Dwarf galaxy with black giant

Astronomers discover a supermassive black hole where it really should not be

The Ultra Compact Dwarf Galaxy M60-UCD1 (bottom right) © NASA / Space Telescope Science Institute / ESA
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Surprising find: At the heart of a compact dwarf galaxy, astronomers have discovered a black hole that is even more massive than the singularity in our Milky Way galaxy. So far, such a pairing was considered impossible. How a dwarf comes to such a "giant" is unclear. Presumably, however, a collision of the dwarf galaxy only later robbed a large amount of their stars, as the researchers report in the journal "Nature".

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Black holes, which arise from massive stars, are abundant in the universe and also in our Milky Way. But singularities that combine more than a million solar masses are much rarer. These extremely massive black holes were previously only known from the centers of larger galaxies. They apparently originated in the early days of the universe and are considered a kind of obstetrician of such galaxies.

Surprising exception

Even in the heart of the Milky Way, such a supermassive black hole exists, it has about four million solar masses, but still accounts for only 0.01 percent of the total mass of our home galaxy. Until now, it was assumed that such "mass-giants" only occur in larger galaxies, where there is enough matter around them to compensate for the black hole in the nucleus. Because observations show that the relationship between the mass of central black holes and the mass of galaxies is always about the same.

Now, however, astronomers around Anil Seth of the University of Utah have discovered the famous exception to this rule - one that challenges the entire rule. Using the Gemini North Telescope on Mauna Kea in Hawaii, the researchers had studied the compact dwarf galaxy M60-UCD1. This star cluster is located some 54 million light-years from Earth and is one of the densest star systems in the universe: it is only a few hundred light-years tall, but contains more than 140 million solar masses. display

Giant in the heart of a dwarf

When the astronomers studied the moving velocities of the stars in this dwarf galaxy, they found that the stars in the center were moving much faster than expected. Such a high speed usually only occurs when there is a supermassive black hole in the center.

According to the researchers, this black hole in M60-UCD1 must have a mass of 21 million sun, making up 15 percent of the total mass of this galaxy. "That makes the galaxy one of the star systems dominated most by a black hole, " says Seth. At the same time, it is the smallest and lightest of all the known galaxies that has such a supermassive black hole.

Star theft among galaxies?

Why such a small galaxy still has such a massive singularity is so far unclear. The researchers suspect, however, that M60-UCD1 was once much larger than you lost a large part of its stars only after a collision with a larger galaxy. "We are convinced that M60-UCD1 once had around 10 billion stars", explains Nadine Neumayer from the Max Planck Institute for Astronomy. "But then it came close to the center of the even larger galaxy M60 and lost by gravitational interaction all the stars and their dark matter in the Au enbereich."

"We know of no other way that could explain the existence of such a massive black hole in such a small object, " adds Remco van den Bosch of the Max Planck Institute for Astronomy. The current observation suggests, however, that many other ultra-compact dwarf galaxies could host such central black holes. "Since there are a huge number of similar ultra-compact dwarf galaxies in the universe, they could all contain supermassive black holes, as we know from the centers of normal galaxies, " says van den Bosch. (Nature, 2014; doi: 10.1038 / nature13762)

(University of Utah / Max Planck Institute for Astronomy, 18.09.2014 - NPO)