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The radio
lobes of Alcyoneus. (Oei et al., arXiv, 2022) |
Astronomers have just found an absolute monster of a galaxy.
Alcyoneus
is a massive radio galaxy 3 billion light-years away that reaches 5 megaparsecs
into space. This structure, which is 16.3 million light-years long, is the
largest known structure of galactic origin.
Our lack
of understanding of these colossi and what causes their remarkable growth is
highlighted by this revelation. However, it may pave the way for a deeper
understanding of not only massive radio galaxies, but also the intergalactic
medium that floats through the vastness of space.
Giant
radio galaxies are just another unsolved riddle in an otherwise enigmatized Universe.
They are made up of a host galaxy (a cluster of stars around a galactic nucleus
containing a supermassive black hole) and huge jets and lobes that erupt from
the galactic core.
These jets
and lobes, interacting with the intergalactic medium, act as a synchrotron to
accelerate electrons that produce radio emission.
The source
of the jets is almost certainly an active supermassive black hole at the
galactic core. When a black hole is guzzling down (or 'accreting') stuff from a
huge disc of material surrounding it, we call it 'active.'
The
accretion disc swirling around an active black hole does not always end up
beyond the event horizon. A small portion of it is diverted from the accretion
disk's inner region to the poles, where it is blasted into space in the form of
ionised plasma jets that travel at a large fraction of the speed of light.
These jets
have the ability to travel great distances before dispersing into massive
radio-emitting lobes.
This
process is pretty normal. Even the Milky Way has radio lobes. What we don't
really have a good handle on is why, in some galaxies, they grow to absolutely
gargantuan sizes, on megaparsec scales. These are called giant radio galaxies,
and the most extreme examples could be key to understanding what drives their
growth.
"If
there exist host galaxy characteristics that are an important cause for giant
radio galaxy growth, then the hosts of the largest giant radio galaxies are
likely to possess them," the researchers, led by astronomer Martijn Oei of
Leiden Observatory in the Netherlands, explain in their preprint paper, which
has been accepted for publication in Astronomy & Astrophysics.
"Similarly,
if there exist particular large-scale environments that are highly conducive to
giant radio galaxy growth, then the largest giant radio galaxies are likely to
reside in them."
The team
went looking for these outliers in data collected by the LOw Frequency ARray
(LOFAR) in Europe, an interferometric network consisting of around 20,000 radio
antennas, distributed throughout 52 locations across Europe.
They
reprocessed the data through a new pipeline, removing compact radio sources
that might interfere with detections of diffuse radio lobes, and correcting for
optical distortion.
The
resulting images, they say, represents the most sensitive search ever conducted
for radio galaxy lobes. Then, they used the best pattern recognition tool
available for locating their target: their own eyes.
This is
how they found Alcyoneus, spewing forth from a galaxy a few billion light-years
away.
"We have discovered what is in projection the largest known structure made by a single galaxy – a giant radio galaxy with a projected proper length [of] 4.99 ± 0.04 megaparsecs. The true proper length is at least … 5.04 ± 0.05 megaparsecs," they write.
Once they
had measured the lobes, the researchers used the Sloan Digital Sky Survey to
try to understand the host galaxy.
They found
that it's a fairly normal elliptical galaxy, embedded in a filament of the
cosmic web, clocking in at around 240 billion times the mass of the Sun, with a
supermassive black hole at its center around 400 million times the mass of the
Sun.
Both of
these parameters are actually at the low end for giant radio galaxies, which
could provide some clues as to what drives the growth of radio lobes.
"Beyond geometry, Alcyoneus and its host are suspiciously ordinary: the total low-frequency luminosity density, stellar mass and supermassive black hole mass are all lower than, though similar to, those of the medial giant radio galaxies," the researchers write.
"Thus,
very massive galaxies or central black holes are not necessary to grow large
giants, and, if the observed state is representative of the source over its
lifetime, neither is high radio power."
It could
be that Alcyoneus is sitting in a region of space that is lower density than
average, which could enable its expansion – or that interaction with the cosmic
web plays a role in the object's growth.
Whatever
is behind it, though, the researchers believe that Alcyoneus is still growing
even bigger, far away in the cosmic dark.
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