Sky & Telescope - January 2025 - 39

Heated gas
falling back
Naming
Odd Radio
Circles
Odd Radio
Circle
Koribalski and Norris came up
with the ORC acronym together, in
a brainstorming session that focused
on J. R. R. Tolkien's Lord of the Rings.
(Orcs are the monstrous foot soldiers of
the enemy in Tolkien's fantasy world.) " I
wanted to make ORC work, but I only
got as far as O for their round/circular
nature and RC for radio circle, "
Koribalski says. " I am quite
sure Ray came up with
O for 'Odd.' "
600,000 light-years
The burst of star formation has ended. The forward shock
wave continues traveling into intergalactic space, resulting
in an ORC. Gas heated by the second shock slowly falls
back toward the galaxy.
More distant
shock wave
may no longer
be visible.
An expanding
shock wave
appears as
an ORC.
on gas supply and the propensity of the galaxy to merge with
others. It can be difficult to tease the two histories apart. But
it does seem indicative that the galaxies at the center of most
ORCs are elliptical: These fuzzy, football-shape galaxies have
probably merged with many galaxies in the past and tend to
host behemoth black holes in their cores. It's possible that
each galaxy's black hole is in fact the outcome of an ancient
merger between two smaller black holes - and such a merger
might be able to power a spherical shock.
Galaxy collisions can also set off shock waves. In cosmological
simulations, investigated by Klaus Dolag (Ludwig
Maximilian University, Germany) and colleagues, major
galaxy mergers result in a central, massive galaxy surrounded
by shock waves of the same size, shape, and relative speed as
ORCs. Also, such powerful mergers shouldn't happen that
often, explaining ORCs' rarity. Only the predicted brightness
Internal
shocks may
appear as
structures
inside the
ORC.
comes up short, unless bolstered by further interactions.
" It's a great scenario, " says Koribalski, " and testable. "
Also under serious consideration is the resurrection of
500,000 light-years
If this scenario pans out, then ORCs provide a pathway to
understanding galaxy evolution. " They'll help us understand
how common these extreme outflows are, " Coil notes.
Not Yet at Journey's End
Bursts of starbirth (and star death) might explain ORC origins,
but the matter is far from settled. Other events besides
starbursts can push out spherical shock waves. Perhaps the
supermassive black hole in the central galaxies is responsible
for the explosive action instead.
The histories of a galaxy's star formation and its supermassive
black hole tend to evolve hand-in-hand, as both rely
" old " intergalactic particles, an idea proposed by Stas Shabala
(University of Tasmania, Australia) and colleagues. Under
this scenario, an elliptical galaxy hosts a supermassive black
hole that shoots out twin jets of speedy plasma, each of which
plows through the thin gas around the galaxy and produces
radio waves. Over time, those jets turn off, the now-intergalactic
particles slow down, and their radio emission fades. But
when a shock wave passes through the area - not necessarily
from the galaxy itself - what's left of the jets lights up.
Norris initially disagreed with the idea and discussed it at
length with Shabala. Now he's a coauthor on the published
study. He currently thinks that the relighting of gas around
old radio galaxies could explain some ORC-like systems.
" It's possible that not one formation scenario explains all
ORCs, " Koribalski notes.
Even the first handful of discoveries demonstrate that
not all ORCs look alike. ORC 1 is isolated, with its galaxy
smack in the center, but ORCs 2 and 3 lie close together on
sk yand tele scope .o r g * JANUARY 2025 39
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