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X-ray tsunami in Perseus galaxy cluster

Not so very long ago or far away in astronomical terms, a near-miss between titanic galaxy clusters might have set into motion an immense wave of hot gas.

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An international team of scientists has discovered a vast wave of hot gas in the nearby Perseus galaxy cluster – a cluster some 11 million light-years across, containing thousands of galaxies – located about 240 million light years away. Like all galaxy clusters, most of the observable matter in the Perseus cluster takes the form of a pervasive gas averaging tens of millions of degrees. This gas is so hot it glows only in X-rays. The newly discovered wave of hot gas is being described as a “tsunami” to indicate its great size and its presence in this pervasive ocean of hot gas. The wave spans about 200,000 light-years, or about twice the diameter of our entire Milky Way galaxy.

To find the X-ray tsunami within the galaxy cluster, the scientists combined data from the Chandra X-ray Observatory with radio observations and computer simulations. A paper describing their findings appears in the June 2017 issue of the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

The researchers say they believe the wave formed billions of years ago, after another, smaller galaxy cluster grazed the Perseus cluster and caused its vast supply of gas to slosh around an enormous volume of space. Lead scientist Stephen Walker at NASA’s Goddard Space Flight Center in Greenbelt, Maryland said:

Perseus is one of the most massive nearby clusters and the brightest one in X-rays, so Chandra data provide us with unparalleled detail. The wave we’ve identified is associated with the flyby of a smaller cluster, which shows that the merger activity that produced these giant structures is still ongoing.

That is, it’s ongoing as of 240 million years ago, which is an eye-blink on astronomical timescales.

A statement from the Chandra X-ray Observatory mentioned that Chandra observations have revealed a variety of structures in the gas of the Perseus galaxy cluster. These structures include vast bubbles blown by a supermassive black hole in the cluster’s central galaxy, NGC 1275, aka Perseus A.

And they include an enigmatic concave feature known to X-ray astronomers as the ‘bay.’

This X-ray image of the hot gas in the Perseus galaxy cluster was made from 16 days of Chandra observations. Researchers then filtered the data in a way that brightened the contrast of edges in order to make subtle details more obvious. An oval highlights the location of what these astronomers called the “bay,” an enormous wave found to be rolling through the hot gas. Image via NASA’s Goddard Space Flight Center/Stephen Walker et al.

The bay is what caught astronomers’ attention for this study. They realized its concave shape couldn’t have formed through bubbles launched by the NGC 1275’s central black hole. Radio observations using the Very Large Array in central New Mexico show that the bay structure produces no emission, the opposite of what scientists would expect for features associated with black hole activity. In addition, standard models of sloshing gas typically produced structures that arc in the wrong direction.

So what might have caused the bay structure. The Chandra statement explained how the scientists went about answering that question:

Walker and his colleagues turned to existing Chandra observations of the Perseus cluster to further investigate the bay. They combined a total of 10.4 days of high-resolution data with 5.8 days of wide-field observations at energies between 700 and 7,000 electron volts. For comparison, visible light has energies between about two and three electron volts.

The scientists then filtered the Chandra data to highlight the edges of structures and reveal subtle details.

Next, they compared the edge-enhanced Perseus image to computer simulations of merging galaxy clusters developed by John ZuHone, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. The simulations were run on the Pleiades supercomputer operated by the NASA Advanced Supercomputing Division at Ames Research Center in Silicon Valley, California.

Image via John ZuHone/Harvard-Smithsonian Center for Astrophysics.

One simulation seemed to explain the formation of the bay, these scientists said. This simulation is shown above. In it, gas in a large cluster similar to Perseus has settled into two components: a “cold” central region with temperatures around 54 million degrees Fahrenheit (30 million degrees Celsius) and a surrounding zone where the gas is three times hotter.

Then, in the simulation, a small galaxy cluster containing about 1,000 times the mass of the Milky Way skirts the larger cluster. It misses the cluster’s center by about 650,000 light years.

In the simulation, the flyby creates a gravitational disturbance that churns up the gas like cream stirred into coffee, creating an expanding spiral of cold gas. After about 2.5 billion years, when the gas has risen nearly 500,000 light years from the center, vast waves form and roll at its periphery for hundreds of millions of years before dissipating.

These waves are giant versions of Kelvin-Helmholtz waves, which show up whenever there’s a velocity difference across the interface of two fluids, such as wind blowing over water. They can be found in the ocean, in cloud formations on Earth and other planets, in plasma near Earth, and even on the sun. Stephen Walker said:

We think the bay feature we see in Perseus is part of a Kelvin-Helmholtz wave, perhaps the largest one yet identified, that formed in much the same way as the simulation shows. We have also identified similar features in two other galaxy clusters, Centaurus and Abell 1795.

Bottom line: The nearby Perseus galaxy cluster is pervaded by extremely hot gas that glows in X-rays. Astronomers have found a giant wave within this gas, which they believe was caused with another, smaller galaxy cluster grazed the Perseus cluster.

Read more from NASA or Chandra

See photos: Kelvin-Helmholtz waves in earthly clouds

Deborah Byrd