Sky & Telescope - December 2020 - 32

gas by absorbing high-energy ultraviolet light from stars and
reradiating it as lower-energy infrared light.
For some astronomers, the inner life of dust is more than
enough to keep them busy. But for others, dust is a means to
an end - or sometimes even a decoy.

The Dirty Windshield of the Milky Way
In 2014, a group of cosmologists made a rather public blunder.
With much fanfare, the team of a project known as BICEP
(Background Imaging of Cosmic Extragalactic Polarization)
announced that they had found an elusive signal from the
dawn of time. Imprinted on the cosmic microwave background, a remnant glow from a time when the entire universe
was roughly 3000° Celsius (5000°F), this signal looked like
one that theorists expected to have been made by the echoes
of gravitational waves released during the epoch of inﬂation, a passing moment when the universe ballooned in size
roughly a trillionth of a trillionth of a trillionth of a second
after the Big Bang.
Early the following year, they issued a mea culpa: The
signal was actually from interstellar dust in the Milky Way
(S&T: May 2015, p. 12).
The BICEP ﬂop was a cautionary tale on many fronts. But
one takeaway is that if astronomers want to understand the
cosmos, they need to understand the dust they're peering
through to see it.
"One of the grad students I worked with in the past said,

p MAGNETIC TRACER Astronomers have used the emission from
polarized dust grains (top) seen with the European Planck spacecraft to
map the Milky Way's magnetic field (bottom).

DECEMBER 2 02 0 * SK Y & TELESCOPE

you have to look through the dirty windshield of the Milky
Way to see anything," says Gordon.
Sadavoy echoes that sentiment. "Depending on what science you're interested in, you need to know the dust in order
to disentangle it from your actual observations," she says.
But dust can also be a powerful tool for revealing things
that might otherwise remain unseen. Take the BICEP saga:
Dust grains ﬁltered background light in a way that mimicked
the sought-after signal because of how the grains aligned
themselves to the magnetic ﬁeld of the Milky Way. That
means astronomers can use the grains to trace out our galaxy's magnetic structure.
In 1949, astronomers John Hall and Alfred Mikesell (U.S.
Naval Observatory) discovered that some starlight was polarized: The lateral oscillation of the light waves had a preferred
direction. What's more, they saw a connection between the
degree of polarization and the amount of reddening. Raw
starlight is not polarized, but dust was ﬁltering it in such a
way that preferentially allowed light waves of a particular
orientation to pass.
Since then, astronomers have recorded polarized light
from all directions in the sky, which in turn provides sweeping vistas of our galaxy's magnetic ﬁeld. Other galaxies have
beneﬁted from a similar treatment. In January, astronomer
Enrique Lopez-Rodriguez (NASA Ames Research Center) and
colleagues observed the polarized glow emitted by warm dust
in the nearby galaxy NGC 1068 and found that the galaxy's
magnetic ﬁeld closely follows its spiral arms.
Astronomers are also using dust to discover large-scale
structures in our galactic backyard. In January, João Alves
(University of Vienna, Austria) and colleagues reported that
many nearby star-forming regions are linked together in a
coherent gaseous thread they dubbed the Radcliffe Wave,
which bobs in and out of the plane of our galaxy (S&T: May
2020, p. 9).
Interstellar dust was the key to ﬁnding this structure. The
team identiﬁed hundreds of stars in front of, within, and
behind dust clouds that permeate these stellar nurseries. They
then combined measurements of how much dust dims and
reddens the starlight with new precision distances to those
stars from the Gaia satellite to trace the 3D structure of the
dust. Astronomers have started using the same idea to revise
the distances to spiral arms in the Milky Way.
This 3D dust mapping is a relatively new tool, says Sadavoy. Historically, the distance to dust has been difﬁcult to
pinpoint, because background starlight reveals all the dust
along a line of sight. "This 3D dust modeling tries to remove
that challenge by taking into account the fact that you're not
just measuring the extinction of stars, but you're measuring extinction of stars as a function of distance," she says.
"That's the new element that we didn't really have before."
But using dust as a tool isn't just limited to the Milky Way
and its neighbors. Dust is also revealing secrets about the
peak of cosmic star formation, which occurred roughly 10
billion years ago.

ESA / THE PL A NCK COLL A BOR ATION (2)

Interstellar Dust

Sky & Telescope - December 2020

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