NGC 6278 and PGC 039620 are two galaxies from a sample
of 1,600 that were searched for the presence of supermassive black holes. These
images represent the results of a study that suggests that smaller galaxies do
not contain supermassive black holes nearly as often as larger galaxies do. The
study analyzed over 1,600 galaxies that have been observed with Chandra over
two decades. Certain X-ray signatures indicate the presence of supermassive
black holes. The study indicates that most smaller galaxies like PGC 03620,
shown here in both X-rays from Chandra and optical light images from the Sloan
Digital Sky Survey, likely do not have supermassive black holes in their
centers. In contrast, NGC 6278, which is roughly the same size as the Milky
Way, and most other large galaxies in the sample show evidence for giant black
holes within their cores.
X-ray: NASA/CXC/SAO/F. Zou et al.; Optical: SDSS;
Image Processing: NASA/CXC/SAO/N. Wolk
Most smaller galaxies may not have
supermassive black holes in their centers, according to a recent study
using NASA’s Chandra X-ray Observatory. This contrasts with the common idea
that nearly every galaxy has one of these giant black holes within their cores,
as NASA leads the world in exploring how our universe works.
A team of astronomers used data from
over 1,600 galaxies collected in more than two decades of the Chandra mission.
The researchers looked at galaxies ranging in heft from over ten times the mass
of the Milky Way down to dwarf galaxies, which have stellar masses less than a
few percent of that of our home galaxy. A paper describing these results has
been published in The Astrophysical Journal and is available here https://arxiv.org/abs/2510.05252.
The team has reported that only about
30% of dwarf galaxies likely contain supermassive black holes.
“It’s important to get an accurate black
hole head count in these smaller galaxies,” said Fan Zou of the University of
Michigan in Ann Arbor, who led the study. “It’s more than just bookkeeping. Our
study gives clues about how supermassive black holes are born. It also provides
crucial hints about how often black hole signatures in dwarf galaxies can be
found with new or future telescopes.”
As material falls onto black holes, it
is heated by friction and produces X-rays. Many of the massive galaxies in the
study contain bright X-ray sources in their centers, a clear signature of
supermassive black holes in their centers. The team concluded that more than
90% of massive galaxies – including those with the mass of the Milky Way –
contain supermassive black holes.
However, smaller galaxies in the study
usually did not have these unambiguous black hole signals. Galaxies with masses
less than three billion Suns – about the mass of the Large Magellanic Cloud, a
close neighbor to the Milky Way – usually do not contain bright X-ray sources
in their centers.
The researchers considered two possible
explanations for this lack of X-ray sources. The first is that the fraction of
galaxies containing massive black holes is much lower for these less massive
galaxies. The second is the amount of X-rays produced by matter falling onto
these black holes is so faint that Chandra cannot detect it.
“We think, based on our analysis of the
Chandra data, that there really are fewer black holes in these smaller galaxies
than in their larger counterparts,” said Elena Gallo, a co-author also from the
University of Michigan.
To reach their conclusion, Zou and his
colleagues considered both possibilities for the lack of X-ray sources in small
galaxies in their large Chandra sample. The amount of gas falling onto a black
hole determines how bright or faint they are in X-rays. Because smaller black
holes are expected to pull in less gas than larger black holes, they should be
fainter in X-rays and often not detectable. The researchers confirmed this
expectation.
However, they found that an additional
deficit of X-ray sources is seen in less massive galaxies beyond the expected
decline from decreases in the amount of gas falling inwards. This additional
deficit can be accounted for if many of the low-mass galaxies simply don’t have
any black holes at their centers. The team’s conclusion was that the drop in
X-ray detections in lower mass galaxies reflects a true decrease in the number
of black holes located in these galaxies.
This result could have important
implications for understanding how supermassive black holes form. There are two
main ideas: In the first, a giant gas cloud directly collapses into a black
hole, which contains thousands of times the Sun’s mass from the start. The
other idea is that supermassive black holes instead come from much smaller
black holes, created when massive stars collapse.
“The formation of big black holes is
expected to be rarer, in the sense that it occurs preferentially in the most
massive galaxies being formed, so that would explain why we don’t find black
holes in all the smaller galaxies,” said co-author Anil Seth of the University
of Utah.
This study supports the theory where
giant black holes are born already weighing several thousand times the Sun’s
mass. If the other idea were true, the researchers said they would have
expected smaller galaxies to likely have the same fraction of black holes as
larger ones.
This result also could have important
implications for the rates of black hole mergers from the collisions of dwarf
galaxies. A much lower number of black holes would result in fewer sources of
gravitational waves to be detected in the future by the Laser Interferometer
Space Antenna. The number of black holes tearing stars apart in dwarf galaxies
will also be smaller.
NASA’s Marshall Space Flight Center in
Huntsville, Alabama, 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.
To learn more about Chandra, visit: https://science.nasa.gov/chandra
Source: NASA's Chandra Finds Small Galaxies May Buck the Black Hole Trend - NASA
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