Sky & Telescope - January 2025 - 37

JURIK PETER / SHUTTERSTOCK.COM
Rings of Power
As EMU team members began to talk about
ORCs at workshops and conferences - even
before the discoveries were officially published
in 2021 - they began to hear ideas
from others about possible explanations
of the phenomenon.
Roland Crocker (Australian National
WHEN BLACK HOLES COLLIDE The merger of two
supermassive black holes is one possibility for initiating
the spherical shock wave that creates ORCs.
Crocker estimates the energy required to
University), who sat in the audience of
a talk Norris gave in Adelaide, Australia,
saw the giant rings as " weird, and cool, and
odd. " But his ears really " perked up " because
he'd been involved in studying two loops on either
side of the Milky Way plane, known as the Fermi Bubbles
(S&T: Apr. 2014, p. 26). Crocker has suggested that sustained
star formation in our galaxy blew out those bubbles, which
span 50,000 light-years end to end. Now, he suggests a similar,
albeit stronger mechanism might make ORCs.
The radio waves coming from ORCs have the characteristics
of synchrotron radiation, emitted by energetic electrons
spiraling around orderly magnetic fields. A powerful shock
wave emanating from the central galaxy could energize intergalactic
particles while sweeping out ambient magnetic fields
to immense scales, explaining the radiation. But where does
the underlying power in that wave come from?
One way to make a powerful shock wave is to have a galaxy
churn out stars at a formidable rate. As the more massive
of these stars explode in quick succession, they generate a
furious wind - particles entangled with magnetic fields -
that can escape the galaxy.
ORCs
ORC Name
ORC 1
ORC 2
ORC 3
ORC 4
ORC 5
ORC 6
ORC 7
Physalis
Cloverleaf
SAURON
J0849−0457
J2223−4834
J1507+3013
make these gargantuan circles is equivalent
to that released by up to 1 billion
supernovae. By the time the star-driven
shock wave creates the large shell visible to
us on Earth, the starburst itself is done. All
that's left is a " red-and-dead " elliptical galaxy
that has halted star formation long ago.
Confirmation of this idea came from a
surprising source: a visible-light spectrum. " We've
been putting in telescope proposals ever since we discovered
these, " Norris says. " And we'd get these comments like, 'Well,
you only see this thing in the radio - why should we be
interested?' " But then Alison Coil (University of California,
San Diego) came across a sensitive MeerKAT image of ORC 1,
published in 2022, showing radio arcs glowing within the
larger ring. She's not a radio astronomer, but she immediately
thought it looked like an outflow.
" My collaborators and I have been studying massive starburst
galaxies that are driving these outflowing galactic winds
for over a decade now, " Coil says. ORC 1, she thought, might
represent a later stage of those galaxies' evolution.
While most of the ORCs are Southern Hemisphere objects,
the one discovered in archival data - ORC 4 - was accessible
from the W. M. Keck Observatory in Hawai'i. It also happened
to be in the sky when Coil had observing time. She and
her team dashed off a 20-minute exposure using the Keck
Redshift
0.551
0.31
0.31
0.45
0.26
0.125
0.3
0.017
0.05
0.55
0.34
0.27
0.08
Lookback time
(millions of
years)
5,540
3,540
3,540
4,720
3,160
1,610
3,450
240
680
5,440
3,810
3,160
1,100
Ring diameter
(millions of
light-years)
1.7
1.2
1.8
1.3
1.0
3.3
1.3
(multiple rings)
(multiple rings)
0.6
0.2
1.2
0.2
Telescope
ASKAP/EMU
ASKAP/EMU
ASKAP/EMU
GMRT
ASKAP
ASKAP
MeerKAT
ASKAP
ASKAP
MeerKAT
ASKAP
ASKAP
VLA/FIRST
RA
21h 03m
20h 58m
20h 58m
15h 55m
01h 02m
10h 27m
19h 14m
11h 37m
01h 31m
08h 49m
22h 23m
15h 07m
*Discovered via machine learning
Right ascension and declination are for equinox 2000.0. Note that ORC 6's position on the sky is not yet published.
Dec.
−62° 00′
−57° 36′
−57° 36′
−27° 26′
−24° 50′
- -
−44° 22′
−54° 33′
−00° 50′
−13° 34′
−04° 57′
−48° 34′
+30° 13′
Type
ORC
Ring with no
central galaxy
Diffuse emission
ORC
ORC
Double ring
ORC
Radio-shell system
Radio-shell system
ORC candidate*
ORC candidate*
ORC candidate*
ORC candidate
sk yand tele scope .o r g * JANUARY 2025 37

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Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025