Galaxy Cluster Collision Explains Cosmic Treble Clef

Astronomers led by Andrea Botteon of Italy's National Institute for Astrophysics reported Wednesday in Astronomy & Astrophysics that two colliding galaxy clusters, after a black hole outburst, produced the "Treble Clef" radio structure, NASA said.

What the team found

NASA's release says the researchers used NASA's Chandra X-ray Observatory to work out what made a radio formation they nicknamed the "Treble Clef," because its shape looks like the musical symbol. Their conclusion, in NASA's words, is that the Treble Clef is likely caused by an outburst from a supermassive black hole, followed by a collision of two galaxy clusters.

The paper was led by Botteon, of the National Institute for Astrophysics (INAF) in Italy, and appeared Wednesday in the journal Astronomy & Astrophysics, according to NASA.

This one was hard to crack. NASA says the formation sits almost directly behind the plane of the Milky Way, so stars, gas and dust from our own galaxy largely block the view from Earth. The team got around that by using telescopes that can see through that material.

Chandra and the German eROSITA X-ray telescope (the name is short for Extended Roentgen Survey with an Imaging Telescope Array) found diffuse, hot gas at the spot. NASA says that showed a galaxy cluster hosts the object. The researchers also determined that the cluster is really two clusters in the middle of merging.

A two-step story

NASA says the authors concluded the Treble Clef likely formed in two stages.

First, an earlier outburst from a supermassive black hole at the center of the cluster drove clouds of charged particles and magnetic fields away at close to the speed of light. That produced bright radio waves, which later faded as the particles lost energy.

Second, the collision between the two clusters compressed or reenergized the cloud, and it brightened again in radio waves. NASA adds that shock waves from the collision, similar to the sonic booms from supersonic jets, may have played a part in that reenergizing.

How they saw the collision

Here is the detail worth reading closely. NASA's release says that to find evidence of the collision in the Chandra data, the authors measured how much the cluster's hot gas departs from a symmetrical shape. Most of the gas is symmetrical and roughly oval. The team subtracted that oval from the original X-ray image and studied what was left over.

In that leftover image, the X-ray glow is brightest on the left and right and faintest in the center. NASA says this implies the clusters collided along the left-right direction as seen in the image. The researchers think the direct collision happened a few hundred million years ago, and that the clusters are now moving away from each other. That timing is the researchers' estimate, as NASA describes it.

The published composite image combines the original X-ray picture with the leftover one. NASA says the hot gas, seen by Chandra and eROSITA, is shown in blue. Radio data from LOFAR (the LOw Frequency ARray, a large network of radio telescopes in Europe) is red and traces the clef shape. Infrared data from the United Kingdom Infrared Telescope appears mostly white.

Context

NASA describes galaxy clusters as some of the largest structures in the universe, holding hundreds of individual galaxies and huge reservoirs of superheated gas. NASA's release also notes that the clef resemblance is pareidolia, the habit of people seeing familiar shapes in random data. There is no scientific link to the musical notation.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program, according to the release. The Smithsonian Astrophysical Observatory's Chandra X-ray Center runs science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

What the result shows is that a black hole outburst and a cluster merger together can light up radio emission in a way neither would alone, and that a structure hidden behind the Milky Way can still be decoded with the right instruments.

What is next

NASA's release does not announce follow-up observations or a schedule for further study of the Treble Clef. The paper itself is out now in Astronomy & Astrophysics, and NASA points readers to nasa.gov/chandra for more on the observatory.

Stay curious.

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