Sky & Telescope - January 2025 - 36

CSIRO
A Tale of ORCs
Norris soon found a third ORC near Kapinska's circle (the
two are probably a pair). By the time his team published the
discoveries in 2021, the researchers had found a fourth circle
in the archives of India's Giant Metrewave Radio Telescope
and a fifth in new ASKAP data. With a growing number of
discoveries, Norris and his CSIRO colleague Bärbel Koribalski
estimated there might be 1,000 ORCs in the sky, hidden in
plain sight because they are so faint and spread out.
Initial appearances were deceiving, though. More than five
years after the first discovery, ORCs still number less than a
dozen, with a number of additional candidates of undecided
status. " I had expected to find many more ORCs, " Koribalski
says, " especially since EMU is now about 20% complete. "
" It's embarrassing, " Norris admits. " I still don't understand
why we haven't found more. "
At first, it was easier to say what ORCs are not than what
they are. For example, Norris saw right away that ORC 1
wasn't simply an imaging artifact, because it had been imaged
multiple times in the EMU survey and appeared in different
places in different fields. A ring of the same size also showed
up in data from other radio arrays.
With artifacts ruled out, the question then became what
created the rings. Unfortunately, the radio waves show no
absorption or emission lines with which to measure the rings'
distance and, therefore, their true size.
The simplest explanation initially was that the phenomena
came from supernovae within our galaxy that had blasted out
bubbles of gas. Those bubbles appear as rings because more
material is piled up along our line of sight on the edges; the
middle region, even when filled in, appears more transparent.
Such gaseous, star-blown shells litter the Milky Way. What's
more, a nearby origin could explain the ORCs' large span on
the sky, because closer things look bigger.
Norris soon ruled this scenario out, though: The handful
of known ORCs didn't cluster along the galaxy's plane,
making the supernova scenario unlikely. His team ruled out
other more extreme possibilities, too, such as that the rings
might come from more distant light bent into a circle by a
foreground galaxy's gravity.
" I actually even checked their proper motion from the two
observations, " Norris says, " in case they were atmospheric or
in the solar system. " They weren't.
A clue came from visible-light images, which show a reddish,
probably elliptical galaxy at the center of all the single
ORCs. (The ORC 2 and 3 pair is an exception: Their galaxy
lies off to one side.) The astronomers initially thought that
these galaxies might be unrelated to the giant wreaths of
radio around them. But as they found new ORCs, each associated
with a central galaxy, the probability of such chance
alignments lowered.
" I think it's the galaxies that really clinched it, " Norris
says. " Once we knew that [the ORCs] had these host galaxies,
then they were clearly extragalactic. "
But if ORCs aren't close by but actually - like the galaxies
at their centers - billions of light-years away, then the circles
have to be vast in size. Based on the distance to the galaxy
at the center of ORC 1, for example, its ring must be about
1 million light-years across. A single star can't make that.
Instead, these might be bubbles blown not by individual stars
but by whole galaxies of them.
PATHFINDER A portion of the 36 dishes that make up ASKAP cluster amidst brush at the Murchison Radio-astronomy Observatory in
Western Australia. ORCs have evaded detection until now because we didn't have the means to find them, but ASKAP's EMU survey has
both the sensitivity and the volume to pick them up. The ASKAP array is a precursor to the much larger Square Kilometre Array being built
in Australia and South Africa (S&T: June 2017, p. 24).
36 JANUARY 2025 * SKY & TELESCOPE

Sky & Telescope - January 2025

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