The discovery of a supermassive black hole
in a relatively small galaxy could help astronomers unravel the mystery
surrounding how the very biggest black holes grow.
Researchers used NASA’s Chandra X-ray Observatory to
identify a black hole containing about 200,000 times the mass of the Sun buried
in gas and dust in the galaxy Mrk 462.
Mrk 462 contains only several hundred million stars,
making it a dwarf galaxy. By contrast, our Milky Way is home to a few hundred
billion stars. This is one of the first times that a heavily buried, or
“obscured,” supermassive black hole has been found in a dwarf galaxy.
“This black hole in Mrk 462 is among the smallest of
the supermassive, or monster, black holes,” said Jack Parker of Dartmouth
College in New Hampshire, who led the study with colleague Ryan Hickox, also
from Dartmouth. “Black holes like this are notoriously hard to find.”
In larger galaxies astronomers often find black holes
by looking for the rapid motions of stars in the centers of galaxies. However,
dwarf galaxies are too small and dim for most current instruments to detect
this. Another technique is to search for the signatures of growing black holes,
such as gas being heated up to millions of degrees and glowing in X-rays as it
falls towards a black hole.
The researchers in this study used Chandra to look at
eight dwarf galaxies that had previously shown hints of black hole growth from
optical data gathered by the Sloan Digital Sky Survey. Of those eight, only Mrk
462 showed the X-ray signature of a growing black hole.
The unusually large intensity of high energy X-rays
compared to low energy X-rays, along with comparisons to data at other
wavelengths, indicates that the Mrk 462 black hole is heavily obscured by gas.
“Because buried black holes are even harder to detect
than exposed ones, finding this example might mean there are a lot more dwarf
galaxies out there with similar black holes,” said Hickox. “This is important
because it could help address a major question in astrophysics: How did black
holes get so big so early in the universe?”
Previous research has shown that black holes can grow
to a billion solar masses by the time the universe is less than a billion years
old, a small fraction of its current age. One idea is that these huge objects
were created when massive stars collapsed to form black holes that weighed only
about 100 times the mass of the Sun. Theoretical work, however, struggles to
explain how they could pack on weight quickly enough to reach the sizes seen in
the early universe.
An alternative explanation is that the early universe
was seeded with black holes containing tens of thousands of solar masses when
they were created – perhaps from the collapse of gigantic clouds of gas and
dust.
A large fraction of dwarf galaxies with supermassive
black holes favors the idea that small black hole seeds from the earliest
generation of stars grew astonishingly quickly to form the billion solar mass
objects in the early universe. A smaller fraction would tip the scales to favor
the idea that black holes began life weighing tens of thousands of Suns.
These expectations apply because the conditions
necessary for the direct collapse from a giant cloud to a medium-sized black
hole should be rare, so it is not expected that a large fraction of dwarf galaxies
would contain supermassive black holes. Stellar-mass black holes, on the other
hand, are expected in every galaxy.
“We can’t make strong conclusions from one example,
but this result should encourage much more extensive searches for buried black
holes in dwarf galaxies,” said Parker. “We’re excited about what we might
learn.”
These results were scheduled to be presented at the
239th meeting of the American Astronomical Society meeting in Salt Lake City,
and were part of a virtual press briefing held on Monday, January 10th.
NASA's Marshall Space Flight Center manages the
Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray
Center controls science operations from Cambridge, Massachusetts, and flight
operations from Burlington, Massachusetts.
Image credit: X-ray: NASA/CXC/Dartmouth Coll./J.
Parker & R. Hickox; Optical/IR: Pan-STARRS
Read more from NASA's Chandra X-ray Observatory.
For more Chandra images, multimedia and related
materials, visit: http://www.nasa.gov/chandra
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