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EI2GYB > ASTRO 28.09.23 00:04l 108 Lines 5835 Bytes #999 (0) @ WW
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Subj: Hidden supermassive black holes reveal their secrets throug
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Hidden supermassive black holes reveal their secrets through radio signals
Date:
September 25, 2023
Source:
Newcastle University
Summary:
Astronomers have found a striking link between the amount of dust
surrounding a supermassive black hole and the strength of the radio emission
produced in extremely bright galaxies.
Astronomers have found a striking link between the amount of dust surrounding a
supermassive black hole and the strength of the radio emission produced in
extremely bright galaxies. The findings are published in the Monthly Notices of
the Royal Astronomical Society.
The team of international astronomers, led by Newcastle University and Durham
University, UK, used new data from the Dark Energy Spectroscopic Instrument
(DESI), which is conducting a five year survey of large scale structure in the
universe that will include optical spectra for ~3 million quasars; extremely
bright galaxies powered by supermassive black holes. They found that quasars
that contained more dust, and therefore appeared redder, were more likely to
have stronger radio emission compared to the quasars that had very little-to-no
dust, appearing very blue.
Almost every known galaxy contains a supermassive black hole, which are black
holes with a mass millions to billions that of our Sun, at its centre,
including our own Milky Way. In some galaxies there is lots of material in the
centre, feeding and growing this supermassive black hole, making it very
energetic and "active." The most powerful type of these active galaxies are
called "quasars," which are some of the brightest objects in the Universe. Most
quasars appear very blue, due to the bright disc of matter that orbits and
feeds the central supermassive black hole which is very bright in optical and
ultraviolet wavelengths. However, astronomers have found that a significant
fraction of these quasars appear very red, although the nature of these objects
is still not well understood.
In order to understand the physics of these red quasars, "spectroscopic"
measurements are required, which can be used to analyse the quasar light at
different wavelengths. The "shape" of the quasar's spectrum can indicate the
amount of dust present surrounding the central region. Observing the radio
emission from quasars can also tell you about the energetics of the central
supermassive black hole; whether it is launching powerful "winds" or "jets"
that might shape the surrounding galaxy.
This new study, led by Dr Victoria Fawcett of Newcastle University, and
previously Durham University, uses spectroscopic observations from DESI to
measure the amount of dust (reddening) in a sample of ~35,000 quasars and link
this to the observed radio emission. They find that DESI is capable of
observing much more extreme red (dusty) quasars compared to similar/previous
spectroscopic surveys, such as the Sloan Digital Sky Survey (SDSS). They also
find that redder quasars are much more likely to have strong radio emission
compared to typical blue quasars.
Dr Fawcett says: "It was really exciting to see the amazing quality of the DESI
data and to discover thousands of these, previously rare, red quasars. I feel
like this study puts lots of the puzzle pieces together in our understanding of
red quasars and definitively links the dust in a quasar to its radio emission.
I think this is the strongest evidence so far that red quasars are a key
element in how galaxies evolve."
This reddening-radio connection is likely due to powerful outflows of gas
driven away from the supermassive black hole, which slam into the surrounding
dust, causing shocks and radio emission. These outflows will eventually blow
away all the dust and gas in the central region of the galaxy, revealing a blue
quasar and resulting in weaker radio emission. This is consistent with the
emerging picture that red quasars are a younger, "blow-out" phase in the
evolution of galaxies. Red quasars may therefore be extremely important for
understanding how galaxies evolve over time.
Dr Fawcett continues "There are still many unanswered questions surrounding red
quasars, such as whether black hole winds or radio jets are ultimately
responsible for this enhanced radio emission. However, with the sample of DESI
red quasars continuing to grow over the next few years of the survey, I am
confident that we are on the brink of fully understanding the nature of these
red quasars."
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