‘Radio jellyfish’ rise in the southern sky

By US Desk
Fri, 04, 21

According to the researchers, that likely means this cosmic jellyfish is actually an odd bird known as a “radio phoenix.”


Galaxy clusters are the largest structures in the universe bound together by gravity. They can contain thousands of galaxies, enormous oceans of hot gas, invisible islands of dark matter and — sometimes — the glowing ghost of a jellyfish or two.

In the galaxy cluster Abell 2877, located in the southern sky about 300 million light-years from Earth, astronomers have discovered one such jellyfish. Visible only in a narrow band of radio light, the cosmic jelly is more than 1 million light-years wide and includes a large lobe of supercharged plasma, dripping with tentacles of hot gas.

The structure’s jelly-like appearance is both “ghostly” and “uncanny”. However, even more astonishing than the space jelly’s shape is how quickly the structure vanishes from view. Whilst it’s bright at regular FM radio frequencies, at 200 megahertz the emission all but disappears. No other extragalactic emission like this has been observed to disappear anywhere near so rapidly.

The universe is swimming with energetic structures that are only visible in radio wavelengths, like the mysterious X-shaped galaxies cartwheeling through space, or the twin blobs at the center of the Milky Way. However, no structure this large has ever been observed in such a narrow band of the radio spectrum.

According to the researchers, that likely means this cosmic jellyfish is actually an odd bird known as a “radio phoenix.”

Like the mythical bird that died in flame and rose again from the ashes, a radio phoenix is a cosmic structure that’s born from a high-energy explosion (like a black hole outburst), fades over millions of years as the structure expands and its electrons lose energy, then finally gets reenergized by another cosmic cataclysm (such as the collision of two galaxies).

To create a radio phoenix, that last cosmic event must be powerful enough to send shockwaves surging through the dormant cloud of electrons, causing the cloud to compress and the electrons to spark with energy again. It could also cause a structure like the jellyfish cluster to glow brightly in certain radio wavelengths, but dim rapidly in others.

Why hummingbirds ‘hum’

Colorful hummingbirds get their name from the hum generated by their fast-moving wings as they hover; these tiny aerodynamic marvels have the fastest wingbeat of all birds, clocking in at around 70 strokes per second (more than 4,000 per minute).

But how exactly do their wings produce a humming noise?

The scientists created the first-ever 3D acoustic model of flying hummingbirds, combining video and audio recordings of the birds in motion with measurements of the forces generated by hummingbird wings as they oscillated. The team traced the humming sound to the upstroke of hummingbird wingbeats.

In most flying birds, the “whoosh” that you hear is the sound of their downstroke — the only wingbeat to generate lift. By comparison, hummingbird wings, which trace a “U” shape in the air as they flap, produce lift on both the downstroke and upstroke. At the speed that hummingbird wings move, these actions and air pressure differences during wingbeats account for the hummingbirds’ humming sound. Variability in the way air moves over feathers and the wing’s overall shape add overtones and nuance to the sound. This make the hum sound pleasant to humans — unlike the more irritating whine of a mosquito or the buzzing of a fly, according to Scholte.