Sky & Telescope - July 2021 - 8

NEWS NOTES

BLACK HOLES

The Magnetic Fields
Around M87's Black
Hole
THE EVENT HORIZON TELESCOPE

(EHT) collaboration has unveiled new
images of the black hole shadow at
the center of the elliptical galaxy M87,
which sits at the center of the Virgo
Cluster some 55 million light-years
away. These images, unlike the iconic
one released in 2019 (S&T: Sept. 2019,
p. 18), include polarized light - photons

that shimmy at only certain orientations as they travel through space. The
new data contain long-awaited information about the behavior of magnetic
ﬁelds near the black hole.
Magnetic ﬁelds are cosmic puppeteers whose marionette strings thread
gas around a black hole and control its
inﬂow. Turbulence in the ﬁelds robs gas
of its angular momentum, enabling it
to fall into the black hole. But if the
ﬁelds are too strong, magnetic pressure
chokes off the ﬂow.
Until now, this picture has been
largely theoretical. But using a planetspanning network of radio telescopes,
the EHT team observed the polarization
of light emitted by gas right around
M87's supermassive black hole. The synchrotron emission comes from electrons
corkscrewing along magnetic ﬁeld lines.
The polarization of that emission is
consistent with a fairly strong magnetic
ﬁeld. But it isn't simply wrapped around
the black hole by the accretion disk; it's
more complex, explains Jason Dexter
(University of Colorado, Boulder).

COSMOLOGY

New Studies: The Universe Is Expanding Faster
TWO INDEPENDENT GROUPS have
measured the Hubble constant, the universe's current rate of expansion, and
their results have deepened the tension
in an ongoing controversy (S&T: June
2019, p. 22).
Observations of the far-away (early)
universe, particularly the cosmic microwave background emitted 380,000 years
after the Big Bang, lead to a current
expansion rate of 67 km/s per megaparsec. But near-universe observations
point to a higher value of 73 km/s/Mpc.
Adam Riess (Space Telescope Science Institute) has been leading an
effort to tie Cepheid variables, a " standard candle " typically found in nearby
galaxies, to Type Ia supernovae, another
standard candle that can be seen from

J U LY 2 0 2 1 * S K Y & T E L E S C O P E

farther away. Improving on previous
work, Riess's team combined precise
distances from the Gaia satellite with
Hubble Space Telescope brightness measurements of 75 Cepheids in galaxies
hosting Type Ia supernovae. In the February 10th Astrophysical Journal Letters,
they report a more precise Hubble constant than any near-universe method:
between 71.6 and 74.4 km/s/Mpc.
The second study, led by John
Blakeslee (NSF's NOIRLab), examined
63 bright, mostly elliptical
galaxies within 300 million
light-years. The team was
looking for brightness differX NGC 1453, one of the giant elliptical galaxies used to calculate the
expansion rate of the local universe

The polarization is also weaker
than the team expected. Synchrotron
radiation ought to be highly polarized
(roughly 70%), but the data show a
maximum of only 30%. Tangles in the
highly magnetized gas near the black
hole likely scramble the signal.
After comparing data to simulations,
the team concluded that the magnetic
ﬁelds around M87 are strong enough to
control the movement of the sparse gas.
Christopher Reynolds (University of
Cambridge, UK), who wasn't involved
with the study, agrees that the data
point to strong magnetic ﬁelds, but he's
hesitant to conclude that a magneticchoking scenario is the only explanation for the observations. " What if the
data are trying to tell us something that
isn't in the lexicon of those models? "
he asks. The EHT astronomers agree,
acknowledging that their set of scenarios is incomplete.
Nevertheless, the new data might be
direct evidence that magnetic ﬁelds can
control the inﬂow of gas - a scenario
that might explain the starvation diet
of our own galaxy's central black hole.
The results appear in the March 20th
Astrophysical Journal Letters.
CAMILLE M. CARLISLE

ences between pixels within each galaxy
image. For nearby galaxies, relatively
few stars are in each pixel, and the
statistical ﬂuctuations in the number of
stars per pixel is higher. For more distant galaxies, more stars per pixel result
in lower pixel-to-pixel variations. So
when one galaxy appears smoother than
another, it's typically farther away.
Surface brightness ﬂuctuations yield
a Hubble constant between 70.2 and
76.4 km/s/Mpc, a result that will appear
in the Astrophysical Journal. While the
range is larger than that from Riess's
team, it still lands on the higher end of
the spectrum. Both studies
support the notion that the
Hubble constant is greater
when measured using objects
in the near universe, indicating new physics at play.
ARWEN RIMMER

M 87'S BL ACK HOLE: EH T COLL A BOR ATION;
NGC 1453: CA R NEGIE-IRVINE G A L A X Y SURV E Y

W This reconstructed image shows the polarized view of the gas-enshrouded supermassive
black hole in M87. Superposed lines indicate
the direction and amount of polarization.

Sky & Telescope - July 2021

Table of Contents for the Digital Edition of Sky & Telescope - July 2021