In this illustration, a massive black hole that’s far greater than stellar mass systems but lighter than supermassive black holes is uncovered in a dwarf galaxy. The black hole is not located at the galaxy’s center, which is a phenomenon only seen in dwarf galaxies with irregular shapes/morphologies, likely indicative of a recent merger or interaction. (SOPHIA DAGNELLO, NRAO/AUI/NSF)

The Smallest Galaxies Have Off-Kilter Black Holes, But Astronomers Know Why

A decade ago, we didn’t know if dwarf galaxies had black holes. Today, half of the ones we see aren’t where we expected.

Ethan Siegel
Jan 13 · 3 min read

Normally, galaxies have supermassive black holes millions to billions of times the Sun’s mass.

The supermassive black hole at the center of our galaxy, Sagittarius A*, flares brightly in X-rays whenever matter is devoured. In longer wavelengths of light, from infrared to radio, we can see the individual stars in this innermost portion of the galaxy. Gas emissions indicated a supermassive black hole of ~2.7 million solar masses, but improved observations of stars at the galactic center revealed a mass of ~4 million solar masses instead. (X-RAY: NASA/UMASS/D.WANG ET AL., IR: NASA/STSCI)

So far, they’ve always been found at the host galaxy’s center, driven there by gravitational interactions and astrophysical dynamics.

When a large number of gravitational interactions between star systems occur, one star can receive a large enough kick to be ejected from whatever structure it’s a part of, while the most massive objects eventually sink to the centers of bound systems. For entire galaxies, this process drives supermassive black holes towards the galactic centers. (J. WALSH AND Z. LEVAY, ESA/NASA)

Their presence is detectable when matter falls in, causing radio and X-ray emission activity.

The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, is shown in three views here. At the top is optical from Hubble, at the lower-left is radio from NRAO, and at the lower-right is X-ray from Chandra. These differing views have different resolutions dependent on the optical sensitivity, wavelength of light used, and size of the telescope mirrors used to observe them. These are all examples of radiation emitted from the regions around black holes, demonstrating that black holes aren’t so black, after all. (TOP, OPTICAL, HUBBLE SPACE TELESCOPE / NASA / WIKISKY; LOWER LEFT, RADIO, NRAO / VERY LARGE ARRAY (VLA); LOWER RIGHT, X-RAY, NASA / CHANDRA X-RAY TELESCOPE)

However, dwarf galaxies ⁠ — much smaller and lower in mass ⁠ — are expected to have black holes measuring just ~10,000–1,000,000 solar masses.

The dwarf galaxy UGC 5340 is forming stars irregularly, likely due to a gravitational interaction with a companion galaxy that is not pictured here. Gravitational interactions often trigger new star formation, leading to the collapse of interior gas clouds. Dwarf galaxies should have intermediate mass black holes within them: more than tens of thousands of solar masses but under a million solar masses. (NASA, ESA, AND THE LEGUS TEAM)

Over 100 dwarf galaxies are now known to possess these black holes, with the first verified one discovered in 2011.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2–10, located 30 million light years away. This is the first dwarf galaxy to be found with a very massive, but not supermassive, black hole in its interior. (X-RAY (NASA/CXC/VIRGINIA/A.REINES ET AL); RADIO (NRAO/AUI/NSF); OPTICAL (NASA/STSCI))

However, solely finding radio emissions isn’t enough: active black holes and star-formation bursts can create that signal.

The dwarf galaxy UGCA 281, shown here as imaged by Hubble in the visible and ultraviolet, is rapidly forming new stars. Radio emissions coming from galaxies could either indicate the presence of a feeding massive black hole, or, as is the case here, a region of active star formation. (NASA, ESA, AND THE LEGUS TEAM)

Researchers led by Dr. Amy Reines just conducted the first large-scale radio survey looking for black holes in dwarf galaxies.

A small section of the Karl Jansky Very Large Array, one of the world’s largest and most powerful arrays of radio telescopes. The radio capabilities of this array, in terms of resolution and sensitivity, place it among the top 2 or 3 arrays in the entire world. (JOHN FOWLER)

Using the Very Large Array, her team surveyed 111 dwarf galaxies, and found 13 of them that showed evidence for massive black holes.

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. (SOPHIA DAGNELLO, NRAO/AUI/NSF; DECALS SURVEY; CTIO)

Remarkably, approximately half of the black holes were not located at the galaxy’s centers, but were significantly off-kilter.

These 13 galaxies, out of the 111 candidates imaged by the VLA, all show evidence for an active massive black hole in their interior. Only approximately half of these galaxies have the black hole’s location aligning with the galaxy’s physical center. (A. E. REINES ET AL. (2019), ARXIV:1909.04670)

The reason is straightforward but fascinating: the quiet galaxies have centered black holes, but merging/interacting galaxies have them off-center.

6 of the 13 dwarf galaxies imaged from the VLA study show a significant offset from the center in the location of the black hole. All of these galaxies show evidence in their morphology for a recent merger or gravitational interaction with a companion galaxy. (A. E. REINES ET AL. (2019), ARXIV:1909.04670)

Perhaps, when they finish settling down, their black holes will be centered after all.


Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is now on Forbes, and republished on Medium on a 7-day delay. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Ethan Siegel

Written by

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Welcome to a place where words matter. On Medium, smart voices and original ideas take center stage - with no ads in sight. Watch
Follow all the topics you care about, and we’ll deliver the best stories for you to your homepage and inbox. Explore
Get unlimited access to the best stories on Medium — and support writers while you’re at it. Just $5/month. Upgrade