Sometimes big surprises come wrapped in tiny galactic packages. That’s the strange find from a team of astronomers who discovered an enormous supermassive black hole in a very unlikely place: at the center of an ultracompact dwarf galaxy.

The galaxy known as M60-UCD1, whose oversized black hole was described in the journal Nature, belies the conventional wisdom – that big galaxies have really big black holes at their center, and small galaxies have relatively smaller black holes. And it could mean that the universe hosts many more supermassive black holes than previously thought, since many could be tucked in the centers of tiny, unassuming ultracompact galaxies.

Your standard black hole left over from the death of a star may weigh several to a few dozen suns. But supermassive black holes can hold the mass of millions or even billions of suns, and they are found at the center of almost every massive galaxy in the universe.

“There seems to be some connection between the evolution of galaxies and that of these [supermassive] black holes, although the nature of the relationship is not well understood,” Amy E. Reines, an astronomer at the University of Michigan in Ann Arbor who was not involved in the study, wrote in a commentary.


Supermassive black holes tend to be a tiny fraction – roughly 0.5% – of a spheroidal galaxy’s overall mass, Reines wrote.

By contrast, the black hole in M60-UCD1 is a whopping 15% of its galaxy’s mass. Imagine your head being 15-30 times bigger than it should be. This strange object appears to defy galactic anatomy.

Researchers led by Anil Seth of the University of Utah discovered the disproportionate black hole by clocking the speeds of the nearby stars circling the center. The faster the stars circle, the bigger the black hole must be – and judging by their speed, this one was sizable, about 21 million times the mass of the Sun.

A black hole that big should be in a galaxy containing 7 billion suns’ worth of mass. But judging by the light coming from it, the galaxy was shockingly tiny: It only had 140 million solar masses.


To put this oversized black hole in perspective, 21 million solar masses is more than five times as massive as the Milky Way’s central black hole, which weighs in at about 4 million suns. And yet, our home galaxy’s total mass is about 50 billion suns — more than 350 times as massive as M60-UCD1. The Milky Way could eat this tiny galaxy for an afternoon snack.

The scientists think that M60-UCD1 was actually once a much bigger galaxy, but much of its mass got stripped off by an even-larger neighboring elliptical galaxy, M60. M60 is enormous – its supermassive black hole holds the mass of 4.5 billion suns, more than 1,000 times as big as the Milky Way’s.

A video, sped up in time and with star-flow depicted in red, illustrates this violent encounter. What remains of M60-UCD1 today is indeed tiny: a blurry pinprick next to M60’s wide disc.

This stripping could explain how many ultracompact dwarf galaxies came to be – and why they are so dense. And it means there could be more than twice as many supermassive black holes in the nearby universe as researchers thought.


“Although this is possible, it is far from certain,” Reines wrote. “Future studies will tell us whether M60-UCD1 is a fluke, or whether other ultra-compact dwarfs are also stripped galactic nuclei that host black holes.”

It’s possible that the galaxy is more massive than it looks based on starlight, if there are a bunch of low-mass or dead stars that don’t produce enough light to be seen. But the researchers think that the high concentration of mass in its center gives the black hole away.

M60-UCD1’s black hole may be enormous for its own galaxy but it’s still tiny compared to the one in the middle of M60. Eventually, M60 will swallow the smaller galaxy and their two black holes will merge.

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