Molly Porter and Megan Watzke
Scientists have taken major steps in their hunt to find
black holes that are neither very
small nor extremely large.
Finding these elusive
intermediate-mass black holes could help astronomers better
understand what the "seeds" for the largest black holes in the early
The new research comes from two separate studies, each using data
Chandra X-ray Observatory and other
Black holes that contain between about one hundred and several
hundred thousand times the mass of the Sun are called
"intermediate mass" black holes or
This is because their
mass places them in between,
While several tantalizing
possible IMBHs have been reported in recent years, astronomers are
still trying to determine how common they are and what their
properties teach us about the formation of the first
supermassive black holes.
One team of researchers used a large campaign called the
Chandra COSMOS-Legacy survey to
study dwarf galaxies, which contain less than one percent the amount
of mass in stars as our Milky Way does. (COSMOS is an abbreviation
of Cosmic Evolution Survey.)
The characterization of
these galaxies was enabled by the rich dataset available for the
COSMOS field at different wavelengths, including data from NASA and
The Chandra data were crucial for this search because a bright,
point-like source of X-ray emission near the center of a galaxy is a
telltale sign of the presence of a black hole.
The COSMOS Legacy Survey
shows data that have provided evidence
for the existence of
intermediate-mass black holes (IMBHs).
Credits: X-ray: NASA/CXC/ICE/M.Mezcua et al.
Read full caption
The X-rays are produced
by gas heated to millions of degrees by the enormous
gravitational and magnetic forces near the black hole.
"We may have found
that dwarf galaxies are a haven for these missing middleweight
black holes," said Mar Mezcua of the Institute of Space
Sciences in Spain who led one of the studies.
"We didn't just find
a handful of IMBHs - we may have found dozens."
Her team identified forty
growing black holes in
Twelve of them are
located at distances more than five billion light years from Earth
and the most distant is 10.9 billion light years away, the most
distant growing black hole in a dwarf galaxy ever seen.
One of the dwarf galaxies
is the least massive galaxy found to host a growing black hole in
Most of these sources are likely IMBHs with masses that are about
ten thousand to a hundred thousand times that of the Sun. One
crucial result of this research is that the fraction of galaxies
containing growing black holes is smaller for less massive galaxies
than for their more massive counterparts.
A second team led by Igor Chilingarian of the
Harvard-Smithsonian Center for Astrophysics (CfA)
in Cambridge, Mass., found a separate, important sample of possible
IMBHs in galaxies that are closer to us.
In their sample, the most
distant IMBH candidate is about 2.8 billion light years from Earth
and about 90% of the IMBH candidates they discovered are no more
than 1.3 billion light years away.
With data from the Sloan Digital Sky Survey (SDSS),
Chilingarian and his colleagues found galaxies with the optical
light signature of growing black holes and then estimated their
They selected 305
galaxies with properties that suggested a black hole with a mass
less than 300,000 times that of the Sun was lurking in the central
regions of each of these galaxies.
Only 18 members of this list contained high quality X-ray
observations that would allow confirmation that the sources are
Chandra and with
XMM-Newton were obtained for ten
sources, showing that about half of the 305 IMBH candidates are
likely to be valid IMBHs.
The masses for the ten
sources detected with X-ray observations were determined to be
between 40,000 and 300,000 times the mass of the (our) Sun.
"This is the largest
sample of intermediate mass black holes ever found," said
"This black hole
bounty can be used to address one of the biggest mysteries in
IMBHs may be able to
explain how the very biggest black holes, the supermassive ones,
were able to form so quickly after the Big Bang.
One leading explanation
is that supermassive black holes grow over time from smaller black
holes "seeds" containing about a hundred times the Sun's mass. Some
of these seeds should merge to form IMBHs.
Another explanation is
that they form very quickly from the collapse of a giant cloud of
gas with a mass equal to hundreds of thousands of times that of the
Mar Mezcua and her team may be seeing evidence in favor of
the direct collapse idea, because this theory predicts that the less
massive galaxies in their sample should be less likely to contain
"Our evidence is only
circumstantial because it's possible that the IMBHs are just as
common in the smaller galaxies but they're not consuming enough
matter to be detected as X-ray sources", says Mezcua's co-author
Francesca Civano of the CfA.
Chilingarian's team has a
"We're arguing that
just the presence of intermediate mass black holes in the mass
range we detected suggests that smaller black holes with masses
of about a hundred Suns exist," says Chilingarian's co-author
Ivan Yu. Katkov of Moscow State University in Russia.
"These smaller black
holes could be the seeds for the formation of supermassive black
Another possibility is
that both mechanisms actually occur.
Both teams agree that to
make firm conclusions much larger samples of black holes are needed
using data from future satellites.