Sky & Telescope - December 2022 - 17

EHT COLLABORATION / ASTROPHYSICAL JOURNAL LETTERS 930:L12 (2022) / CC BY 4.0
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THE RING Radio photons whizzing around the black hole just outside the event horizon draw a thin circle called the photon ring. Left: This
snapshot from a simulation shows one possibility for what the photon ring around Sgr A* would look like with perfect resolution. Center: The same
simulation, averaged to match the cadence of EHT observations. Right: What the EHT's reconstructed image of this ring might look like, combining
various methods' results.
suffered many dramatic mergers with other galaxies, which
funnel gas into galactic centers. If Sgr A* has grown by eating
a random assortment of clouds, then there's been nothing to
spin it up, he explains.
The second EHT inference is that Sgr A* leans on its
throne. Instead of pointing straight up along the rotation
axis of our galactic pinwheel, the black hole's spin axis lies
more sideways, angled at most 50° from our line of sight.
(An angle of around 30° looks likely.) Researchers can't tell
whether it's leaning toward us or away - we're either looking
down at its forehead or up at its chin.
Astronomers already had reason to suspect this result:
Infrared observations of hotspots looping around Sgr A* at
breakneck speed also suggest the black hole cants away from
us (S&T: Feb. 2019, p. 11). That two different experiments
give us a similar result is consoling.
Nor is Sgr A* the only supermassive black hole listing
sideways. Observations of accreting black holes in spiral galaxies
have revealed a range of orientations, says Geoff Bower
(Academia Sinica Institute of Astronomy and Astrophysics,
Hilo), who coined the cowardly lion moniker. The varied tilts
are unsurprising, he explains, because a black hole's orientation
depends on how it grew. Mergers with other black holes
could cause a jumble of spin tilts, and for less active black
holes like Sgr A* there may have been no hefty gas stream to
force the black hole into a particular orientation. Thus there's
no reason Sgr A* ever had to point straight up.
Some have wondered what this orientation means for
the Fermi bubbles, the giant dumbbell-shaped outflows that
extend up and out from the galactic center for tens of thousands
of light-years (S&T: Feb. 2021, p. 60). Many astronomers
suspect that a fantastic roar from the black hole blew
the bubbles a few million years ago. Hsiang-Yi Karen Yang
(National Tsing Hua University, Taiwan), who uses computer
simulations to investigate the bubbles' formation, says that
outflows from tilted jets would still be funneled up and out of
the Milky Way's disk by the dense surrounding gas, creating
bubbles that align with the galaxy's rotation axis rather than
that of the black hole. She's currently working to simulate
this scenario, although preliminary results suggest that creating
the bubbles this way doesn't as easily explain the lobes'
appearance, she says.
The Lion's Mane
Then there's the inflowing gas itself. The gas in the disk feeding
a black hole drags magnetic fields with it as it spirals in
toward the event horizon. Scenarios for the conditions in
the disk fall into two categories: SANE and MAD. Standard
and normal evolution (SANE) disks are more turbulent, with
weak and messy magnetic fields threading them. Magnetically
arrested disks (MADs) are packed with fields all stuck upright,
which can choke the gas flow and serve as a highway for
magnificent jets.
Twenty years ago, MADs were astrophysical insanity. Narayan
and his collaborators fought to publish the scenario in
scientific journals against referees' complaints that magnetic
field lines would never behave like this. Even Narayan didn't
think they'd happen in the real universe. " I thought it was
just a cute idea, " he says.
But younger scientists in his group saw the idea's potential,
particularly Sasha Tchekhovskoy (now Northwestern University),
who as a graduate student demonstrated that a MAD
with a " booming jet " would arise naturally. Even so, Narayan
didn't expect MADs to be common.
The EHT results might indicate otherwise: Overall, the
EHT team favors a MAD feeding Sgr A*. " That's huge! "
exclaims Michael Johnson (Center for Astrophysics, Harvard
& Smithsonian). " People generally expected that it was SANE,
turbulent, and nonspinning, because we don't see powerful
jets. We're saying it's MAD, strongly magnetized, and
spinning. " The same holds for M87*, but given its jet, that's
expected. " If this is true, then MAD systems are everywhere. "
sk yand tele scope .o r g * DECEMBER 2022 17

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