Sky & Telescope - May 2024 - 25

Lighting Up the Cavern: The Path Forward
JWST's first year has revolutionized our view of the early
universe. First, we have significantly expanded the black hole
horizon: The most distant supermassive black hole we've
detected, that in GN-z11, lies only 440 million years after the
Big Bang. Second, we've observed a vast population of smaller
black holes actively accreting inside young and small galaxies;
these objects are 10 to 100 times more numerous than
expected, based on the number of observed giant quasars.
Perhaps many of them will stay " small " and never reach
billion-solar-mass proportions. Third, these smaller black
holes seem to be 10 to 100 times more massive than expected
with respect to the stellar mass of their hosts.
What are the consequences, thus far, for our theories of
the early universe? By detecting more distant supermassive
black holes, the expansion of the black hole horizon will
eventually allow us to pinpoint the formation mechanism of
the first black holes, also known as seeds (S&T: Nov. 2022,
p. 16). These seeds were either heavy, with an initial mass on
the order of 100,000 solar masses, or light, with a mass up to
a few thousand - or a combination of both types. Light seeds
would have had to grow furiously to match the early supermassive
black holes we've found; heavy seeds would slowly
pace their race to greatness.
To understand how the first population of black holes
formed, we have two pathways: Either we discover incredibly
massive black holes at even higher redshifts, impossible
to explain with the light-seed scenario, or we obtain a better
census of quasar precursors and their galactic hosts and
determine how their masses evolved with time. JWST is well
under way on both pathways.
The little red dots also highlight tensions between observations
and theoretical work. In the pre-JWST era, theorists
predicted many more black holes than observers did,
based on the number uncovered so far. Some theorists have
welcomed the discovery of the little-red-dots population as
bringing theory and reality closer together. But some observers
worry that we may now have swung too far the other
way. For now, it remains unclear how many of the little red
dots are in fact black holes.
Some early JWST observations also suggested that the universe
became full of massive galaxies earlier than expected,
based on how bright these galaxies are (S&T: May 2023, p. 9).
This tension has already eased thanks to modifications in
how astronomers measure the stellar mass of galaxies at high
redshift, by improving the estimate of the masses with which
stars form and by better accounting for the role of interstellar
dust. Possibly, the presence of glowing, overly massive
black holes will reduce estimates of the stellar content of
these galaxies even further, better reconciling them with our
expectations.
Furthermore, over-massive early black holes may be the
best indication yet that they formed from heavy seeds. When
a heavy seed forms, the galaxy may take a while to build up
its stars and catch up, size-wise, either through mergers with
other galaxies or via in situ star formation. Thanks to these
combined processes, the system would slowly progress toward
the familiar 0.1% ratio of masses in our cosmic neighborhood.
If this picture is accurate, then many galaxies may
undergo a " hidden little monster " phase.
To conclude, JWST's first year of operations has revealed a
mother lode of faraway black holes. Some of the observations
obtained thus far are puzzling and in conflict with what we
thought the early universe would look like. Remember - we
are still in the phase of exploring a dark cavern, barely lit by
starlight. Our eyes - and minds - are still adjusting to the
entirely new environment that JWST is showing us.
Some of these oddities will probably be re-incorporated
into our previous expectations. Others will disrupt our
vision of the cosmos. Reassuringly, in this dark and damp
cavern, we have not yet found something truly bizarre, like
water flowing uphill. But the magnificent young cosmos
that JWST is showing us is already changing the history of
astronomy forever.
¢ FABIO PACUCCI is an astrophysicist at the Harvard & Smithsonian
Center for Astrophysics, where he holds the Clay and
Black Hole Initiative fellowships.
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Sky & Telescope - May 2024

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