The newly-discovered radio galaxy, dubbed Alcyoneus, has a projected length of 16.3 million light-years (4.99 Mpc).
“Most galactic bulges hold a supermassive black hole that grows by accreting gas, dust and stars from its surroundings,” said Leiden Observatory astronomer Martijn Oei and colleagues.
“The black hole ejects a fraction of its accretion disk plasma from the host galaxy along two collimated, magnetized jets that are aligned with its rotation axis.”
“The relativistic electrons generate, through spiral motion, synchrotron radiation that is observed by radio telescopes.”
“The two jets either fade gradually or end in hotspots at the end of diffuse lobes, and ultimately enrich the intergalactic medium with cosmic rays and magnetic fields.”
“The full luminous structure is referred to as a radio galaxy,” the astronomers said.
“The giant radio galaxy definition accommodates our limited ability to infer a radio galaxy’s true proper length from observations,” they added.
“A radio galaxy is called a giant radio galaxy if and only if its proper length projected onto the plane of the sky exceeds some threshold, usually chosen to be 2.3 or 3.3 million light-years.”
“Currently, there are about a thousand giant radio galaxies known, the majority of which have been found in the northern sky.”
“About one hundred exceed 6.5 million light-years and ten exceed 9.8 million light-years; at 16 million light-years, the literature’s projectively longest is J1420-0545.”
The new giant radio galaxy was first identified in the images captured by the Low-frequency Array (LOFAR) telescope as part of the LOFAR Two-metre Sky Survey.
Dubbed Alcyoneus, the object is at least 16.3 million light-years long, and consists of the central jet-like structure and two outer radio lobes.
The radio source is associated with SDSS J081421.68+522410.0, an elliptical galaxy some 3 billion light-years away in the constellation of Lynx.
“The picture of the two plasma plumes is special, because never before we saw a structure this big made by a single galaxy,” the researchers said.
“The discovery shows that the sphere of influence of some galaxies reaches far from their direct environment.”
According to the authors, Alcyoneus most probably inhabits a filament 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,” they said.
“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.”
The discovery is reported in a paper to be published in the journal Astronomy & Astrophysics.
Martijn S.S.L. Oei et al. 2022. The discovery of a radio galaxy of at least 5 Mpc. A&A, in press; arXiv: 2202.05427
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