaxy clusters of different sizes have a grazing collision, the
smaller cluster will begin to plow through the larger one. (Because of its
superior mass, the bigger cluster has the upper hand when it comes to
gravitational pull.) As the smaller cluster moves through, its hot gas is
stripped off due to friction. This leaves behind a wake, or tail, that
trails behind the cluster. After the center of the smaller cluster passes by
the center of the larger one, the gas in the tail starts to encounter less
resistance and overshoots the center of its cluster. This can cause the tail
to "slingshot" as it flies to the side, curving as it extends away from the
cluster's center.
The newest Chandra data contains evidence—including the brightness of the
X-rays and the temperatures they represent—for one of these curving
"slingshot" tails. Previous studies of Abell 1775 with Chandra and other
telescopes hinted, but did not confirm, that there was an ongoing collision
in this system.
A new image of Abell 1775 contains X-rays from Chandra (blue), optical data
from the Pan-STARRS telescope in Hawaii (blue, yellow, and white), and radio
data from the LOw Frequency ARray (LOFAR) in the Netherlands (red). The tail
is labeled in this image along with a region of gas with a curved edge,
called a "cold front," that is denser and cooler than the gas it is plowing
into. The tail and the cold front all curve in the same direction, creating
a spiral appearance. A separate labeled image shows the field of view of the
Chandra data.
Astronomers previously found that Abell 1775 contains an enormous jet and
radio source, which is also seen in this new composite image. This jet is
powered by a supermassive black hole in a large elliptical galaxy in the
cluster's center. New data from LOFAR and the Giant Metrewave Radio
Telescope (GMRT) in India reveals that the radio jet is actually 2.6 million
light years long. This is about twice as long as astronomers thought it was
before and makes it one of the longest ever observed in a galaxy cluster.
The structure of the jet changes abruptly as it crosses into the lower
density gas in the upper part of the image, across the edge of the cold
front, implying that the collision has affected it.
According to the new study, the gas motions inside the cluster could be
responsible for other structures detected by observing Abell 1775 in radio
waves, such as two filaments located near the origin of the jet (one of
these is labeled). The LOFAR and Chandra data have also enabled the
researchers to study in great detail the phenomena that contribute to
accelerating electrons both in this galaxy's jet and in the radio emission
near the center of the larger cluster.
There is an alternate explanation for the appearance of the cluster. As a
small cluster approaches a larger one, the dense hot gas of the larger
cluster will be attracted to it by gravity. After the smaller cluster passes
the center of the other cluster, the direction of motion of the cluster gas
reverses, and it travels back towards the cluster center. The cluster gas
moves through the center again and "sloshes" back and forth, similar to wine
sloshing in a glass that was jerked sideways. The sloshing gas ends up in a
spiral pattern because the collision between the two clusters was
off-center.
The Botteon team favors the slingshot tail scenario, but a separate group of
astronomers led by Dan Hu of Shanghai Jiao Tong University in China favors
the sloshing explanation based on data from Chandra and ESA's XMM-Newton.
Both the slingshot and sloshing scenarios involve a collision between two
galaxy clusters. Eventually the two clusters will fully merge with each
other to form a single, larger galaxy cluster.
Further observations and modeling of Abell 1775 are required to help decide
between these two scenarios.
A paper describing the results by Botteon's team has been published in the
journal Astronomy and Astrophysics and is
available online. The separate work on the "sloshing" theory led by Dan Hu has been
accepted for publication in The Astrophysical Journal and is also
available online.
References:
“Nonthermal phenomena in the center of Abell 1775: An 800 kpc head-tail,
revived fossil plasma and slingshot radio halo” by A. Botteon, S.
Giacintucci, F. Gastaldello, T. Venturi, G. Brunetti, R. J. van Weeren, T.
W. Shimwell, M. Rossetti, H. Akamatsu, M. Brüggen, R. Cassano, V. Cuciti, F.
de Gasperin, A. Drabent, M. Hoeft, S. Mandal, H. J. A. Röttgering and C.
Tasse, 11 May 2021, Astronomy and Astrophysics.
“The Merger Dynamics of the Galaxy Cluster A1775: New Insights from Chandra
and XMM-Newton for a Cluster Simultaneously Hosting a Wide-angle Tail and a
Narrow-angle Tail Radio Source” by Dan Hu, Haiguang Xu, Zhenghao Zhu, Chenxi
Shan, Yongkai Zhu, Shida Fan, Yuanyuan Zhao, Chengze Liu, Hoongwah Siew,
Zhongli Zhang, Liyi Gu, Melanie Johnston-Hollitt, Xi Kang, Qinghua Tan,
Jiang Chang and Xiang-ping Wu, 18 May 2021, The Astrophysical Journal.
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