An area of space broadcasting at FM radio frequencies looks like a jellyfish – but that's not what astronomers find really surprising. The discovery's most remarkable feature is its invisibility at higher frequencies. The radiation drops off with frequency a thousand times as fast as its nearest counterparts.
Most studies have been conducted at frequencies around 1.4 Gigahertz – instruments capable of venturing into the lower frequency range were generally not very sensitive. The recent construction of facilities that pick up lower frequencies is opening our eyes to aspects of the universe we had no idea existed, some truly astonishing in scale.
Curtin University PhD student Torrance Hodgson found the latest example while studying the galaxy cluster Abell 2877 with the Murchison Widefield Array (MWA) "We looked at the data, and as we turned down the frequency, we saw a ghostly jellyfish-like structure begin to emerge," Hodgson said in a statement. Yet above about 200 MHz, there was nothing to see. The object has been dubbed the USS Jellyfish (for Ultra Steep Spectrum).
“We thought it might be a mistake,” Hodgson's supervisor Professor Melanie Johnston-Hollitt told IFLScience. However, after upgrades to the MWA improved its resolution they reexamined the area. The jellyfish's existence was unquestionable, as was the speed with which its brightness fades as frequency increases. “This is bizarre,” Johnston-Hollitt told IFLScience, “Absolutely extreme”.
Large differences in radiation strength over small frequency variations are a mark of artificial sources, one of the things the Search For Extra-Terrestrial Intelligence (SETI) looks for. However, this was clearly nothing like last year's Proxima Centauri signal. The jellyfish is around 1.2 million light-years across – more than half the distance from the Milky Way to the Andromeda Galaxy. It's certainly not an intergalactic billboard advertising candied jellyfish.
Instead, Hodgson and Johnston-Hollitt argue in The Astrophysical Journal, it's what astronomers call a “phoenix”, albeit an unusual one. Phoenixes occur when supermassive black holes spit out an immense amount of charged particles. Initially, their deceleration emits radiation, which fades as they cool. Then something stirs the intracluster medium, re-accelerating the electrons and causing them to radiate again.
The phoenixes we are familiar with have been caused by dramatic events such as the collision between two immense superclusters, accompanied by shock-waves in the surrounding gas.
Events like that don't produce the falls with frequency like this one, however. The paper proposes that what we see matches the spectrum from a more gentle reactivation of a cluster's surrounding electrons, which they refer to as a “sloshing”. Johnston-Hollitt told IFLScience it is not yet clear what caused this but; “Galaxies have their own weather,” and many things stimulate the modest movements matching a low-frequency phoenix.
The extra twist to this story is that the electrons have been spat out of two separate supermassive black holes in the same cluster, creating a “polyphoenix”. Johnston-Hollitt told IFLScience that even though we've never seen this before, it's one aspect that isn't surprising. “If on average there is one supermassive black hole per major cluster, statistically there will be some with two,” she said.
The black hole sources are near the jellyfish “bell”, with the “tentacles” dangling down towards the cluster's center. The morphology's cause remains a mystery, and Johnston-Hollitt said, “If we saw another one it would probably have a completely different shape.”
The MWA is a predecessor of the immense Square Kilometer Array (SKA) telescope to be built in the near future. Johnston-Hollitt believes the SKA will reveal many more USS objects. Moreover, with 10-15 times the resolution of the MWA, it will allow us to learn much more about the causes of this one.