In 2011, astronomers announced the first, surprising discovery of a massive black hole in a dwarf galaxy. Now, many of these same astronomers have announced the discovery of a further 13 of these wandering monsters.
In a paper published to the Astrophysical Journal, Amy Reines of Montana State University and colleagues at the US National Radio Astronomy Observatory said these black holes are located less than 1bn light years from Earth. They are found in galaxies 100 times less massive than our own Milky Way, making them some of the smallest known to host black holes with an average mass of around 400,000 times that of our sun.
The discovery was made using the Very Large Array (VLA) that also made the first discovery back in 2011. The galaxies were selected from the NASA-Sloan Atlas catalogue because they had stars totalling less than 3bn times the mass of the sun, which is about equal to the Large Magellanic Cloud. The VLA was then able to make new and more sensitive, high-resolution images of 111 of the selected galaxies.
‘We must broaden our searches’
“We were very surprised to find that, in roughly half of those 13 galaxies, the black hole is not at the centre of the galaxy, unlike the case in larger galaxies,” Reines said.
She and her fellow researchers said this shows that the galaxies have likely merged with others early in their history. This is consistent with computer simulations predicting that roughly half of the massive black holes in dwarf galaxies will be found wandering in the outskirts of their galaxies.
“This work has taught us that we must broaden our searches for massive black holes in dwarf galaxies beyond their centres to get a more complete understanding of the population and learn what mechanisms helped form the first massive black holes in the early universe,” Reines said.
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