Intermediate-Mass Black Hole with Torn-Apart Star (Artist’s Impression)
Hubble Observation of Intermediate-Mass Black Hole
Ground-Based View of J2150−0551 Region
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New data from the NASA/ESA Hubble Space
Telescope have provided the strongest evidence yet for mid-sized black
holes in the Universe. Hubble confirms that this “intermediate-mass”
black hole dwells inside a dense star cluster.
Intermediate-mass black holes (IMBHs) are a long-sought
“missing link” in black hole evolution. There have been a few other IMBH
candidates found to date. They are smaller than the supermassive black
holes that lie at the cores of large galaxies, but larger than
stellar-mass black holes formed by the collapse of massive stars. This
new black hole is over 50 000 times the mass of our Sun.
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IMBHs are hard to find. “Intermediate-mass black holes
are very elusive objects, and so it is critical to carefully consider
and rule out alternative explanations for each candidate. That is what
Hubble has allowed us to do for our candidate,” said Dacheng Lin of the University of New Hampshire, principal investigator of the study1.
Lin and his team used Hubble to follow up on leads from NASA’s Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton), which carries three high-throughput X-ray telescopes and
an optical monitor to make long uninterrupted exposures providing highly
sensitive observations.
“Adding further X-ray observations allowed us to understand the total energy output,” said team member Natalie Webb of the Université de Toulouse in France. “This helps us to understand the type of star that was disrupted by the black hole.”
"In 2006 these high-energy satellites detected a powerful
flare of X-rays, but it was not clear if they originated from inside or
outside of our galaxy. Researchers attributed it to a star being torn
apart after coming too close to a gravitationally powerful compact
object, like a black hole.
Surprisingly, the X-ray source, named 3XMM
J215022.4−055108, was not located in the centre of a galaxy, where
massive black holes normally reside. This raised hopes that an IMBH was
the culprit, but first another possible source of the X-ray flare had to
be ruled out: a neutron star
in our own Milky Way galaxy, cooling off after being heated to a very
high temperature. Neutron stars are the extremely dense remnants of an
exploded star.
Hubble was pointed at the X-ray source to resolve its
precise location. Deep, high-resolution imaging confirmed that the
X-rays emanated not from an isolated source in our galaxy, but instead
in a distant, dense star cluster on the outskirts of another galaxy —
just the sort of place astronomers expected to find evidence for an
IMBH. Previous Hubble research has shown that the more massive the
galaxy, the more massive its black hole. Therefore, this new result
suggests that the star cluster that is home to 3XMM J215022.4−055108 may
be the stripped-down core of a lower-mass dwarf galaxy that has been
gravitationally and tidally disrupted by its close interactions with its
current larger galaxy host.
IMBHs have been particularly difficult to find because they
are smaller and less active than supermassive black holes; they do not
have readily available sources of fuel, nor do they have a gravitational
pull that is strong enough for them to be constantly drawing in stars
and other cosmic material and producing the tell-tale X-ray glow.
Astronomers therefore have to catch an IMBH red-handed in the relatively
rare act of gobbling up a star. Lin and his colleagues combed through
the XMM-Newton data archive, searching hundreds of thousands of sources
to find strong evidence for this one IMBH candidate. Once found, the
X-ray glow from the shredded star allowed astronomers to estimate the
black hole’s mass.
Confirming one IMBH opens the door to the possibility that
many more lurk undetected in the dark, waiting to be given away by a
star passing too close. Lin plans to continue this meticulous detective
work, using the methods his team has proved successful.
“Studying the origin and evolution of the intermediate mass black holes will finally give an answer as to how the supermassive black holes that we find in the centres of massive galaxies came to exist,” added Webb.
Black holes are one of the most extreme environments humans are aware of, and so they are a testing ground for the laws of physics and our understanding of how the Universe works. Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favoured home? With a confident conclusion to one mystery, Lin and other black hole astronomers find they have many more exciting questions to pursue.
Notes
[1] The results are published in the Astrophysical Journal Letters and were a result of the HST Program GO-15441
More information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study consists of D. Lin, J. Strader, A. J. Romanowski, J. A. Irwin, O. Godet, D. Barret, N. A. Webb, J. Homan, and R. A. Remillard.
Image credit: ESA/Hubble, M. Kornmesser
Links
Contact
Dacheng Lin
University of New Hampshire
Durham, New Hampshire, USA
Email: dacheng.lin@unh.edu
Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: bethany.downer@partner.eso.org
Source: ESA/Hubble/News
