Sky & Telescope - October 2023 - 15

After the spectacular images of SMACS 0723, our team
turned JWST's gaze to another galaxy cluster, WHL0137, and
the most strongly magnified galaxy known in the universe's
first billion years: the Sunrise Arc. At such a great distance,
most galaxies appear as faint infrared smudges. However,
thanks to the serendipitously perfect placement of the galaxy
behind the cluster WHL0137, the Sunrise Arc is gravitationally
lensed and magnified by a factor of hundreds, which
reveals the smallest details visible anywhere in the universe's
first billion years, including individual star clusters.
Based on our data and analysis, we found that some of
these clusters are young and actively forming stars, as we
expected in the early universe, but others are decidedly old
and well-established, with ages of 30 million years or more.
We determined that these old star clusters are gravitationally
bound, meaning that they are likely to persist forever: Simulations
show that once stars become drawn together in a tight
swarm, almost nothing can disrupt them.
Without gravitational lensing, the Sunrise Arc might have
seemed unremarkable, because the light from both the young
and old stars would have blurred together and led to measurements
of stellar ages somewhere in between. Instead, the old
star clusters in the Sunrise Arc and the Sparkler give us clues
to the history of the globular clusters in our Milky Way today
(S&T: July 2021, p. 14). The surprising diversity of ages shows
us how our early observations and ideas can change with a
deeper, more focused look.
Surprising Disks
Over the past 13 billion years, the
universe has expanded in size by
a factor of eight, stretching and
redshifting the light to wavelengths
eight times longer than the photons
were emitted with. Sun-like
stars are invisible to Hubble at that
distance, with their mostly yellow
light redshifted to infrared wavelengths
beyond Hubble's vision.
That is why JWST was designed to
see in the infrared. Within distant
galaxies, Hubble can only see
the young, hot stars that burned
bright in ultraviolet light. JWST's
infrared instruments reveal a
full stellar census of each distant
galaxy, including stars of all colors
and ages. This new technological
capability is what enables us to see
these older star clusters in the early
universe for the first time.
With Hubble, most distant
galaxies appeared clumpy, with
young star clusters actively forming
new stars. That remains true for
most galaxies we see, but JWST has revealed that some distant
galaxies also have massive stellar disks that were previously
invisible to Hubble. Rather than finding all early galaxies are
patchy and unsettled, JWST has shown that some galaxies
may have formed disks 13 billion years ago, much earlier than
anticipated.
Our own Milky Way is more than 13 billion years old.
Could it have had its disky shape for most of that time?
Recent work using the Gaia space telescope suggests yes, our
galaxy's disk first formed about 13 billion years ago (S&T:
Aug. 2023, p. 34). Astronomers have wondered whether that
makes the Milky Way unusual. But JWST's snapshots of
distant galaxies suggest that such rapid development is more
common than we expected. We'll soon learn more: Observers
have now identified more than 700 galaxies that appear
to shine at us from the universe's first 600 million years, and
the analysis of those galaxies' shapes has only just begun.
Oases Calmed by Black Holes
By revealing the older stars in distant galaxies, JWST has also
found entire galaxies that are surprisingly old and dormant,
having not formed new stars for tens or even hundreds of
millions of years. The early universe is small and densely
packed with gas ready to compress and form stars, yet JWST
is revealing that some galaxies are taking a peaceful break
from star formation as early as 700 million years after the Big
Bang. How have these galaxies found temporary calm during
such tumultuous times?
It appears these distant galaxies
Young, massive clusters
Older, globularcluster
progenitor
Earendel
SUNRISE ARC This knotted streak of light is a distant
galaxy, its image lensed, magnified, and multiplied by the
gravity of the cluster WHL0137 (facing page, composite
using eight filters). The above image is colorized to make
emission from star-forming regions stand out in green
(white circles). The four regions are actually the same
star-forming complex. Star clusters also make multiple
appearances (one circled here in orange). Earendel is the
most distant star ever detected, seen when the universe
was 900 million years old.
generated their calm from within
- not by finding inner peace,
but rather with an inner dragon
that burns them out. One distant
galaxy, GS-9209, appears to have
actively formed stars for 200 million
years but then gone dormant
for a full 450 million years before
we observe it 1.25 billion years
after the Big Bang. At its core,
a supermassive black hole, with
the mass of a half billion Suns, is
blasting out radiation as material
spirals inward and attempts
to feed the black hole. In nearby
galaxies, we see such jets heat up
surrounding gas, preventing it
from cooling and forming stars
for billions of years. However, we
did not expect supermassive black
holes to be common in the early
universe, since they would not
have had much time to form and
grow (S&T: Jan. 2017, p. 24).
Recently, one of us (Larson) discovered
a supermassive black hole
sk yand tele scope .o r g * OCTOBER 2023 15
CLUSTER: NASA / ESA / CSA / D. COE (STSCI), IMAGE PROCESSING Z.
LEVAY (STSCI); SUNRISE ARC CLOSE-UP: NASA / ESA / CSA / D. COE
(STSCI), IMAGE PROCESSING J. DEPASQUALE (STSCI)
https://skyandtelescope.org/
Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023
