Astronomers using NSF’s Karl G. Jansky Very Large Array (VLA) at the National Radio Astronomy Observatory have observed 111 nearby dwarf galaxies within one billion light-years from Earth. They have identified 13 dwarf galaxies that host very massive black holes. These galaxies, more than 100 times less massive than our own Milky Way, are among the smallest galaxies known to host massive black holes. In roughly half of these galaxies, the black hole is not at the center of the galaxy. The black holes have an average mass of about 400,000 solar masses.
An 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. Image credit: Sophia Dagnello, NRAO / AUI / NSF.
“All of the black holes I had found before were in the centers of galaxies,” said Dr. Amy Reines, an astrophysicist at Montana State University.
“These were roaming around the outskirts. I was blown away when I saw this.”
Dr. Reines and her colleagues 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 astronomers 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 totaling 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 their 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.
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. Image credit: Sophia Dagnello, NRAO / AUI / NSF / DECaLS Survey / CTIO.
“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,” Dr. Reines said.
“This indicates that the galaxies likely have merged with others earlier in their history.”
The results confirm predictions from recent computer simulations that postulated that black holes may often be off-center in dwarf galaxies due to the way galaxies interact as they move through space.
The findings may change how astronomers look for black holes in dwarf galaxies in the future.
“We need to expand searches to target the whole galaxy, not just the nuclei where we previously expected black holes to be,” Dr. Reines said.
The team’s paper was published in the Astrophysical Journal.
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Amy E. Reines et al. 2020. A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from High-resolution Radio Observations. ApJ 888, 36; doi: 10.3847/1538-4357/ab4999
