Black holes can be confused with stars

A new kind of celestial object could contain traces of cosmic catastrophes

Black hole

Rochester (USA) / Garching - Extremely compact star clusters, in which the stars move at unusually high speeds, could serve astronomers as a kind of cosmic fossil. Because from the movement of the stars in such clusters one can read the history of cosmic catastrophes, the collisions and mergers of galaxies. According to the calculations of a team of researchers from the USA and Germany, around one hundred of the compact star clusters should be able to be found in the nearby Virgo galaxy cluster.

When galaxies collide, the supermassive black holes at their centers can also collide. But the black holes do not merge: As they approach, enormous distortions of space-time occur, which exert a recoil on the smaller of the two black holes and catapult it out of the merging galaxies at high speed. It pulls stars from its environment with it, which then form an extremely compact star cluster around the black hole.

"The speed at which the black hole was ejected can then be read off how fast the stars in this cluster are moving," explains David Merritt of the Rochester Institute of Technology. Together with his colleagues Jeremy Schnittman from the Johns Hopkins University in Baltimore and Stefanie Komossa from the Max Planck Institute for Extraterrestrial Physics in Garching, Merritt has investigated the connections between the movement of stars in the compact clusters and previous cosmic catastrophes. The researchers present their results in the current issue of the "Astrophysical Journal".

"Tracking down such objects would be comparable to discovering the DNA of a long-extinct species," says Komossa. Astronomers may have seen compact clusters before - but confused them with ordinary globular clusters. The difference can only be seen by examining the velocity distribution of the stars in the clusters. Merritt, Schnittman and Komossa estimate that there are around one hundred compact star clusters around supermassive black holes in the nearby Virgo galaxy cluster that are bright enough for such observations.