Discovery of a Mighty Black Hole in a Dwarf Galaxy
21 September 2014


A team of astronomers, using data from NASA’s Hubble Space Telescope HST) and ground-based observatories, has unexpectedly found a monster black hole residing inside one of the smallest known galaxies.

The black hole is five times more massive than the one at the center of our giant Galaxy, the Milky Way. It is inside one of the densest galaxies known so far, the dwarf galaxy M60-UCD1, which has 140 million stars within a diameter of about 300 light-years or 0.2% the Milky Way’s diameter.

To an observer inside M60-UCD1, the night sky would sparkle with at least 1 million stars visible to the naked eye. For comparison, about 4,000 stars can be perceived with the unaided in Earth’s nighttime skies.

The finding demonstrates there are many other compact galaxies in the universe that contain supermassive black holes. The observation also indicates dwarf galaxies may actually be the ripped remnants of larger galaxies that were destroyed during collisions with other galaxies.

The team of researchers, led by astronomer Anil Seth, used the HST and the Gemini North 8-meter optical and infrared telescope in Hawaii to observe M60-UCD1 and measure its black hole’s mass. The detailed HST images provide information about the galaxy’s size and stellar population. Gemini measures the movements of the stars as affected by the black hole’s powerful gravitation. These data are used to compute the mass of the black hole.

Black holes are small extremely dense objects that have a gravitational pull so strong that even light, travelling at a speed of almost 300,000 km/s, cannot escape. Supermassive black holes, those with masses of at least one million times the mass of the Sun, are thought to reside at the centers of many galaxies.

The black hole at the center of the Milky Way is four million times more massive than the Sun. However, it comprises less than 0.01% of the Galaxy’s total mass. By comparison, M60-UCD1’s monster black hole, whose mass is 21 million solar masses, is an astounding 15% percent of the total mass of its mother galaxy.

One explanation is that M60-UCD1 was once a large galaxy containing 10 billion stars, but then it passed very close to the center of an even larger galaxy, termed M60, and during that fatal encounter, all the stars and matter in the outer part of M60-UCD1 were pulled away, and captured by M60.

Seth’s team believes that M60-UCD1 may ultimately collide, and merge with M60, which has its own enormous black hole weighing 4.5 billion solar masses or more than 1,000 times the mass of the Milky Way’s black hole. When the merger occurs, the black holes in the two galaxies will probably merge. Both galaxies are located about 50 million light-years away from Earth.

References and Further Reading

Aymen Mohamed Ibrahem
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