Sky & Telescope - January 2025 - 40

ORCS 2 AND 3: R. NORRIS ET AL. / PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF AUSTRALIA 2021;
CLOVERLEAF: XIAOYUAN ZHANG & MATTHIAS KLUGE (MPE), BÄRBEL KORIBALSKI (CSIRO)
A Tale of ORCs
ible from the Southern Hemisphere.
Others, too, are turning to different instruments. While
DOUBLE ORC Unlike ORC 1, ORCs 2 and 3 appear close together on
the sky and, as later work showed, share a host galaxy, which is offset to
one side of both circles (not visible in this radio image).
the sky and are probably related. They only have one associated
galaxy, and instead of lying in the middle of either ring
or even between the two, it lies off to one side of both rings.
Koribalski and Norris now think that these two formed in a
way distinct from single ORCs, perhaps by the resurrection
mechanism Shabala proposed.
Other teams have since found a couple of relatively nearby,
multiply ringed systems. " These are very likely related to
ORCs, in my opinion, " Koribalski says. But she cautions
against reaching firm conclusions just yet. " We need more
data (always) and more simulations. "
The Road Goes Ever On
Astronomers continue to study the known complement of
ORCs. Coil is doing so with an upgraded Keck instrument,
called the Keck Cosmic Reionization Mapper. By taking longer
exposures at longer wavelengths of ORC 4, she hopes to
better understand the extent and state of the gas flow around
its galaxy. She's using a similar instrument, the Multi-Unit
Spectroscopic Explorer on the Very Large Telescope in Chile,
to explore gas flows around the other ORCs that are only visthe
original ORCs evaded X-ray detection, Esra Bulbul (Max
Planck Institute for Extraterrestrial Physics, Germany) did
manage to collect high-energy photons coming from hot gas
in a nearer ORC-like system, dubbed the Cloverleaf for its
overlapping circles around a single elliptical galaxy. Using the
European Space Agency's XMM-Newton Observatory, she
and colleagues saw two bright dollops of X-ray emission: one
around the central galaxy itself and one off to the side, unrelated
to any radio or visible light. Combined with the lopsided
shapes of nearby galaxies, as seen in visible light, the X-ray
data suggest an ongoing maelstrom as two galaxy groups
come together. The overlapping radio circles might again represent
Shabala's idea, with shock waves lighting up remnants
of ancient jets sent out from the central galaxy's black hole.
Primarily, though, the focus is on simply finding more
ORCs. " We really need to get more examples, " Norris says.
" We're really hampered. "
He notes that searches at longer wavelengths using the
Low Frequency Array (LOFAR) based in the Netherlands
haven't found anything yet, which is something of a mystery.
If ORCs are emitting synchrotron emission, then they should
emit more radiation at LOFAR's longer wavelengths than at
the shorter radio wavelengths collected by ASKAP. " They can
see the ones we know about, " he says, " but they haven't seen
any new ones, and that surprises me. "
Machine-learning searches of EMU data haven't turned
up any classical, one-ring ORCs, either, although they have
found some of the nearer multiple-ring systems. " So far, all
the classical ORCs have been found by eye, " Norris says. " But
I think that's about to change, as machine-learning algorithms
become increasingly powerful. "
Citizen-science projects are also an obvious, albeit laborious,
next step. Eleni Vardoulaki (National Observatory of
Athens, Greece) and Hongming Tang (Tsinghua University,
China) announced in mid-2024 that Radio Galaxy Zoo: EMU
(https://is.gd/GalaxyZooEMU) is up and running. Just a
month after the project's launch, users have already classified
more than 12,500 radio sources; they can also note any
distinctly odd shapes in the project's online forum.
In the meantime, most of the ORCs currently known
owe their discovery to a single individual: Koribalski. Even
now, she still looks for the giant circles on a regular basis,
downloading each ASKAP field as it comes in and scanning
through some 20,000 radio sources by eye. She's also working
with Norris to compile current observations and ideas into a
review article for a scientific journal.
" It would be great to discover these things when you're at
THE CLOVERLEAF This system of giant, overlapping radio rings
(pink) also emits X-rays (blue). Visible light is shown as white to yellow.
Some of the X-ray-emitting regions are associated with the central elliptical
galaxy; others aren't associated with any source of radio or visible
light and might instead come from ionized gas in between galaxies.
40 JANUARY 2025 * SKY & TELESCOPE
the start of your career - I could have built an entire career
on them, " says Norris, who has semi-retired. " But I will leave
that to my younger colleagues. "
¢ News Editor MONICA YOUNG relishes the fictional astromythology
in Tolkien's Lord of the Rings and Silmarillion.

https://www.is.gd/GalaxyZooEMU

Sky & Telescope - January 2025

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