By a stroke of luck, a team led by Dutch Ph.D. student Martijn Oei has
discovered a radio galaxy of at least 16 million light-years long. The pair
of plasma plumes is the largest structure made by a galaxy known thus far.
The finding disproves some long-kept hypotheses about the growth of radio
galaxies.
A supermassive black hole lurks in the center of many galaxies, which slows
down the birth of new stars and therefore strongly influences the lifecycle
of the galaxy as a whole. Sometimes, this leads to tumultuous scenes: the
black hole can create two jet streams, that catapult the building material
for baby stars out of the galaxy at almost the speed of light. In this
violent process, the stardust heats up so much that it dissolves into plasma
and glows in radio light. The international team of researchers from Leiden
(The Netherlands), Hertfortshire, Oxford (both UK), and Paris (France) have
now collected that light—with the pan-European LOFAR telescope, whose
epicenter lies in a marshy Dutch 'radio dark' nature reserve, where your
smartphone deliberately loses signal.
Record length
The picture of the two plasma plumes is special, because never before
scientists saw a structure this big made by a single galaxy. The discovery
shows that the sphere of influence of some galaxies reaches far from their
direct environment. How far, exactly? That is hard to determine.
Astronomical pictures are taken from a single viewpoint (Earth), and
therefore do not contain depth.As a result, scientists can only measure a
part of the radio galaxy length: a low estimate of the total length. But
even that lower bound, of more than 16 million light-years, is gargantuan,
and comparable to one hundred Milky Ways in a row.
Visible with the naked radio eye
Because Earth does not occupy a special place in the universe, it was never
very likely that such a largest galactic structure would reside in our own
backyard. And indeed: the radio giant is three billion light-years away from
us. Despite that mind-boggling distance, the giant looms as large in the sky
as the moon—an indication that the structure had to have a record length.
The fact that the radio eyes of the LOFAR telescope only saw the giant just
now, is because the plumes are relatively faint. By reprocessing a set of
existing images in such a way that subtle patterns stood out, the scientists
were suddenly able to spot the giant.
The giant Alcyoneus
The researchers named the giant structure Alcyoneus, after the son of
Ouranos, the Greek primordial god of the sky. This mythological Alcyoneus
was a giant that fought against Heracles and other Olympians for supremacy
over the cosmos. In the world-famous Pergamon Altar in Berlin, a sculpture
of this Alcyoneus is carved out.
Ghostly dance
Alcyoneus' plumes possibly reveal information about the mostly elusive
filaments of the Cosmic Web. The Cosmic Web is another name for the
contemporary, grown-up universe, that looks like a network of threads and
nodes that astronomers call filaments and clusters, respectively. The
galaxies in filaments and clusters are clearly visible themselves, but
detecting the medium between galaxies has only been successful in
clusters—barring a handful of exceptions. Could Alcyoneus change this?
Because Alcyoneus, just like the Milky Way, inhabits a filament, its plumes
feel a headwind while moving through the medium. This subtly changes the
direction and shape of the plumes: They perform a slow dance with an
invisible partner. For many years, scientists have proposed that the shapes
of and pressures in the plumes of radio galaxies could relate to filament
properties, but never before did they find an example where that connection
is as plausible as with Alcyoneus. Namely, Alcyoneus' plumes are so big and
rarefied that the surrounding medium can relatively easily mold them.
Black holes are cosmic mainstays
The Cosmic Web retains its form because the attractive force of gravity is
compensated by the heat pressure of the medium in filaments and clusters. In
the past two decennia it has become clear that the glowing stardust that jet
streams eject from galaxies, keeps the Web warm. In this way, the central
black holes in galaxies contribute to sustain the large-scale structure of
our universe. That is extra noteworthy because black holes are very small
compared to filaments and clusters. It is as if something the size of a
marble regulates the Earth's temperature.
Mysterious origin
What has given Alcyoneus its record length, remains a mystery for now. The
scientists first thought of an exceptionally massive black hole, an
extensive stellar population (and so a lot of stardust), or extraordinarily
powerful jet streams. Surprisingly enough, Alcyoneus appears to be less than
average on all these aspects compared to its smaller sisters and brothers.
In the times ahead, the team will therefore now investigate whether the
environments of radio galaxies could explain the growth of giants instead.
Reference:
Martijn S.S.L. Oei et al, The discovery of a radio galaxy of at least 5 Mpc.
arXiv:2202.05427v1 [astro-ph.GA],
arxiv.org/abs/2202.05427. Accepted for publication in Astronomy & Astrophysics.
Tags:
Space & Astrophysics