This is what we astronomers call a blob, or a smudge, if you want to get really technical.
It may not look like much from here, but what do you expect for something near the
literal edge of the observable universe. If you were there when this light was emitted,
you’d A. be at the beginning of time, and B. be looking at an entire galaxy containing an
enormous black hole at its heart. It’s the most distant black hole we’ve semi-directly
detected. That’s cool enough on its own, but as an added bonus this one smudge may have
solved the mystery of the origin of the supermassive black holes in our universe.
Introducing Abell 2744. It’s a giant cluster of galaxies around 4 billion light years
away. Basically our back yard, cosmologically speaking. The most
interesting thing about Abel 2744—at least for us today—isn’t what’s in the cluster itself,
but what’s behind it. The cluster lives in a vast halo of dark matter that warps
space across its 4-million-light year span. Light from distant objects traveling through
the cluster is deflected and magnified. The cluster is a stupendous gravitational lens that
enables us to see much further than our puny human-made telescopes would normally allow.
That’s why the Hubble Space Telescope studied Abel 2744 in detail, and why it was one of the early
must-dos for the James Webb Space Telescope. And this is where our smudge got interesting.
JWST observes deep into infrared wavelengths, and showed that the object has IR colours consistent
with a galaxy whose normally visible light has been stretched out by the expanding universe,
increasing or redshifting its wavelength by a factor of over 10. Higher redshift means larger
distance, and a redshift of 10.1 means this light has been traveling for around 13.2 billion years,
coming to us from a time near the beginning of time, when the universe was less than 3.5% its
current age. That earned our smudge the name UHZ1, for ultra-high redshift galaxy number
one—where the z is the symbol for redshift. That’s cool, but JWST has now found galaxies
quite a bit more distant than this. The really exciting moment came when we pointed another
orbiting satellite at the smudge. This is the image by the Chandra X-ray telescope. It turns
out there are a LOT of X-rays coming from this little blob. Now X-rays are much harder to bring
to a sharp focus so the 4 pixels of X-ray light aren’t coming from across this spread-out region,
but rather from somewhere inside this region. There’s really only one known way for a galaxy to
be blasting out X-rays at this level, and that’s by harboring a quasar. To refresh your memory,
a quasar is when the supermassive black hole—the SMBH—that lives at the center of
every galaxy starts feeding. As matter swirls towards the black hole it’s superheated until
it outshines the entire surrounding galaxy. And just before it reaches the black hole,
conditions get so crazy that the space outside the black hole glows bright with high-energy
X-rays. Based on the amount of X-ray light and the distance, we can estimate that the black hole
must be 40 million times the Sun’s mass. That’s 10 times the mass of the Milky Way’s own SMBH.
UHZ1 is the most distant quasar ever discovered, which also makes it the
earliest known quasar and so the earliest black hole that we actually have evidence for.
So with that out of the way, let’s get to the good stuff. To understand why UHZ1 is so exciting,
you have to understand one of the biggest debates in cosmology - the origin of supermassive black
holes. Essentially every galaxy in the modern universe has at its core a black hole that’s
100,000 to several billion times the mass of the Sun. We also see much smaller black holes—so
called stellar black holes—that can be in the rough range of 10 to 100 solar masses. Oddly,
we don’t see black holes in the middle range of 100 to 100,000. We’ll come back to why that’s odd.
Now, we know how to make stellar black holes—they’re what you get
when the core of a massive star collapses on itself after the star dies. So how do
you make a supermassive black holes grew? That’s the big question. It
could be they grew from the very first stellar corpses in the early universe,
gulping down gas and merging with other black holes for billions of years to reach their current
enormous sizes. But over the last decade or so, as we looked to greater and greater distances,
we started to find quasars shining out from the first billion years of cosmic time—quasars powered
by supermassive black holes that should not have had enough time to get that big.
There are two potential solutions to this conundrum. Either the black holes started
small and grew way, way faster than we thought they could. Or they started much bigger than
the black holes that form in stellar deaths in the modern universe. These are the small seed
and heavy seed models respectively. UHZ1 is going to help us choose between them.
Let’s start with the small seed model. Can a stellar corpse grow into a quasar engine in
less than a billion years? Well, it’s tricky. There’s a limit to how fast a black hole can
feed even with an endless supply of gas. As gas spiraling into a black hole heats up,
it blasts out radiation which pushes on the infalling gas and counters the
black hole’s gravity. Bigger black holes can eat more and radiate more,
but there’s always an approximate upper limit that increases with that mass—it’s called the
Eddington limit. You can calculate how big a black hole would grow feeding at this maximum
rate for a billion years—and the a nswer is … not big enough to explain those early quasars.
That said, there are various tricks we can include to make the small seed model work.
There are scenarios in which black holes can feed faster than the Eddington limit,
although they still need to feed non-stop to reach supermassive status so quickly—which
itself is a problem. Or you can form a lot of black holes close to each other and have them
merge very quickly. Or you can start out with really, really big stars leaving behind black
holes that are maybe 100 or even 1000 times larger than are produced today. In the modern universe,
all the heavy elements released in past supernovae cause gas clouds to fragment as they collapse,
leading to smaller stars. But the pristine hydrogen and helium that
filled the universe at the beginning held together as it collapsed, probably
leading to gigantic stars and commensurately massive, but still not supermassive black holes.
With a combination of these tricks—fast and consistent feeding and biggish stellar black
hole seeds—it’s possible to make supermassive black holes in under a billion years. This
still doesn’t explain why we don’t see any black holes in the 100 to 100,000 solar
mass range. If SMBHs grew from stellar black holes by passing through this range then you’d
expect some stragglers to still be in that range, but that doesn’t seem to be the case.
OK, so let’s try the heavy seed model. What if the seeds of supermassive black holes weren’t
even born from the hyper-dense cores of dead stars? In fact it's actually a misconception that you
need extremely dense matter to make a black hole. For any given mass, there’s a particular size that
mass needs to be crushed into in order for it to form a black hole. This is the Schwarzschild
radius. For the Sun, it’s around 3 km, and a solar mass crammed within that radius would
have the same density as a typical 3000 meter tall mountain crammed into a cubic centimeter.
On the other hand, a typical chonky supermassive black hole of a billion
solar masses has a Schwarzschild radius the size of Neptune’s orbit. A billion suns
crammed into that region would have the density of … cotton candy. That’s right,
fill a solar system with cotton candy and you’d get an instant giant black hole.
The early universe did not contain significant quantities of cotton candy,
as far as we can tell anyway. But it was filled with a lot of gas that would
later form stars and galaxies. As that gas started to collapse to make those things,
the resulting protogalactic clouds may have had cores dense enough to collapse directly
into black holes without ever forming a star. Again, this only works in the early universe
where the unpolluted hydrogen and helium doesn’t tend to fragment as it collapses.
And as it happens, theoretical calculations of the expected sizes of these direct collapse
black holes are right around 100,000 solar mass mark. That gives us a way to
go straight from gas to the smallest SMBHs without having to grow from stellar seeds
through the intermediate range. And once you have a small SMBH, a pretty sensible rate of
feeding can turn it into a large SMBH in mere hundreds of millions of years.
OK, great. Between our two models - small versus heavy seed, it’s looking like the
heavy seed model feels more plausible. So let’s see what our new player—UHZ1—has to say about this.
Previous record holders for most distant quasar, like J1342+0928, J0313–1806, and J1007+2115,
are all similarly massive, and they had already put the ‘small seed’ formation theory under a
lot of stress in the past few years. But they were all found right around 700 million years
after the big bang—just on the boundary of the amount of time needed to grow a small seed,
assuming some very creative astrophysics to allow a really cosmic bulking phase.
But UHZ1 shaves 200 million years of the allowed growth time. If it was marginally
plausible to grow a small seed into an SMBH before, now it’s feeling nearly impossible.
Let’s look a bit more closely at this object to try to make this more concrete. Based on
the JWST images, we can get an idea of the amount of starlight versus quasar light in this object.
Even those images are dominated by quasar light. The galaxy has around 40 million
Suns worth of mass in stars, (the same as it's mass in the black hole) and that's around 1000 times less than the stars in the Milky Way. This isn’t
unusual in itself. We expect galaxies from so long ago to still be building up mass and
converting gas into stars. But for most of the universe’s history we see a pretty tight
correlation between the number of stars in a galaxy and the mass of its central black
hole. That has been taken by some to indicate that galaxies and their central black holes
grow together in relative lock-step. UHZ1 suggests otherwise—it tells us that the
black hole starts relatively enormous and then the stellar mass catches up.
UHZ1 may represent the first in a new class of object called an OBG - for overly-massive black
hole galaxy, or outsized black hole galaxy. OBGs are objects hypothesized to exist in the early
universe in which the black hole and stellar masses were around the same. The amount of
infrared and X-ray light coming from UHZ1 is consistent with simulations of OBG formation,
so maybe this class of object is no longer hypothetical. If so,
that tells us a lot. OBGs only work if their central black hole is formed
by direct collapse, not through the growth of a stellar remnant.
Overall, things are looking pretty good for the heavy seed model, and
quite a bit less good for the small seed model. But it’s not quite a smoking gun. For example,
maybe UHZ1 is weird. Perhaps its SMBH formed in an unusual way—for example,
from a rare and massive primordial black hole or in an exotic environment like a quasistar. But
the fact that observations of UHZ1 line up so well with the direct collapse and the OBG model would
tend to disfavor these alternate explanations, at least in UHZ1s case. And even if UHZ1 is an
OBG it doesn’t mean other supermassive black holes couldn’t have formed from small seeds.
But in general the case for the latter as an important source of SMBHs is being whittled away.
So what’s next? Well, we keep looking. UHZ1 is the first of its
kind. But in astronomy where there's one, there's often many and hopefully soon we'll find the many.
As JWST and Chandra continue their good work, we’ll develop a clearer and
clearer picture of what the earliest times were like as we discover more distant
glimmers of the earliest stars and quasars that first lit up a newly-born spacetime.
Hey Guys. Two things before we go. First I wanted to tell you about fascinating fails over on PBS
Terra. It’s a show about scientific discoveries that were made by accident. Their latest episode
is about - the story of Penzias and Wilson and their discovery of cosmic background radiation.
There’s a link in description! And if you go over, tell them (politely) that Space Time sent you.
Next, I wanted to let you know that because you asked, the classic black hole orbits design is
now available on the Space Time Merch store as a t-shirt and for the first time a sweatshirt.
There's no better way to perpare for your next surprise encounter with a black hole,
then to carry a geodesic map of the exterior with you at all times. And
now you can be warm at the same time. There’s a link in the description to our Merch store!