The name “USS Jellyfish” may evoke visions of a Starfleet vessel designed to accommodate gelatinous marine aliens. But it is actually the name of a very real, newly discovered structure in outer space that is like nothing that has ever been observed before.
The enormous entity, which extends for more than one million light years, is made of aging plasma that has taken on the shape of a jellyfish, at least from our perspective on Earth. Located in Abell 2877, a cluster of galaxies some 340 million light years away, the USS Jellyfish is the first known example of a “polyphoenix,” a radio source of immense complexity and mystery, according to a recent study published in The Astrophysical Journal.
Torrance Hodgson, a PhD student at the Curtin University node of the International Centre for Radio Astronomy Research who led the study, first noticed the bizarre structure in 2017 while studying radio sources captured by the Murchison Widefield Array (MWA), an extremely sensitive telescope located in the Australian outback.
The USS Jellyfish is so strange that he assumed it must’ve been an error in his observations, and he initially told his advisor Melanie Johnston-Hollitt, the second author on the study, to “please ignore the obvious bug in my program that produced the outlier,” Hodgson said in an email.
“But on her insistence, we sat down together and found the original radio images, located the mystery source, measured it manually, and sure enough, this thing was real!” Hodgson continued. The researchers could only make out “a bright blob” at that point, so they had “a lot of theories but very few ideas of what it could be,” he recalled.
To get a better handle on the nature of the blob, Hodgson and his colleagues observed Abell 2877 with both the Australia Telescope Compact Array and the new and improved MWA “phase 2” survey, in 2018.
“Months and months later, after some very difficult processing of the data, finally we got the images of the Jellyfish, and at the much higher resolution of phase 2,” Hodson said. “It was spectacular!”
The new observations confirmed that the USS Jellyfish is brightest at a very narrow band of low frequencies under 200 MHz, making it invisible to most radio telescopes in the world, which capture higher frequencies. The “steepness” of this sudden loss of radio visibility, from low to high frequencies, is what gives the structure its name: though “USS” makes it sound like an intergalactic starship, the abbreviation actually stands for Ultra-Steep-Spectrum.
“As you turn up the frequency dial, its brightness drops off exponentially, quickly rendering it completely undetectable to conventional, higher-frequency radio telescopes,” Hodgson said.
Scientists expected that a new generation of low-frequency radio telescopes, such as the MWA, might reveal these weird USS sources. Surprisingly, these radio sources haven’t really materialized in new data as anticipated, leaving astronomers puzzled by their absence. The USS Jellyfish is now a “staggering” exception, Hodgson said, as it has the steepest spectrum ever observed at these low frequencies.
“After being disappointed to find so few low frequency sources, we find something that is more than six times steeper—on a logarithmic scale!—than your typical radio source,” Hodgson noted. “And that's a real mystery: why does this source exist at such an extreme outlier position, when there's almost no other sources of intermediate spectral steepness in between?”
One explanation is that the USS Jellyfish is the product of a rare and mind-boggling sequence of events that has played out over the course of two billion years. The structure is strewn between galaxies that may have contained energetic cores known as active galactic nuclei (AGN). These nuclei spew out enormous amounts of light and plasma due to intense interactions between the supermassive black holes at galactic centers and any material that ends up falling into disks around them.
There are no observable AGN near the USS Jellyfish now, but that might mean that they aged out and became dormant. In their ancient active phase, they may have laid the groundwork for the glowing USS source we see today by blasting plasma across Abell 2877, which eventually took on the tentacled jellyfish shape.
Though the electrons in that plasma lost energy over time, a more recent shockwave—or some other type of energizing event—could have brightened the structure again in these exact radio frequencies.
“To be so steep, the source either has to be exceptionally old, except then it should have faded from simply expanding into space; or triggered by a shock that is exceptionally weak, except then it shouldn't be so bright!” Hodgson said.
“To solve this we've had to do a bit of cosmic archaeology, and combine both ideas,” he continued. “It is indeed exceptionally old, at more than two billion years, and by luck it's remained highly undisturbed over that time, until just recently where the ancient black hole ejecta of multiple galaxies has mixed and been very gently reaccelerated by some still unknown shock passing through the cluster to reignite the source.”
This type of phenomena is known as a “radio phoenix,” so named because the ashes of an older structure are reignited by a new energizing event. Smaller radio phoenixes have been observed before, but the USS Jellyfish is the first known example of a “polyphoenix,” a word that Hodgson coined to express the multilayered nature and gigantic scale of the new entity.
For now, it’s not clear what origins or mechanisms account for this hypothetical shock that may have lit up the USS Jellyfish. Hodgson and his colleagues hope to follow up on their research by conducting X-ray observations of the cluster to see if any lingering traces of an energizing event might be detectable.
The team is also looking forward to the eventual completion of the Square Kilometre Array (SKA), a massive radio telescope that will span continents, allowing astronomers to study the sky at radio frequencies that are far lower than anything available today. Unfortunately, the SKA won’t be ready to open its eyes and ears until the 2030s, at the earliest.
“I'm learning science requires patience!” Hodgson said.
In the meantime, the team plans to continue searching for any other polyphoenixes that might be out there, hidden in the cosmic wilds. It may be that there is a whole menagerie of undiscovered USS sources that may one day join this spectacular space jellyfish.