Sky & Telescope - May 2024 - 23

velous story of its own. Astronomers discovered GN-z11 in
2016 with the Hubble Space Telescope. For nearly seven years,
until the advent of JWST, GN-z11 held the record for the most
distant object known. In late 2022, it lost its place to another
galaxy named JADES-GS-z13-0 (S&T: Oct. 2023, p. 12).
But GN-z11 still had a story to tell. In 2023, astronomers
used JWST to study the galaxy's spectrum in detail. A spectrum
not only breaks apart the light seen from an astronomical
object into its constituent wavelengths, much like a
rainbow, but it also reveals how the material producing the
light is moving with respect to the observer. The spectrum of
GN-z11 showed that the gas in the innermost region of the
galaxy was moving at roughly 1,000 km/s (more than 2 million
mph) - the fingerprint of a massive, central black hole.
Using this information, the researchers estimated the mass
of this " small and vigorous black hole in the early universe, " as
they called it, was some 1.5 million solar masses. That makes
it one-third as massive as the black hole at the center of the
Milky Way today - a tiny object to be detected so far away.
In another plot twist, astronomers discovered that the
central black hole in GN-z11 is probably swallowing gas
furiously, turning itself into a floodlight. The rate at which a
black hole can accrete gas from its environment has a speed
limit, the so-called Eddington limit, whose namesake, Sir
Arthur Eddington, was a renowned 20th-century English
astronomer. Above the Eddington limit, the infalling gas's
own glow pushes material out and away from the black hole,
controlling how much the black hole can accrete.
In the high-redshift universe, with abundant gas to be
accreted, the Eddington limit is like the speed limit on a
highway: generally obeyed but not unbreakable. Most black
holes accrete around the speed limit. Some accrete somewhat
higher or lower than that. GN-z11, however, seems to
be accreting at about five times its Eddington limit. This rate
is remarkable: If, billions of years ago, GN-z11 continued
growing at this pace, it would reach the superlative mass of
1 billion solar masses by redshift 9.5, more than 13 billion
years ago. We are unaware of such extremely massive objects
at those redshifts; they must be scarce if they exist. SuperEddington
accretion might also be an episodic event, turning
on and off and keeping black hole growth in check.
The tiny supermassive black hole in GN-z11 became visible
to our telescopes because it's radiating away a vast amount
of energy. What we see in its spectrum also suggests that we
may be staring down at the accretion disk from above, hence
observing the black hole from its most luminous side.
GN-z11 is not the only remarkable black hole that JWST
has discovered in the very early universe. For example, the
crown for the farthest black hole known belonged previously to
CEERS 1019, a 10-million-solar-mass object shining at redshift
8.7, 13.1 billion years ago. Astronomers discovered this remarkable
source in the Cosmic Evolution Early Release Science
(CEERS) JWST survey. CEERS 1019 also accretes slightly above
the Eddington limit, indicating that violating the speed limit
could be common in this early population of compact objects.
When your eyes adjust to darkness,
you begin to discern a more numerous
population of dimmer stars. Astronomers
are having the same experience with the
early universe.
Countless Feeble Candles
GN-z11 and CEERS 1019 are relatively midsize when it comes
to supermassive black holes. Astronomers have found dozens
of giant quasars a little later in cosmic history, roughly 1 billion
years after the Big Bang. Before the advent of JWST, black
hole hunters' focus was on finding these extremely massive -
and bright - objects as far away as possible.
Yet the giant quasars discovered at very high redshift are
rare, extraordinary objects. Consider the distance between
the Milky Way and the Andromeda Galaxy, some 2.5 million
light-years. Multiply this distance by 1,000 and construct a
cube from this side. In the early universe, at redshift 6 (12.8
billion years ago), we estimate that there is just a single,
bright quasar in this immense volume. At redshift 10 (13.2
billion years ago), these objects are so rare that there may
only be one in the entire universe.
But there are other things in that great expanse besides
the majestic quasars. When you look up at the sky on a clear
and dark night, your eyes immediately see the brightest stars.
Then, when your eyes adjust to darkness, you begin to discern
a more numerous population of dimmer stars. Astronomers
are having the same experience with the early universe: They
are now discovering a population of smaller, fainter supermassive
black holes.
When analyzing the first deep images obtained by JWST,
different teams started to notice tiny red dots popping out
everywhere. These sources were distinctively crimson compared
to other sources in the field, because their emission at
longer, redder wavelengths was stronger. They also appeared
small in physical and angular size - so small, in fact, that
although the dots emitted as much light as a whole galaxy
does, that light came from a region that's between the size of
a large star cluster and the smallest dwarf galaxies.
After a careful spectral analysis, researchers discovered
that some of these peculiar sources were young galaxies hosting
black holes at their centers. Most were observed between
redshift 4 and 7, when the universe was between 770 million
and 1.6 billion years old. Astronomers discovered tens of
specimens of this large population and lovingly named them
" little red dots " or " hidden little monsters. "
Instead of billions of solar masses, these little monsters
commonly are black holes of 10 to 100 million solar masses.
If accreting at their Eddington rates, then these somewhat
smaller black holes generate a luminosity of about 100 billion
times that of our Sun; the most massive quasars we know of
blaze with a luminosity of almost 1,000 trillion times that of
our Sun.
The discovery of a population of fainter, less massive black
sk yand tele scope .o r g * MAY 2024 23
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Sky & Telescope - May 2024

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