‘Wandering’ Black Holes Found in Dwarf Galaxies

Robert Lea
Jan 5 · 4 min read

Newly discovered massive black holes located away from the centre of their respective galaxies may give us vital clues about how black holes formed in the early Universe.

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 accretion disc and generates jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF

The observation of 13 newly discovered massive black holes in dwarf galaxies less than a billion-light-years from earth could aid scientists’ understanding of the mechanism that lies beneath the formation of such black holes in the young universe. Even though we understand the mechanism behind the formation of black holes in later epochs of the universe, how massive primordial black holes — or ‘seed’ black holes — formed is still something of a mystery.

The dwarf galaxies in question are over 100 times less than the mass of the Milky Way and as such, are the smallest galaxies observed to possess a black hole. These black holes are theorised to possess a mass equivalent to 4x10⁵ solar masses — or 400,000 of our Suns.

“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,” explains Amy Reines of Montana State University.

Henize 2–10: a dwarf galaxy that hides a massive secret ( Reines et al. (2011))

Searching the Darkness for Darkness

In 2011, Reines and her team used the Very Large Array (VLA) telescope to observe the first instance of a massive black hole in a dwarf galaxy, surprising the scientific community in the process. The black hole with the tremendous mass of one million solar masses lies at the centre of the dwarf starburst galaxy Henize 2–10 — located 34 million-light-years from Earth. The discovery came as a shock as it confirmed that galaxies undergoing extreme bursts of star formation can also harbour massive black holes.

The starbursting blue compact dwarf galaxy Henize 2–10. (a) An optical colour image from the Digital Sky Survey shows that the outer parts of the galaxy are blue but have a smooth light distribution. This proves to have an exponential profile. The burned-out central parts contain the ongoing starburst. The tiny rectangle in (a) shows a scaled version of panel (b) from Reines et al. (2011). It is an HST image showing Hα emission in red, the I-band stellar continuum in green, and U band in blue. The white box shows the field of panels © and (d), also from Reines et al. (2011). © Shows radio emission contours (green) superposed on a continuum-subtracted, HST Hα image. Radio and Hα emission from ionized gas regions match. The radio point source (beam size in the inset) is coincident with the X-ray source (not shown). Panel (d) is an HST Hα image that does not have continuum subtracted; it shows that the AGN (+) is not coincident with any of the superstar clusters that are part of the ongoing starburst. Image credit: Reines et al (2011), DSS, John Kormendy

These qualities make such dwarf galaxies analogous to the earliest galaxies in the young universe, suggesting that such early galaxies could also have harboured black holes. Thus, making the study of dwarf galaxies possessing black holes an important stepping stone to the understanding of the mechanism that formed primordial black holes. As a result, the team’s 2011 discovery inspired a search for similar situations using radio telescopes by astronomers across the globe.

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

Reines and her team continued their search by selecting a sample of dwarf galaxies from the NASA-Sloan Atlas — a catalogue of galaxies collated visible-light telescopes — each with stars holding a collective mass of no more than 3 billion solar masses. This is roughly equivalent to the Large Magellanic Cloud — a satellite galaxy of the Milky Way.


Book ’Em, Dano!

The scientists further narrowed down their selection by highlighting 111 candidates that also appeared in the National Radio Astronomy Observatory’s Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey, conducted between 1993 and 2011. The VLA was then used to make new, more sensitive observations and thus, more detailed high-resolution images of these 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,” Reines continues. She points out that the team’s new observations have yielded further surprises. “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.”

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). (Sophia Dagnello, NRAO/AUI/NSF)

Reines believes that these ‘wandering’ off-centre black holes give a strong clue as to mergers in these galaxies’ respective histories. A conclusion that is consistent with computer simulations that predict approximately half of the massive black holes in dwarf galaxies should be found on the outskirts of their home galaxy. The results demonstrate that massive black holes need not exist in the centres of their host 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 concludes.

Reines will present her work at the American Astronomical Society’s meeting in Honolulu, Hawaii. Her study, currently available as a preprint, will then be published in the Astrophysical Journal.


Rob is freelance science journalist from the UK, specialising in physics, astronomy, cosmology, quantum mechanics and obscure comic books.

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The Cosmic Companion

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Robert Lea

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Freelance science journalist. BSc Physics. Space. Astronomy. Astrophysics. Quantum Physics. SciComm. ABSW member. WCSJ Fellow 2019. IOP Fellow.

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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