A huge wave of hot gas in the Perseus galaxy cluster

X-ray image of the hot gas in the Perseus galaxy cluster (Image NASA's Goddard Space Flight Center/Stephen Walker et al.)
X-ray image of the hot gas in the Perseus galaxy cluster (Image NASA’s Goddard Space Flight Center/Stephen Walker et al.)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes the discovery of a large wave of hot gas in the Perseus galaxy cluster that extends for about 200,000 light years. A team of astronomers led by Dr. Stephen Walker of NASA’s Goddard Space Flight Center combined observations with NASA’s Chandra X-ray Observatory and others at radio frequencies with computer simulations to study it.

The Perseus galaxy cluster, also known as Abell 426, is relatively close at 240 million light years from Earth and extends for about 11 million light years. The hot gas wave has an extension twice the Milky Way diameter, yet it looks small compared to the whole cluster where it was found. That’s normal because the cluster is formed by over a thousand galaxies, making it one of the largest clusters in this area of ​​the universe.

Most of the matter observed in galaxy clusters is in the form of hot gas, with an average temperature of tens of millions of degrees. For this reason, this gas is only visible at X-rays and the Perseus cluster is the brightest at those frequencies. The Chandra Observatory is the ideal instrument to study this gas and allowed to find a variety of structures within it.

There are interesting structures such as large bubbles blown by the supermassive black hole in the central galaxy of the cluster, NGC 1275. However, the one that attracted the most attention is a curious concave wave formation, called “bay” jargon, indicated in an oval in the image.

The astronomers investigated the bay structure by combining many days of Chandra observations that included a total of 10.4 days of high resolution data and 5.8 days of wide-field observations at energies ranging between 700 and 7,000 electron volts. Chandra’s data were filtered to highlight the structures’ edges and reveal smaller details.

The next step in the research was to compare Chandra’s processed data with computer simulations on merging galaxy clusters developed by astrophysicist John ZuHone of the Harvard-Smithsonian Center for Astrophysics in Cambridge, made with the Pleiades supercomputer at NASA Ames Research Center.

In the latest top 500 supercomputer ranking of November 2016, Pleiades placed at No. 13 with its 7.100 TFlops peak. In 2011, Pleiades was in the Top 10 but the competition in this field is fierce and despite being upgraded in the following years it lost some position in the raking.

One of the simulations provided a possible explanation for bay formation, based on the fact that the gas in a large cluster like Perseus settled in a colder central region, where this means 30 million degrees Celsius (about 54 million degrees Fahrenheit), and another around it where the gas has a temperature three times hotter. A much smaller cluster might have passed near Perseus causing a gravitational turbulence that created a cold gas spiral.

This event had an impact after about 2.5 billion years when the gas rose 500,000 light years from the center and large waves formed that began rolling to the periphery for hundreds of millions of years before dissipating. This is a case of Kelvin-Helmholtz instability, a kind of fluid-dynamic instability that occurs when the different layers of a fluid are in motion relative to each other.

It wouldn’t be the first Kelvin-Helmholtz instability discovered in space but it would be the biggest. The researchers found a relationship between the size of the waves and the strength of the cluster’s magnetic field: if it’s too weak the waves reach much larger dimensions, if it’s too powerful they don’t form at all.

For this reason, Kelvin-Helmholtz instabilities of this type can help investigate the magnetic fields of galactic clusters that couldn’t be measured in any other way. In essence, these are phenomena with enormous space and time extensions that can offer new information on the largest structures in the universe.

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