A remarkable ring of bright X-ray sources — black holes or neutron stars — has been discovered in a galaxy approximately 300 million light years from Earth. This ring was forged when one galaxy smashed through the middle of another, creating ripples in the gas. The discovery is reported in a paper in the Astrophysical Journal (arXiv.org preprint).
New data from NASA’s Chandra X-ray Observatory reveal the presence of very bright X-ray sources, most likely binary systems powered by either a stellar-mass black hole or neutron star, in the remarkable ring of the galaxy AM 0644-741. In this composite image, X-rays from Chandra (purple) have been combined with optical data from the NASA/ESA Hubble Space Telescope (red, green, and blue). Image credit: X-ray – NASA / CXC / INAF / A. Wolter et al; optical – NASA / STScI.
“Where did the ring of black holes or neutron stars in this galaxy — the so-called ring galaxy AM 0644-741 — come from? We think that it was created when one galaxy was pulled into another galaxy by the force of gravity,” said Dr. Anna Wolter of the Osservatorio Astronomico di Brera and co-authors.
“The first galaxy generated ripples in the gas of the second galaxy, AM 0644-741, located in the lower right. These ripples then produced an expanding ring of gas in AM 0644-741 that triggered the birth of new stars. The first galaxy is possibly the one located in the lower left of the image.”
The most massive of these fledgling stars will lead short lives of millions of years.
After that, their nuclear fuel is spent and the stars explode as supernovas leaving behind either black holes with masses about 5-20 times that of the Sun, or neutron stars with a mass approximately equal to that of the Sun.
“Some of these black holes or neutron stars have close companion stars, and siphon gas from their stellar partner,” the astronomers said.
“This gas falls towards the black hole or neutron star, forming a spinning disk like water circling a drain, and becomes heated by friction.”
“This superheated gas produces large amounts of X-rays that NASA’s Chandra X-ray Observatory can detect.”
While a ring of black holes or neutron stars is intriguing in itself, there is more to the story of AM 0644-741.
All of the X-ray sources detected in the ring of this galaxy are bright enough to be classified as ultraluminous X-ray sources (ULXs). This is a class of objects that produce hundreds to thousands of times more X-rays than most ‘normal’ binary systems in which a companion star is in orbit around a neutron star or black hole.
Until recently most astronomers thought that ULXs generally contained stellar-mass black holes, with the possible presence in some cases of intermediate-mass black holes that contain over a hundred times the mass of the Sun.
However, this thinking was overturned when a few ULXs in other galaxies, including Messier 82 and Messier 51, were found to contain neutron stars.
Several other explanations besides intermediate-mass black holes have been suggested for the intense X-ray emission of ULXs.
They include unusually rapid growth of the black hole or neutron star, or geometrical effects arising from the funneling of infalling material along magnetic field lines.
“The identity of the individual ULXs in AM 0644-741 is currently unknown,” the researchers said.
“They may be a mixture of black holes and neutron stars, and it is also possible that they are all black holes or all neutron stars.”
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Anna Wolter et al. 2018. The X-Ray Luminosity Function of Ultraluminous X-Ray Sources in Collisional Ring Galaxies. ApJ 863, 43; doi: 10.3847/1538-4357/aacb34
