A ring galaxy with a lot of ultraluminous X-ray sources

An article published in the “Astrophysical Journal” describes the detection of a series of X-ray sources in the ring of the galaxy AM 0644-741. A team of researchers led by Anna Wolter from INAF-Osservatorio Astronomico di Brera, Italy, used observations from NASA’s Chandra X-ray Observatory to discover those ultraluminous sources concluding that the ring containing them consists of binary systems that include black holes or neutron stars and that the ring formed following a collision between galaxies.

Approximately 300 million light years from Earth, the galaxy AM 0644-741, also known as the Lindsay-Shapley Ring, is quite particular. Its classification is of a lenticular galaxy, therefore intermediate between elliptic and spiral, unbarred and therefore devoid of a central bar-shaped structure, but also of a ring galaxy for the structure subject of this research. This type of shape is unusual and is probably the consequence of a collision between galaxies.

The origin of the shape of the galaxy AM 0644-741 is important for its consequences. That’s because in such cases the collision occurs between galaxies of very different size with the smaller one that passes through the larger one generating an outward shock wave that leads to that shape but also to a compression of interstellar gas and to a consequent increase in star formation.

The image (X-ray: NASA/CXC/INAF/A. Wolter et al. Optical: NASA/STScI) combines Chandra’s observations with others of the Hubble space telescope. The X-ray emissions detected by Chandra are shown in purple while the ones at visible light frequencies detected by Hubble are shown in red, green and blue. At the bottom left there’s another galaxy that could be the “culprit” of the shape of AM 0644-741.

In addition to the X-ray sources in the ring of AM 0644-741, others of different types are visible, identified as black holes. One is the supermassive black hole at the center of the galaxy, another one appears inside the galaxy but actually the source is behind it, a good 9.1 billion light years from Earth.

The X-ray sources discovered in the ring of the galaxy AM 0644-741 are of the type classified as ultraluminous sources (ULX). These sources emit amounts of X-rays a thousand times greater than the typical binary systems consisting of a normal star and a black hole or a neutron star, the remains of the most massive stars that died in supernovae. In the past, astronomers thought that such powerful sources were due to very massive black holes, even of intermediate mass with masses hundreds of times the Sun’s. However, the identification of ULXs generated in systems with neutron stars changed the situation with new questions.

For now the ULXs in the galaxy AM 0644-741 haven’t been identified therefore further studies will be needed to try to confirm or deny some of the various hypotheses about their origin. It could be black holes or neutron stars that grow rapidly stealing gas from their companions or the effects of materials falling along magnetic field lines. The studies may also concern ULXs that were discovered in the ring galaxies NGC 922, Arp 147, Arp 143, Arp 148, Arp 284 and the Cartwheel Galaxy.

In these seven galaxies, 63 X-ray sources were identified, of which 50 are of the ULX type. The average for these ring galaxies is higher than that found in other types of galaxies, a further reason for interest. Binary systems have been studied for years but it’s possible that in some cases there are still processes that aren’t known well enough that lead to the generation of a ULX. This type of study crosses path with that of the evolution of ring galaxies: according to previous research, the one in AM 0644-741 will keep on expanding for another 300 million years and then will start disintegrating.

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