4 Nov 2016

Galaxy collision releases supermassive Black Hole

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With the Very Long Baseline Array telescope, astronomers were able to spot the remnants of a cosmic encounter involving two galaxies, one larger than the other.

Based on their observations, the smaller galaxy was left with a nearly naked supermassive black hole fleeing at more than 2,000 miles per second, leaving a trail of debris behind.

James Condon and colleagues estimate the galaxies to be part of a galaxy cluster situated more than 2 billion light-years away from planet Earth. The encounter itself occurred millions of years ago and left the smaller galaxy stripped of almost all its gas and stars.

After passing through the larger galaxy, the smaller of the two dwindled in size to just about 3,000 light-years across. For reference, the Milky Way is about 100,000 light-years across.

When Two Galaxies Collide

Originally, the astronomers were looking for supermassive black hole pairs orbiting each other. Specifically, they were looking for supermassive black holes that are not galaxy centres, which is a sign of two galaxies merging. It is thought that larger galaxies attain their size by consuming their companion galaxies. When two galaxies get close enough, their black holes orbit each other before eventually merging.

"We've not seen anything like this before," said Condon of fleeing black hole.

As part of their original goal, the astronomers sought to capture high-resolution images of more than 1,200 galaxies using the VLBA. These galaxies have been identified previously by sky surveys carried out with infrared as well as radio telescopes and, based on the researchers' observations, nearly all have supermassive black holes at their centres.

However, one object from a galaxy cluster called ZwCl 8193 stood out. Known as B3 1715+425, this object was a supermassive black hole enveloped by a galaxy fainter and much smaller than what the astronomers expected. Additionally, it was speeding away from a larger galaxy's core, leaving ionized gas in its wake.

From this, the researchers concluded that the object, a nearly-naked supermassive black hole, was what remained of a galaxy that passed through one much larger than it was. They also said B3 1715+425 will lose more of its mass along the way and stop making new stars.

According to Condon, the speeding remnant will probably be invisible in about a billion years. Given this idea, there could be more objects left over from earlier encounters between galaxies in the universe that can no longer be detected today.
This doesn't deter Candon and his colleagues though. They said they will keep looking, taking advantage of the high-quality images they can get from the VLBA.

The VLBA Telescope

Dedicated in 1993, the VLBA is part of the Long Baseline Observatory, which is a National Science Foundation facility. It features 10 dish antennas, each 25 meters in diameter, scattered from St. Croix in the Caribbean to Hawaii. When all the 10 antennas are working together, they result in a single telescope with the highest resolution available to astronomers today.

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