Small Galaxy Boasts Big Black Hole

Astronomers have detected a supermassive black hole in the center of a tiny galaxy — where it has no right to be.

Don't be fooled by the small size of the ultracompact dwarf galaxy M60-UCD1 — it harbors a supermassive black hole, according to research published in the September 18th Nature. The new finding makes M60-UCD1 the smallest and least massive galaxy known to contain such a gargantuan black hole and is the first concrete evidence for how ultracompact dwarf galaxies form.

ultracompact dwarf galaxy

This Hubble Space Telescope image shows the massive elliptical galaxy Messier 60, which overshadows the tiny dwarf galaxy M60-UDC1 (inset). M60-UCD1 is only 300 light-years across yet contains a black hole roughly 10 times more massive than the one that sits in the Milky Way’s center.
NASA / ESA / A. Seth (University of Utah)

M60-UCD1 is located in the Virgo Cluster, about 54 million light-years from Earth. It is one of the most massive and brightest ultracompact dwarf galaxies, objects that can shove up to 200 million solar masses into a radius 160 light-years or less. They are similar in size to globular clusters but are at least 10 times more massive.

Ultracompact dwarf galaxies have perplexed astronomers since they were discovered over a decade ago. Scientists were uncertain how these tiny galaxies formed. Two competing theories emerged: either they were unusually massive star clusters, or they were the remnants of larger galaxies that had been stripped down to their cores by the gravitational pull of massive neighbors.

The study is the first to provide observational evidence for the stripped-down theory, by revealing the presence of a supermassive black hole. Supermassive black holes have more than a million times the mass of the Sun and are found at the centers of large galaxies, but usually not in dwarf galaxies. But at 21 million times the mass of the sun, M60-UCD1’s black hole is “remarkably massive,” says study coauthor Anil Seth (University of Utah). “It’s about five times more massive than the Milky Way’s black hole, in spite of the fact that this object — this ultracompact dwarf galaxy — is 500 times smaller and a thousand times less massive” than the Milky Way.

The existence of the supermassive black hole indicates that M60-UCD1 was once a larger galaxy, and that many of its stars were later ripped away by interactions with its neighbor, the much larger elliptical galaxy M60. Given the black hole's mass, the original galaxy likely had a central bulge roughly 100 times more massive than the current galaxy's total stellar mass.

“They’ve really, finally, after a decade, got some solid evidence that it’s one model rather than the other, that at least some of them must have been at the center of a galaxy in the past,” says Michael Drinkwater (University of Queensland, Australia), who was not involved in the research.

M60-UCD1’s black hole is particularly impressive given the galaxy’s small size. It makes up a whopping 15 percent of the galaxy's total mass, whereas the black hole in our Milky Way makes up only a fraction of a percent of the total mass of the galaxy. Most galaxies follow the Milky Way’s pattern, although exceptions exist.

Characterizing such a tiny object is a difficult task. The astronomers used the Gemini North 8-meter telescope on Hawaii’s Mauna Kea to discern the motions of stars in the galaxy. They found that the stars in the center of the galaxy were orbiting at roughly 175 kilometers per second (390,000 mph), much faster than expected and indicating the presence of a black hole.

“Immediately, as soon as I saw the stellar motions map, I knew that there was something exciting,” says Seth. “It had a larger black hole than even we had considered as our maximum case.”

The result could have some broader implications, the astronomers say. There are many ultracompact dwarf galaxies — around 50 are known in the nearest galaxy clusters — and these objects have a common peculiarity. Many ultracompact dwarf galaxies are more massive than expected based upon their luminosities — a possible indication of a black hole, but not the only plausible explanation.

However, in the case of M60-UCD1, once the astronomers included the black hole in their calculations, the galaxy’s stellar mass matched what astronomers would expect, given its luminosity. This result, they argue, indicates that the same process is likely also the explanation for the high mass estimates for other ultracompact dwarf galaxies.

Another possible explanation for the unexpectedly large masses is that the average mass of stars in these dwarfs is much higher than in standard galaxies.

“Really, until we go and measure a few more, it’s hard to know the relative importance of the two origins,” says Drinkwater.

And measuring a few more will be the next step in this research. Most of the known ultracompact dwarf galaxies are too faint to study with this method, but the next generation of telescopes should allow astronomers to look for more supermassive black holes in other objects of this type.

The result indicates a new place for scientists to search for black holes. If the astronomers are correct and many ultracompact dwarf galaxies do contain black holes at their centers, then the team predicts that the true number of massive black holes in the local universe may be twice the current estimate.

This is still speculation at this stage, Drinkwater says, but it’s an idea that would be interesting to investigate.

“This certainly opens up that possibility that they’re there, and that’s very exciting,” said Seth.

 

Reference: A. C. Seth et al. "A supermassive black hole in an ultra-compact
dwarf galaxy." Nature. September 18, 2014.


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One thought on “Small Galaxy Boasts Big Black Hole

  1. Peter WilsonPeter Wilson

    Two competing theories emerged: either they were unusually massive star clusters, or they were the remnants of larger galaxies that had been stripped down to their cores by the gravitational pull of massive neighbors.

    First, not to quibble, but it’s not the “gravitational pull of massive neighbors” that may cause the stripping, it’s the “close encounter” that would do it. Given M60’s black hole weighs in @ 4.5 billion solar masses, one can calculate how close UDC1 would have had to pass in order to have been stripped down to its current size.

    As to the two theories, the obvious question: What do the simulations say?

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