Sky & Telescope - October 2023 - 17

galaxies in the universe's first 700 million years. More distant
galaxies become more challenging because they are fainter
and their spectra redshift to even longer wavelengths, where
even JWST is less sensitive.
Emma Curtis-Lake (University of Hertfordshire, UK) holds
the record for the most distant galaxy confirmed with a spectrum:
JADES-GS-z13-0 is observed 98% of the way back to the
Big Bang, when the universe was just 320 million years old.
Curtis-Lake and the JADES team have confirmed that galaxies
existed 13.5 billion years ago (S&T: Apr. 2023, p. 8). Finding a
galaxy this distant so early in JWST's mission was very impressive
and also comfortably within theoretical expectations.
The current picture emerging is that we are doing well
identifying galaxies 320 to 600 million years after the Big
Bang. Spectroscopy has now confirmed at least 20 galaxies at
these distances. We celebrate these discoveries as refreshing
and even a bit surprising after the number deficits suggested
by Hubble at this early epoch. Most astronomers did not
expect to find and confirm galaxies this distant so soon with
JWST. And we certainly were not expecting to find galaxies
even more distant.
But astronomers have identified some galaxies that might
date back to even earlier times. One galaxy discovered in the
first images from the CEERS survey appeared to date back to
just 250 million years after the Big Bang. Not only was this
galaxy shockingly early, but it was also unexpectedly bright.
This brightness gave astronomers confidence they could
obtain a quality spectrum to confirm the light's redshift, and
from it calculate the true age. Their proposal was accepted,
and observations were scheduled for March 24, 2023. It was
the moment of truth. Would we need to rethink some of our
basic understanding of early galaxies and the universe? Or
would this galaxy turn out to be closer to us in a more recent
era, as some predicted?
When the data came down, the result was immediate: We
actually observe this galaxy 1.2 billion years after the Big
Bang, much later than initially estimated. Spectral signposts
of oxygen and hydrogen glare at us intensely from afar. These
elements glow so brightly that they affect the colors we see
in the images in a specific way that mimics a more distant
galaxy. This could only happen for a galaxy at that specific
distance if it's also very dusty and bursting with intense star
formation to generate those strong spectral features. The
universe had played a clever cosmic prank, and astronomers
nearly fell for it.
JWST images have already revealed many more galaxies we
need to check, including other very distant candidates that
could yet rattle our understanding of the universe. It's not
that we don't expect to find galaxies 250 million years after
the Big Bang - we do, just not yet. They should be so rare and
faint that it should take us longer to find them with JWST,
using one of three methods: We can observe much larger
areas of the sky with surveys like COSMOS-Web, already
under way; we can use gravitational lensing to magnify the
distant universe, as programs like Cosmic Spring, PEARLS,
The Far Stuff Is Made
of Star Stuff
Carl Sagan famously said we are all made of star
stuff. But the star stuff in our bodies wasn't around
in the beginning. The Big Bang produced hydrogen
and helium and little else (S&T: May 2022, p. 20). It
took generations of stars forming heavier elements
and spreading them throughout the cosmos to
eventually enrich our Sun and our planet with the
chemical stuff of life.
JWST is revealing our elemental history in
greater detail and at earlier times than ever before.
By taking spectra of distant galaxies, JWST can
see the fingerprints of different elements and measure
their quantities. Galaxies in the early universe
had lower amounts of heavy elements like oxygen
compared with galaxies today. But these heavy
elements were still there.
One of the holy grails for JWST is to discover
the very first pristine stars and galaxies composed
of only hydrogen and helium. We have yet to find
them. We find heavy elements in place as early as
430 million years after the Big Bang in the galaxy
GN-z11. Its spectrum reveals at least six different
elements: hydrogen, oxygen, carbon, neon,
magnesium, and nitrogen. The most common
elements in the air we breathe - nitrogen and
oxygen - were already plentiful enough in GN-z11
to be observed in its light that traveled almost 13.4
billion years to reach us.
Seeing heavy elements like carbon so early in the
universe was unsurprising to astronomers. Most
of us worry we'll never see pristine stars. Heavy
elements help gas cool and condense. Without
these elements, pristine gas must come together
in greater volumes before gravity will cause it to
collapse, fuse atoms, and catch fire. That's why we
expect long-theorized " Population III " stars to be
oversized, burn extra hot, and die young, exploding
after just a few million years and spreading their
star stuff - including the heavy elements forged in
their cores - throughout their host galaxies. Subsequent
generations of stars would form enriched
with elements like carbon, nitrogen, and oxygen,
and these are the stars we see.
Of course, it is only the beginning of JWST's
mission, and we will continue to analyze the
spectra of distant galaxies to measure the buildup
of star stuff over cosmic time and search for those
pristine Pop III stars.
sk yand tele scope .o r g * OCTOBER 2023 17
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Sky & Telescope - October 2023

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