Sky & Telescope - October 2023 - 16

Galaxies Galore
Did JWST Break the Universe?
One of the first science results from JWST appeared to break
our understanding of galaxy formation - or even cosmology.
Six anachronistic galaxies were determined to be too massive
too soon (S&T: Dec. 2022, p. 8). One appeared to be as
massive as our Milky Way just 700 million years after the Big
Bang. It was as if you'd planted a sunflower garden and woke
up the next morning to find some of the plants had already
grown to be eight feet tall.
This result was reported in the first weeks of JWST science
and generated enticing headlines. Papers with proposed
solutions quickly followed. Some accepted the results at face
value, explaining how they could upend our current cosmological
model, Lambda CDM (S&T: Mar. 2022, p. 14).
Additional matter and dark energy in the early universe could
accelerate the growth of galaxies and the expansion of the
universe, these solutions suggested. But that proposal would
also imply that the universe is a billion years younger than
the current age estimate of 13.8 billion years, which would be
hard to reconcile with measurements.
Other papers presented less sensational solutions. Perhaps
EARLY DISKS With only a month of JWST data, astronomers found a
dozen dusty disk galaxies shining at us from more than 10 billion years
ago. The farthest appears as it was when the universe was only 1 billion
years old. These galaxies are invisible to the Hubble Space Telescope.
earlier than previously thought possible. Observed just 570
million years after the Big Bang, this black hole has the mass
of about 10 million suns. That's a few times the mass of the
black hole at the center of our own Milky Way, but in a tiny
galaxy about 5% the size of our own. Other teams have spotted
hints of large black holes in small galaxies at even earlier
times. It's hard to imagine black holes growing so big so early
in cosmic history. How could a black hole have achieved this
remarkable growth spurt?
First, it must have started out big. There are primarily
two ways this could happen. One option is that it might be a
remnant of one of the very first stars to form in the universe,
called Population III stars. We have yet to see one of these
stars, but we believe each one was big, burned hot, and died
young in a supernova, finally leaving behind a black hole with
the mass of 100 suns. But at this mass, the object would still
not have enough time to reach the observed mass of 10 million
suns by steadily eating gas. Instead, it would have needed
several bursts of intense feeding to quickly grow by a factor of
a million, which seems implausible.
Alternatively, the black hole could have started out from
an even bigger seed, as an enormous gas cloud that skipped
forming stars altogether and instead collapsed directly to
form a black hole with a mass closer to 30,000 suns. Such
black holes are still theoretical and would require very specific
conditions to form. But if such an object did arise, then
theoretically it could have grown quickly but steadily to the
observed size. More JWST observations already under way will
help shed light on this conundrum.
16 OCTOBER 2023 * SKY & TELESCOPE
smaller stars were less likely to form at these times, some
argued. Radiation from the cosmic microwave background
has a temperature of 2.7 kelvin today, but it was a bit warmer
(about 30K) when these galaxies existed. The radiation would
have kept gas a bit warmer, suppressing the formation of
smaller stars. If many small stars are missing, then these
galaxies would actually have much less mass than originally
measured, perhaps only 10% the mass of our Milky Way.
Alternatively, jets from supermassive black holes may contribute
some of the light we see, leading us to overestimate
how many stars, and thus how much mass, the galaxies have.
This adjustment could also bring the galaxy masses down to
one-tenth of the Milky Way's. While such heft would still be
impressive for the time, current theories do allow for a few
galaxies that massive 13 billion years ago.
However, two survey teams recently estimated the masses
for hundreds of early galaxies and found no clear signs of
overly massive objects: All the galaxies appear to fit comfortably
within predictions. Future JWST observations at longer,
mid-infrared wavelengths - which can tell us more about the
stellar populations and whether there's a big black hole present
- will help us tease apart how massive the galaxies really are.
Too Soon?
JWST is revealing exciting numbers of galaxies in the early
universe. So just how far back have we seen? As we did with
Hubble, astronomers are already pushing the limits of JWST
- we spot tiny, faint galaxies in the images, and we struggle
to discern their distances from their feeble light. Confirming
their distances requires spectroscopy, which spreads the light
from each galaxy into a rainbow, each color its own wavelength.
By detecting the fingerprints of elements at specific
wavelengths, we can measure a galaxy's redshift.
Some of the very first JWST spectra swiftly confirmed
E. J. NELSON ET AL. / ASTROPHYSICAL JOURNAL LETTERS 2023
Sky & Telescope - October 2023

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