Artist's conception of a dwarf galaxy, its shape distorted, most likely by a past interaction with another galaxy, and a massive black hole in its outskirts (pullout). The black hole is drawing in material that forms a rotating disk and generates jets of material propelled outward. Credit: Sophia Dagnello, NRAO/AUI/NSF. Hi-Res File
Artist's conception of a dwarf galaxy, its shape distorted, most likely by a past interaction with another galaxy, and a massive black hole in its outskirts (bright spot, far right; no pullout). Credit: Sophia Dagnello, NRAO/AUI/NSF. Hi-Res File
Visible-light images of galaxies that VLA observations showed to have massive black holes. Center illustration is artist's conception of the rotating disk of material falling into such a black hole, and the jets of material propelled outward.
Credit: Sophia Dagnello, NRAO/AUI/NSF; DECaLS survey; CTIO. Hi-Res File
Roughly half of the newly-discovered black holes are not at the centers of their galaxies
Astronomers seeking to learn about the mechanisms that formed massive
black holes in the early history of the Universe have gained important
new clues with the discovery of 13 such black holes in dwarf galaxies
less than a billion light-years from Earth.
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. The scientists expect that the black holes in these smaller
galaxies average about 400,000 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,
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 Amy Reines of
Montana State University.
Reines and her colleagues used the National Science Foundation’s Karl
G. Jansky Very Large Array (VLA) to make the discovery, which they are
reporting at the American Astronomical Society’s meeting in Honolulu,
Hawaii.
Reines and her collaborators 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 3 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 center 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 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,” Reines
said.
Reines worked with James Condon, of the National Radio Astronomy
Observatory; Jeremy Darling, of the University of Colorado, Boulder; and
Jenny Greene, of Princeton University. The astronomers are publishing
their results in the Astrophysical Journal. (Preprint )
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
Media Contact:
Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu
