Astronomers discovered a black hole concealed inside a huge star cluster, B023-G078, in the Andromeda galaxy, our nearest neighboring galaxy.
With a mass of one hundred thousand solar masses, this
black hole is smaller than any other black hole yet larger than those developed
when stars explode.
This makes the black hole the only verified black hole
with an intermediate-mass.
B023-G078 was regarded as a massive star cluster.
However, experts contend that the nucleus is empty. The nucleus of stripped
galaxies is the relics of smaller galaxies that collided with larger ones and
had their surrounding stars stripped away by gravitational forces.
Senior author Anil Seth, associate professor of
astronomy at the University of Utah and co-author of the study, said, “We have
very good detections of the biggest, stellar-mass black holes up to 100 times
the size of our sun, and supermassive black holes at the centres of galaxies
that are millions of times the size of our sun, but there aren’t any
measurements of black between these. That’s a large gap. This discovery fills
the gap.”
Lead author Renuka Pechetti of Liverpool John Moores
University, who started the research while at the U, said, “Previously, we’ve
found big black holes within massive, stripped nuclei that are much bigger than
B023-G078. We knew that there must be smaller black holes in lower mass
stripped nuclei, but there’s never been direct evidence. I think this is a
pretty clear case that we have finally found one of these objects.”
Seth said, “I knew that the B023-G078 object was one
of the most massive objects in Andromeda and thought it could be a candidate
for a stripped nucleus. But we needed data to prove it. We’d been applying to
various telescopes to get more observations for many years, and my proposals
always failed. When we discovered a supermassive black hole within a stripped
nucleus in 2014, the Gemini Observatory gave us the chance to explore the
idea.”
The researchers acquired data from the Gemini
Observatory and photos from the Hubble Space Telescope for their investigation.
Using the data, they modeled the light profile of B023-G078 to determine its
mass distribution.
Near the core of a globular cluster, the
characteristic light profile has the same form as the outside areas. B023-G078
is different. The light in the center is spherical and becomes flattered as it
moves outward. The chemical composition of the stars also varies, with the
central stars containing more heavy elements than those towards the object's
periphery.
Seth said, “Globular star clusters form at the same
time. In contrast, these stripped nuclei can have repeated formation episodes,
where gas falls into the centre of the galaxy and forms stars. And other star
clusters can get dragged into the centre by the galaxy’s gravitational forces.
It’s kind of the dumping ground for a bunch of different stuff. So, stars in
stripped nuclei will be more complicated than globular clusters. And that’s
what we saw in B023-G078.”
Using the object's mass distribution, the scientists
projected the speed at which stars inside the cluster should be travelling at
any given position. They then matched it to their own data. They discovered
that the fastest-moving stars orbited the center.
When scientists constructed a model without a black
hole, the central stars moved too slowly relative to their observations. When
the black hole was introduced, the speeds matched the data. The black hole adds
to the evidence that this item is a nucleus that has been stripped.
Pechetti said, “The stellar velocities we are getting
gives us direct evidence that there’s some dark mass right at the centre. It’s
tough for globular clusters to form big black holes. But if it’s in a stripped
nucleus, then there must already be a black hole present, left as a remnant
from the smaller galaxy that fell into, the bigger one.”
Seth said, “We know big galaxies generally form from
the merging of smaller galaxies, but these stripped nuclei allow us to decipher
the details of those past interactions.”
Reference: Research Paper
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