Sky & Telescope - December 2020 - 33

Dusty Star-Forming Factories
According to astronomer Caitlin Casey (University of Texas,
Austin), the history of the universe as told through starlight
is biased and inaccurate. The light from cold dust in distant
galaxies, however, has revealed a major hidden character in
this drama.
About 3 billion to 4 billion years after the Big Bang, the
universe was populated with galaxies enshrouded in so much
dust that no starlight escaped. Instead, the galaxies blazed
with dustlight, the thermal glow of interstellar dust heated by
massive crops of bright young stars.
The dust goes hand-in-hand with the star formation, says
Casey. Lots of new stars leads to lots of dust production as the
most massive stars quickly age and die off.
With no starlight to guide them, astronomers can only
ﬁnd and study these early galaxies with telescopes sensitive to
millimeter-wavelength light, which is emitted from brutally
cold gas and dust. At these wavelengths, the galaxies glow
brightly, whereas in images taken with visible light, they are
nowhere to be seen.
Dustlight has revealed that these obscured galaxies
dominated star formation at a time when the universe, on
average, produced stars at a rate 10 times greater than it does
today. Whereas the Milky Way churns out a few new stars per
annum, Casey says, these stellar factories produced roughly
5,000 stars every year. And they have few modern parallels.
While there are a handful of comparable galaxies in the modern universe, "they were a thousand-fold more common 10
billion years ago," Casey says.
It's not yet clear why these galaxies were so proliﬁc. One
potential culprit is galaxy collisions, says Casey, but it's
uncertain how widespread such mergers were 10 billion years

Hubble

ALMA
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HIDDEN G A L A XIES: UNIV ERSIT Y OF TOK YO / CE A / N AOJ;
DUST TE X T URE: SERGIO34 / SHU T TERSTOCK.COM

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p HIDDEN GALAXIES Sections of sky that appear empty to the Hubble
Space Telescope contain massive, star-forming galaxies when viewed in
the submillimeter range with the Atacama Large Millimeter/submillimeter
Array in Chile.

80%
Fraction of starlight
in the Milky Way that
is absorbed and
reradiated by dust

ago. It's also possible that galaxies back in the day simply had
more gas to fuel star formation than their modern counterparts. This mystery is driving Casey and colleagues to peer
farther out into space - and hence further back in time - to
pinpoint when these galaxies and their dust ﬁrst came on the
cosmic scene.
"We don't know if these are still very common in the
universe's ﬁrst billion years," she says. She envisions that,
eventually, "we'll start to see galaxies that have very high star
formation rates but that are not yet polluted by dust." Such
galaxies would have existed in a brief window of time after
the ﬁrst generation of stars switched on but before enough
of them (or the second wave that followed) dirtied up those
pristine infant galaxies.
Pinpointing the onset of cosmic dust could thus provide a
window into the ﬂeeting yet foundational epoch of the ﬁrst
stars. The ﬁrst stars had to have formed without any help
from dust. With no dust to cool the primordial gas, the ﬁrst
stars were likely bloated monstrosities hundreds of times
more massive than the Sun.
But getting that galaxy census will require instruments
that can survey large swaths of the sky at once, something
that current millimeter-wavelength facilities aren't good at
doing. A new camera under development for the Large Millimeter Telescope in Mexico, she says, will be a boon to this
kind of work and enable astronomers to continue a century of
investigations that have shown dust to be crucial in moving
elements around the cosmos and helping stars form.
Only dust could have revealed this hidden chapter in
the story of cosmic evolution. It's a lesson that astronomers
continue to be reminded of, from the far reaches of the visible
universe to familiar stellar nurseries sitting right next door:
Interstellar dust is everywhere, and to ignore it is to risk missing a vital element of where we come from.

¢ CHRISTOPHER CROCKETT is a freelance science journalist
living in Arlington, Virginia. Formerly an astronomer, he found
research to be too tedious and now writes about it instead.
sk yandtelescope.org * DECEMBER 2 02 0

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Sky & Telescope - December 2020

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