Astronomers have discovered 13 massive black holes in dwarf galaxies less than a billion light-years from the Earth, says a new study, adding that these dwarf galaxies, more than 100 times less massive than our own Milky Way, are among the smallest galaxies known to host massive black holes.
According to the study, published in the Astrophysical Journal, the researchers expect that the black holes in these smaller galaxies average about 4,00,000 times as heavy as the Sun.
"We hope that studying them and their galaxies will give us insights into how similar black holes in the early Universe formed and then grew, through galactic mergers over billions of years, producing the supermassive black holes we see in larger galaxies today, with masses of many millions or billions of times that of the Sun," said study researcher Amy Reines from Montana State University in the US.
For the findings, Reines and her colleagues used the US National Science Foundation's Karl G. Jansky Very Large Array (VLA) to make the discovery.
They used the VLA to discover the first massive black hole in a dwarf starburst galaxy in 2011. That discovery was a surprise to astronomers and spurred a radio search for more.
The scientists started by selecting a sample of galaxies from the NASA-Sloan Atlas, a catalog of galaxies made with visible-light telescopes.
They chose galaxies with stars totalling less than three billion times the mass of the Sun, about equal to the Large Magellanic Cloud, a small companion of the Milky Way.
From this sample, they picked candidates that also appeared in the National Radio Astronomy Observatory's Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey, made between 1993 and 2011.
They then used the VLA to make new and more sensitive, high-resolution images of 111 of the selected galaxies.
"The new VLA observations revealed that 13 of these galaxies have strong evidence for a massive black hole that is actively consuming surrounding material. 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.
The scientists said this indicates that the galaxies likely have merged with others earlier 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.