Basic Guide to Astronomy

Basic Guide to Astronomy

By: Govert Schilling December 7, 2021 0

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It takes a long time for galaxy clusters to collide. Now, astronomers have discovered a pair of merged star clusters in an unprecedented intermediate stage.

Astronomers saw for the first time a relatively short period of time, which occurred during the collision of massive clusters of galaxies. This dramatic smashing computer simulation successfully reproduced the new observations.

A team led by Zhang Xiaoyuan (Leiden University and SRON Netherlands Institute for Space Research) used NASA’s Chandra X-ray Observatory to map the distribution of hot gas in the merged star cluster named ZwCL 2341+0000, which has approximately Pisces is 3 billion light-years away.

The team asked Chandra to stare at the remote star cluster pair for 57 hours and collect thousands of X-ray photons. Observations show that there is a sharp cone-shaped structure of hot gas between the colliding star clusters, one of which is about three times the mass of the other. "This is really unexpected," said co-author Aurora Simionescu (SRON and Leiden University). "I have never seen anything like this before."

When two star clusters collide, their respective galaxies "pass through" each other relatively undisturbed. Just like the large amount of dark matter in a star cluster, galaxies are "collision-free," which means they will only be affected by mutual gravity. However, the gas in the hot X-ray clusters does collide, resulting in bow shock waves and so-called "cold fronts" at the interface between gas volumes of different temperatures.

In the early stages of the merger, the appearance of these structures was rather blunt—a famous example is the bullet star cluster. In the final stage, these structures tend to curl backwards like breaking waves, making them look like tongues or slingshots. Astronomers often observe these two shapes-blunt "bullet" and wavy "tongue"-but the sharp cone seen in ZwCl 2341+0000 is new.

Zhang and his colleagues got in touch with John ZuHone (Harvard and Smithsonian Center for Astrophysics), who has been conducting computer simulations of colliding and merging galaxy clusters since 2011. According to ZuHone, this simulation reveals properties such as the mass and mass of galaxy clusters. Density affects the distribution of the X-ray emitting gas, as do collision parameters (such as angle and velocity).

In 2019, ZuHone and Bryan Brzycki (University of California, Berkeley) published more detailed simulations that also included the effects of magnetic fields. "When magnetic field lines surround a cold front, they tend to inhibit the development of velocity disturbances," ZuHone said. The magnetic field keeps the gas consistent, and the result is a narrower structure with relatively sharp edges, perhaps more than a million light-years away.

The new MHD simulation tailored for the ZwCl 2341+0000 case successfully reproduced the cone structure as a relatively short-lived feature that lasted for hundreds of millions of years. ZuHone said that eventually, the gas will return to the gravitational pull of dark matter, “swaying around like wine in a wine glass.”

The new Chandra observations and the latest computer simulation results are published in Astronomy and Astrophysics (preprints available here). The author writes that the cluster "is likely to be in a short-lived phase that is rarely observed and provides an example of a complex transition between bullet-like morphology and the development of a slingshot tail."

According to Simionescu, the new image of ZwCl 2341+0000 provides an early glimpse of how the famous bullet ball might change its shape in hundreds of millions of years. ZuHone added that cluster collisions occur on very slow time scales, but the benefit of computer simulations is that they allow you to speed up the time and put an observation snapshot into a one billion-year-old movie.

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Author: Editor of Sky and Telescope December 9, 2021

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