In the center of most galaxies lies a supermassive black hole. Some of these
are actively feeding on the gas and dust around them, expelling excess
energy as powerful jets that are seen as quasars across the entire
observable Universe. A new study led by astronomers at the Cosmic Dawn
Center reviewed this process using new techniques—and the results may change
how we think about the diets of these cosmic behemoths.
Located in the center of galaxies, supermassive black holes are millions or
even billion times more massive than our Sun. With their extreme
gravitational pull, they are able to engulf vast amounts of gas, dust, and
perhaps even stars that wander into their vicinity.
Physics tells us that this material tends to form a disk as it is drawn
towards the black hole in a phenomenon called "accretion." Now these
accretion disks are some of the most uninviting, violent places in the known
Universe, with velocities approching the speed of light, and temperatures
far in excess of the surface of our Sun. This heat produces radiation which
we see as light, but the conversion of heat to light is so efficient—about
30 times more efficient than nuclear fusion—that physicists don't quite
understand how.
Hungry cosmic behemoths
The dietary patterns of black holes have wide range. Some, like the one in
our own Galaxy, aren't very hungry and don't seem to have accretion disks.
But we see other galaxies with ravenous hunger whose supermassive black
holes have grown extremely hot accretion disks so bright that they outshine
all of the stars in their galaxy.
Only recently have we obtained our first picture of an accretion disk from
the Event Horizon Telescope, a worldwide network of radio telescopes.
However, this accretion disk belongs to a very nearby galaxy. We cannot
repeat this experiment with more distant galaxies as the disks are simply
too small and so are unresolved, even by the largest telescopes.
Variability is key
Fortunately another method of probing the size and structure of distant
accretion disks seems promising: Although we cannot resolve the disks'
various components, we can study how its intensity varies in time. By
studying the variations in the disks' light we can piece together a picture
of the accretion disks of even the most distant galaxies.
This is what DAWN Ph.D. Fellow John Weaver has done, looking into past
observations of more than 9,000 galaxies with bright accretion disks—the
so-called quasars—from the observational program "Sloan Digital Sky Survey."
When the source is not resolved, the observed light from the accretion disk
will be "contaminated" by light from the galaxy hosting the black hole. This
unwanted light from the host galaxies has largely been ignored by previous
studies. However, by using a new model for the variations in the quasar
light, John Weaver and his collaborator Keith Horne, professor of astronomy
at the University of St Andrews, were able to separate the light of the
accretion disk from that of the host galaxy.
In other words, the model allowed them to more directly see the light from
the accretion disk around supermassive black holes, even in galaxies
billions of lightyears away.
Obscured by dust
What Weaver and Horne found was that cosmic dust near the accretion disk was
likely blocking their view. Using several different models of cosmic dust to
account for, and remove, its obscuring effects, they were able to determine
how hot the accretion disk is, both near the black hole and far from it at
the edges of the disk.
This difference in temperature between the hot inner disk and the cold outer
disk has been theoretically predicted. However, what Weaver and Horne found
observationally was a very different picture of the temperature of the disk:
the disks turned out to be even hotter near the black hole than predicted.
These unexpected findings were published today in the Monthly Notices of The
Royal Astronomical Society and suggest that our assumptions and theoretical
models need to be revised—with consequences for our understanding of
supermassive black holes altogether.
Not only do we have more to learn about supermassive black holes, but the
variations in their ravenous hunger are a marvelous demonstration that our
Universe is a far more dynamic place than one would expect looking at the
static night time sky.
Reference:
John R Weaver et al, Dust and the intrinsic spectral index of quasar
variations: hints of finite stress at the innermost stable circular orbit,
Monthly Notices of the Royal Astronomical Society (2022).
DOI: 10.1093/mnras/stac248
Tags:
Space & Astrophysics