Black hole 'flees' from galactic merger

Astronomers searching for signs of galactic mergers have spotted a supermassive black hole apparently 'fleeing' from one such collision.

Published: November 7, 2016 at 12:00 pm

This deep Hubble image of the galaxy cluster ZwCl 1715.5+4229 (also known as ZwCl 8193) shows the full extent of the giant galaxy 2MASX 17171926+4226571 and some of its gravitationally lensed background galaxies.


The white ‘X’ marks the VLBA radio position, the white ‘+’ marks the Hubble optical position of the giant galaxy’s core, and the white box bounds the zoomed image at right. Right: this shallower zoomed Hubble image of the galaxy cluster reveals the core of the giant galaxy, the smaller galaxy, several cluster galaxies, and extended interaction debris.

The Hubble position of the giant galaxy’s core is marked by the white ‘+,’ and optical identification of B3 1715+425 is enclosed by a white circle centered on its Hubble position.

Image credit: J.J. Condon et al / NASA / ESA / Hubble / NSF / VLBA.

Astronomer Jim Condon explains how his team made the discovery. Credit: NRAO/AUI/NSF

A supermassive black hole has been spotted speeding away from a large galaxy, possibly having survived a violent galactic collision

It is thought that there is a supermassive black hole at the centre of most galaxies, including our own.

Supermassive black holes are millions to billions of times more massive than our own Sun, and can often grow even more massive as galaxies collide and their central black holes merge.

Using the National Science Foundation's Very Long Baseline Array (VLBA), a group of astronomers were searching for supermassive black holes not located at the centre of a galaxy, as this could be a sign of a collision between two galaxies having occurred.

Searching in a cluster of galaxies called ZwCl 8193 two billion lightyears from Earth, the team found a supermassive black hole surrounded by a proportionately small galaxy, and concluded that it must have been the remnant of a larger galaxy that was torn apart during a galactic collision.

This remnant is about 3,000 lightyears across while our own Milky Way, for example, is about 100,000 lightyears across.

The black hole in question – designated B3 1715+425 - was spotted speeding away from the larger galaxy about 3,200 kilometres per second, suggesting it was part of a smaller galaxy that collided with the larger one.

As this happened, the smaller galaxy was probably mostly destroyed save for its central supermassive black hole, which burst through the other side of the larger galaxy carrying a galactic remnant with it.

"We were looking for orbiting pairs of supermassive black holes, with one offset from the centre of a galaxy, as telltale evidence of a previous galaxy merger," says James Condon, of the National Radio Astronomy Observatory.


"Instead, we found this black hole fleeing from the larger galaxy and leaving a trail of debris behind it."

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