The M60-UCD1, discovered by NASA's Hubble Space Telescope in 2013, is one of the smallest known galaxies. But now the space agency has discovered that the dwarf galaxy is harboring a “monster” black hole.
The diameter of M60-UCD1 is about 300 light years – just 1/500th
of our galaxy’s width. However, it is packed with 140 million
stars, which also makes it one of the densest galaxies.
For comparison, NASA explains, the nighttime sky we see from
Earth’s surface shows 4,000 stars. If we lived inside the
newly-discovered M60-UCD1, our nighttime sky would be covered
with at least one million stars “visible to the naked
eye.”
But what really surprised astronomers is the supermassive black
hole they found inside M60-UCD1.
Lurking in the smallest galaxy, the black hole is five times the
mass of the one at the center of our Milky Way galaxy. It has a
mass equal to 21 million suns, and is 15 percent of the small
galaxy's total mass – but less than 0.01 percent of the Milky
Way’s total mass.
“That is pretty amazing, given that the Milky Way is 500
times larger and more than 1,000 times heavier than the dwarf
galaxy M60-UCD1,” University of Utah astronomer Anil Seth,
lead author of an international study on the dwarf galaxy, said
in Nature’s Thursday publication.
The finding has prompted astronomers to consider rethinking dwarf
galaxy theories.
They have now grown to suggest that dwarf galaxies may, in fact,
be the stripped remnants of larger galaxies that were torn apart
during collisions with other galaxies. Until the discovery, they
thought that tiny galaxies were small islands of stars born in
isolation.
“We don’t know of any other way you could make a black hole
so big in an object this small,” Seth said.
The observation also suggests that there are many other compact
galaxies in the universe that contain supermassive black holes.
It could be 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, M60. As a result, the stars and dark matter
in the outer part of the galaxy were torn away and became part of
M60.
It is possible that M60-UCD1 may eventually be pulled to fully
merge with M60, which has its own black hole that is more than
1,000 times bigger than the black hole in our galaxy.
Should this happen, the black holes in both galaxies – which are
50 million light-years away – would also likely merge.