Chandra X-ray Observatory Analyzes 'Treble Clef' Galaxy Cluster Collision

Astronomers have used NASA’s Chandra X-ray Observatory to identify the cause of a radio wave formation nicknamed the “Treble Clef” due to its resemblance to the musical symbol. The findings, published in the Astronomy & Astrophysics journal, indicate the structure was likely formed by an outburst from a supermassive black hole followed by a collision between two galaxy clusters.

Observing Through the Milky Way

Identifying the source of the Treble Clef was challenging because the formation lies almost directly behind the plane of the Milky Way. According to NASA, stars, gas, and dust from the home galaxy largely block the view from Earth. To overcome this, researchers utilized telescopes capable of peering through the intervening material.

Data from NASA’s Chandra and the German Extended RÖntgen Survey with an Imaging Telescope Array (eROSITA) detected diffuse, hot gas at the location of the Treble Clef. This detection confirmed that a galaxy cluster hosts the object. The researchers further determined that this galaxy cluster is actually two clusters in the process of merging. Galaxy clusters are among the largest structures in the universe, containing hundreds of individual galaxies and reservoirs of superheated gas.

A Two-Stage Formation Process

The study, led by Andrea Botteon of the National Institute for Astrophysics (INAF) in Italy, concluded that the Treble Clef likely formed through a two-stage process. First, an earlier outburst from a supermassive black hole in the center of the galaxy cluster drove clouds of charged particles and magnetic fields away at close to the speed of light. This event produced bright radio waves that later faded as the particles lost energy.

Subsequently, the collision between the two clusters compressed or reenergized the cloud, causing it to become brighter again in radio waves. The researchers suggest that shock waves, described as similar to the sonic booms from supersonic jets, may have played a role in this reenergizing process.

Analyzing X-ray Data

To reveal evidence of the cluster collision, the authors employed a special technique to examine how much the cluster’s hot gas deviates from a symmetrical shape. Most of the hot gas is symmetrical and shaped approximately like an oval. The authors removed this symmetrical oval shape from the original X-ray image. The remaining X-ray emission in the residual image shows it is brightest on the left and right, and faintest in the center.

The brighter X-rays in the residual image, toward the left and right of the cluster’s center, imply that the collision between the two clusters likely occurred along the left-right direction as seen in the image. The researchers think the direct collision between the two clusters occurred a few hundred million years ago, and the clusters are now moving away from each other.

Why it matters

The study demonstrates how multi-wavelength observations can uncover the history of cosmic events obscured by the Milky Way. By combining X-ray data from Chandra and eROSITA with radio data from the LOw Frequency ARray (LOFAR) and infrared data from the United Kingdom Infrared Telescope, astronomers were able to reconstruct the sequence of a black hole outburst and a subsequent cluster merger. The resemblance to a treble clef is a case of pareidolia, the phenomenon when people see familiar shapes in random data, since there is no scientific connection to the musical notation.

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Sources

• science.nasa.gov: NASA’s Chandra Breaks Down ‘Treble Clef’s’ Galaxy Cluster Collision - NASA Science

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