Astronomers discover 13 black holes in dwarf galaxies with half of them mysteriously lurking at the edges
Astronomers have discovered that massive black holes in dwarf galaxies can exist at unusual places -- along the edges of their galaxies. This is uncommon since most massive black holes are thought to occupy the central position, as in the case of larger galaxies.
During their recent hunt, scientists discovered 13 active and massive black holes in dwarf galaxies, just less than a billion light-years from Earth. And roughly half of these black holes were not found at the center. Instead, they were found lurking around the fringes of their host galaxy, say scientists.
"This work has taught us that we must broaden our searches for massive black holes in dwarf galaxies beyond their centers to get a more complete understanding of the population and learn what mechanisms helped form the first massive black holes in the early Universe," says Amy Reines of Montana State University and lead author of the study.
The team believes that studying these black holes might help them unravel the mysterious origins of supermassive black holes, with masses many millions of times the mass of our 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, producing the supermassive black holes we see in larger galaxies today, with masses of many millions or billions of times that of the Sun," adds Reines.
Dwarf galaxies are smaller than a tenth of the size of our Milky Way. For instance, the Large Magellanic Cloud, located about 160,000 light-years from Earth, is a dwarf galaxy and a small companion of the Milky Way. It contains about a tenth of the mass of the Milky Way, and has about 10% of its stars.
Scientists did not know that dwarf galaxies hosted massive black holes until Reines and her colleagues discovered one in 2011. “We have shown that even small galaxies can have massive black holes and that they may be more common than previously thought. This is really exciting because these little galaxies hold clues to the origin of the first ‘seeds’ of supermassive black holes in the early Universe,” she explained in an earlier statement.
This discovery spurred a hunt for black holes hiding in dwarf galaxies. So Reines and her colleagues used the National Science Foundation's Karl G. Jansky Very Large Array (VLA) to help in this search.
From the high-resolution images of 111 selected galaxies, the team found that 13 of them showed strong evidence for a massive black, as they were vociferously consuming their surrounding material -- dust and gas. "We were very surprised to find that, in roughly half of those 13 galaxies, the black hole is not at the center of the galaxy, unlike the case in larger galaxies," Reines says.
This indicates that the galaxies likely have merged with others earlier in their history, according to the team. These results agree with predictions from computer modeling. Getting a good census of dwarf galaxies with massive black holes is an important first step to resolving the question of the origins of massive black holes, says Reines.
The study has been published in the Astrophysical Journal.