In 2011, scientists using the South
Pole Telescope in Antarctica observed more than two-dozen distant galaxy clusters,
twelve of which had never been seen before. Among them was the Phoenix Cluster, a group of
about a thousand galaxies bathing in blistering radiation- with its biggest and brightest
galaxy unlike anything we’d seen before. In the decade that followed, the
Phoenix Cluster enjoyed a meteoric rise to become one of the most extensively
studied galaxy clusters in the universe, shattering a number of cosmic records for X-Ray
emittance, gas fractions and star formation. But lately, it has taken on a significance of
another kind… because, at the heart of the Phoenix Cluster’s unique central galaxy, lies perhaps the
largest black hole in the universe… one with a mass claimed to exceed entire galaxies, and dwarf
the inner-Solar System with its event horizon. The question is, why do people think that this
black hole is the largest- and more to the point, who are those people, and what information
did they use to draw their conclusion? Furthermore, just how large can the
biggest black holes really grow? And if this one is not the largest, which is? These are the questions we will be answering as
we search for the universe’s Biggest Black Holes. Black holes are volumes of mass which are so
densely compressed that their gravitational influence deforms the space surrounding
them. Inside a region of warped spacetime, the force of attraction becomes so great that the
escape velocity needed to leave exceeds the speed of light, and thus not even massless photons
can break free to meet the eyes of an observer. Instead, they fall straight towards the centre,
resulting in a lightless sphere of un-rendered space around the concentration, known as an Event
Horizon. Though the concept might sound ominous, the science is anything but- any time we
compress enough mass into a sufficiently miniscule volume of space, an event horizon
of sapped light will form around it. And in a universe dominated by gravity, there are a number
of ways to achieve a gravitational collapse. Mini-quasar binary systems
like GRO-1655-40 provide us with compelling evidence that black holes form
from the leftover cores of collapsing stars. When a giant star goes supernova, only its outer
layers ignite, leaving the inert core exposed, which collapses in on itself in
the absence of an energy source. If this collapsing core weighs more than a
few times the mass of our sun, then it will continue to compact and compress its volume until
an event horizon forms around it. This gives us a Stellar Black Hole, with a mass ranging from
a few times that of our sun to a few-dozen. By far the smallest and most common kind,
the Milky Way alone may be littered with the corpses of millions of its bygone stars.
While the majority of these remain invisible and undetectable in the vastness of space, some
stellar black holes light up as they feed on their surrounding gas. Black holes grow via accretion-
the shredded matter closest to the event horizon, under the greatest tidal influence, collects into
a thin, rotating disk along the black hole’s axes. On the inside of this disk, a small fraction
of excited particles will eventually experience enough drag force to guide them below the
event horizon, where they plunge towards the centre and grow the black hole’s mass. But
the majority of the disk’s matter will remain outside, instead orbiting the horizon at
obscenely high velocities and temperatures. Thus, the process of accretion is a fairly
slow-burner for a Stellar Black Hole, allowing it to accrue a few-dozen extra Solar
Masses of material over many millions of years. Eventually, it may reach 100 Solar Masses,
at which point it would be classed as an Intermediate Mass Black Hole. Considerably
rarer than their stellar counterparts, Intermediate Mass Black Holes start at a century
of Solar Masses, for the largest collapsed stars, but they can grow to tens of thousands,
for those beasts we see lying at the hearts of star clusters and disrupted
proto-galaxies. But at this insane weight, Black Holes of an intermediate mass are
surely too large to have been built up by accretion alone- there must have been another
mechanism supplementing their runaway growth. And indeed, we do see evidence of such a
mechanism, realised when black holes collide. When two of these monsters find each-other in space,
they soon begin spiralling in on one-another as their orbits decay, ultimately destined to join
their event horizons and merge their masses. This facilitates the snowballing-like growth of
gargantuan black holes on a variety of scales- in binary systems, during chance encounters,
and even during the collision of galaxies. Lying at the hearts of most galaxies,
we find the largest type of black hole- a Supermassive Black Hole, with a mass
greater than at least 100,000 suns. Almost every large galaxy houses one
of these beasts in its nucleus- and for particularly large, mature galaxies
like the Milky Way, Andromeda, and M-87, they can grow to millions of times the sun’s mass-
large enough to swallow stars in a single gulp. These black hole nuclei are thought to have
played a vital role in establishing the shape of their host galaxies, particularly in the core
region, with the highest concentrations of stars. And thus, where we find massive, over-sized
galactic cores, we can expect to find similarly massive monster black holes. As such, it is easy
to see why the nucleus of the IC 1101 galaxy, has long been viewed as a principal candidate
for the largest black hole in space. IC 1101 was once thought to be the largest galaxy,
with a diameter of around six-million light years, according to early, overly-generous
estimates. This figure has been reined in by more than half in recent years, but in
any case, the galaxy still boasts one of the largest nuclei we’ve ever seen- extending
about 13 and a half thousand light years. And this massive galactic bulge weighs in
favour of a similarly massive black hole at the centre of it all, believed to have
a mass in the region of 40-billion Suns, corresponding to an event horizon radius exceeding
the orbit of Neptune dozens of times over. For a black hole this insanely large, “supermassive”
doesn’t do it the proper justice… and so, for those monsters whose mass exceeds ten-billion
Solar Masses, we refer to them as Ultra-Massive Black Holes, or Stupendously Large Black
Holes, a.k.a. “SLABs” in special cases. And like the black hole at the heart of IC 1101,
many of the largest SLABs we’ve pinpointed tend to lie around this 40 billion Solar Mass
mark. A handful with even greater masses have been proposed, but none with conclusive
evidence derived from direct observations. In fact, scientists aren’t even sure whether
black holes larger than about 50 billion Solar Masses can exist, due to the physics playing
out in the accretion disks that grow them. Above around 50 or 60 billion Solar Masses,
the associated disk feeding such a black hole would become so enormous that its
matter would likely condense into stars, long before it reached the event horizon.
The resulting radiation emitted by these stars would then severely hamper the conditions
in the rest of the accretion disk for feeding, curtailing Ultramassive Black Hole growth around
this threshold. Thus, we weren’t expecting to find a black hole larger than about sixty-billion
Solar Masses… that was, until we found TON-618. The 618th entry in the Tonantzintla
Catalogue refers a radio-loud Quasar- the brightest and most energetic type
of galaxy, which is drowned out by its own blinding emissions, stirred up
by its black hole’s accretion disk. This enduring light source shines from a depth
of more than ten-billion light years, yet it is so luminous that it was mistaken for a local
blue star in the Milky Way galaxy for more than a decade following its discovery. However, in 1970,
radio surveys revealed the source to be a quasar- powered by a colossal, accreting black hole.
When scientists attempted to reverse-engineer the properties of this black hole from its spectral
data, they discovered that matter must be crossing its horizon at more than 7,000km per second,
in order to generate its profile of emissions. Such ungodly speeds and their associated
temperatures could only have been the product of the gravitational influence
exerted by a record-breaking black hole, with an unfathomable mass of sixty-six
billion times that of our sun. At such an extraordinary weight, the event horizon
of this beast would stretch for little under 400 billion kilometres, with a radius more than forty
times the distance between Neptune and the sun. Unfortunately, however, this estimate has
been rolled back somewhat in recent years, now more aligned with the mass of IC 1101’s
black hole, of around 40 billion Solar Masses. But either way, it is TON-618 that has enjoyed the
most success in capturing the hearts and minds of amateur astronomers. Appearing in size comparison
videos, news articles, and on discussion forums, this black has become something of a
“people’s champion”, a designation which instantly springs to mind when contemplating
the largest black holes in the universe. And yet lately, there have been murmurs of
a new champion, with a mass that smashes TON-618’s previous record… and its name is now
also starting to crop up on sites like YouTube, Reddit and Wikipedia, claimed to be
the new largest black hole in space… the monster lying at the heart of
the Phoenix Cluster of galaxies. The Phoenix Cluster is some way
closer to home than TON-618, about half the lookback distance
at 5.8 billion light years. As we mentioned, this 1,000-member galaxy
grouping was discovered in 2011, and in the years since has rapidly risen to become one of
the most intensively studied galaxy clusters. The Phoenix Cluster is unique in a number
of ways- it was the most X-Ray-luminous cluster that had been identified at
the time, with some of the highest fractions of gas and rates of star formation,
particularly in its dominant central galaxy. Ordinarily, Brightest Cluster Galaxies, which
lie at the hearts of certain types of clusters, are not very hospitable environments for star
formation, igniting new stars at a rate even slower than the dormant Milky Way. While they
do house large quantities of intra-cluster gas, this matter rarely cools to a temperature at which
it can break out in prolific star-birth. Rather, BCGs tend to be hives of hot plasma, with an old
stellar population flooded by ionising radiation, owing to ejections from an enormous, centralised
black hole. As the gravitational focal point of its galaxy cluster, a BCG is usually grown via the
accumulation of progenitor galaxies, concentrating at the heart of the cluster along with their
black holes. This leads to a proliferation of seismic black hole merger events, eventually
yielding a tremendously oversized beast tens of billions of times the mass of the sun, surrounded
by multitudes of gas upon which it can feed. And as matter crosses its event horizon,
enormous amounts of gravitational potential energy are released, manifesting in the
form of a series of high-energy outflows. These outflows, erupting from a pair of
astrophysical jets, cascade through their surrounding interstellar gas for thousands
of light years, heating it up in the process. This keeps the majority of a BCG’s interstellar
medium too hot and energised to condense, strongly suppressing the formation of new
stars… ultimately rendering the galaxy’s morphology as some variant of an elliptical
galaxy, with flat-lining rates of star-birth. But rather unusually, that’s not what we
see in the Phoenix Cluster’s BCG- Phoenix A. In fact, this galaxy is a stellar factory,
grinding out stars at a rate hundreds of times higher than the Milky Way. Phoenix
A is classified as type II Seyfert Galaxy, the second-most energetic type of Active Galactic
Nucleus, where the core is overflowing with excess radiation and light, but not enough to drown out
the rest of the host galaxy, like within a quasar. Phoenix A derives most of its
centralised luminosity from a packed population of freshly fused stars, atop
thick blankets of radiant star-forming gas. The first evidence of abnormally high
star-formation rates in this galaxy was detected in 2012, and soon after, NASA’s Chandra
Telescope saw trillions of Solar Masses’ worth of rapidly-cooling gas concentrated around the BCG,
greater than the mass of all other one-thousand cluster constituents combined. This gas expels
heat as it glows in X-Radiation wavelengths, allowing it to cool to a temperature
better-suited to the production of stars. As mentioned, the Phoenix Cluster was the most
X-Ray luminous that had ever been identified- with the highest rate of cooling gas ever seen
in a BCG- more than 3,000 Solar Masses a year. And this cooling is what enables the unusually
high star formation rates seen in Phoenix A, as it grinds out over 600 suns’
worth of new stars per year- compared to the measly single Solar
Mass churned out by the Milky Way. And when the Hubble Telescope focused its eyes
on Phoenix A, it saw these stars arranged into an enormous series of great filaments,
measuring 330,000 light years by 160,000- both longer than the entire Milky Way galaxy,
and the largest such filaments ever discovered. Such unusual features point to a lack of black
hole activity in the galaxy’s recent past. If it were any other way, we wouldn’t see
such big breakouts of star formation within the swathes of cooling gas. However, if we
observe Phoenix A in another wavelength, we do see signs of active galactic outbursts,
imprinted upon the cluster’s emissions profile. Above and below the stellar filaments,
we see a pair of cavities carved in the structure’s X-Ray footprint, echoing areas
which have been emptied of their cooling gas, most likely due to an eruption
from its black hole feeding. The larger pair of cavities, in particular,
point to an extremely powerful outburst about a hundred-million years ago, which sent an enormous
shockwave cascading through the galactic medium, hollowing out the region of star
birth by dispelling cooling gas. The question is, what could’ve caused this
black hole to undergo such a powerful outburst? To answer this, we need to probe the apparent
“double-personality” of the galactic nucleus, which derives around half of its energy from a
pair of astrophysical jets, like a radio galaxy, while the other half is powered by friction
in the accretion disk, like a quasar. Scientists believe this fifty-fifty split is
likely to be the result of Phoenix A’s nucleus “flipping” from Radio Galaxy Mode to Quasar Mode.
A hundred-million years before we are observing this galaxy, its radio jets would’ve ignited
into a hyper-luminous bout of quasar activity, producing the shockwave which
permeated its surroundings, before settling back into “Radio-Mode”
for a second, less significant eruption tens of millions of years later, echoed
by the inner cavities. And ever since, the black hole has been lying dormant, unable to
reheat its surrounding gas… allowing it to cool, and enabling this brief, fleeting window
of star formation we see to commence. So, where does the proposed size of Phoenix A*
come into all of this? Why, almost a decade since these revelations, has it only now been
assigned as the new largest black hole? Well, it is the size and scale of
these larger cavities that point to an eruption amongst the
largest we’ve ever detected, the likes of which could only have been produced
by a black hole with the most extreme properties. With this in mind, in 2015, scientists at the Max
Planck Institute for Radio Astronomy developed a new framework for locating the biggest
black holes in the universe by searching at the hearts of galaxy clusters. In the paper,
the authors highlight the black hole at the heart of the Phoenix Cluster as a candidate
SLAB, which based on their model, may warrant a mass in the order of a hundred-billion Solar
Masses- more than double the theoretical limit. And it is this mass prediction that has shaken
up conversations surrounding the largest black hole in recent months. At a hundred-billion
Solar Masses, Phoenix A* would be more than 24,000 times the mass of our galaxy’s central
black hole, Sag. A*, and about 7% of the mass of the entire Milky Way galaxy… not to mention more
than double the mass of the Triangulum Galaxy. Such a description-defying weight translates
to an event horizon stretching for more than half a trillion kilometers, with a
diameter [3,900 AU] one-hundred times greater than the median distance
between Pluto and the sun [40 AU]. Such a beast of a black hole would present a
serious challenge to what we thought was possible… however, one has to examine the context of
this estimate before getting carried away, as it is not based on any direct
measurement of Phoenix A’s dynamics. In fact, this paper gave the same ballpark
estimate of a hundred-billion Solar Masses to no less than three central cluster
black holes; not just to Phoenix A*, but to IC 1101*, and the black hole at
the heart of another BCG, Holmberg 15A. But we know that IC 1101’s black hole
is probably not quite as large as a hundred-billion Solar Masses, more likely
to be in the range of 40 billion, perhaps. And similarly, our most intricate and precise
measurements to date of the Holmberg 15A galactic nucleus also suggest a black hole of around
40 billion Solar Masses- not one-hundred. With this in mind, it would not be unreasonable
to cast doubt over the claim of Phoenix A’s black hole being so large. In fact, the apparent
inability of the black hole to prevent star birth en messe with its ejections, has some proposing
that Phoenix A* may be undersized compared to its cluster, and thus unable to sufficiently
reheat its gas. In reality, the true mass of the Phoenix A central black hole is likely to
be much closer to the 40 billion Solar Mass mark predicted for our other Stupendeously Large
Black Holes. But as enthusiasts, we are always intrigued by the thought of a new record-breaking
discovery, and a new largest entity in space. Unfortunately, however, the notion that
Phoenix A* exceeds 100 billion Solar Masses, which has been so widely touted as a “new
largest black hole” in recent months, is not based on any “new” research or discoveries. It is
in fact an outdated, indirectly derived estimate, which has been plucked from an older paper for
the purpose of inflating a new largest black hole. And ever since, this figure has been swept up by
the Internet, gaining virality in a manner similar to TON-618, appearing in black hole comparison
videos, news articles, and cropping up in online discussions- often citing the 100-billion
Solar Mass figure given by this paper. But the truth is that the Phoenix A
central black hole is probably not the largest we’ve ever found, and is almost
certainly less than a hundred-billion suns. The problem is, is that we don’t yet have
a reliable direct means to conclusively pin down its true mass. Sadly, the more distant
Active Galactic Nuclei at higher redshifts are notoriously difficult to study, and we are
limited to indirect estimates for the time being. But new, more reliable and innovative techniques
are being developed and applied to other galactic cores slightly closer to home. Just this year were
scientists able to pin down the mass of another ultramassive black hole, some 2.5 billion light
years distant. Early in 2023, scientists used measurements of gravitational lensing to constrain
a precise estimate for the mass of the monster at the heart of the Abell 1201 Galaxy Cluster
BCG, as being about 32 billion Solar Masses. Though slightly less massive
than some of the aforementioned, this figure comes with a much lower
margin of error, and will likely pave the way for the technique to be expanded
to more distant Active Galactic Nuclei. There’s also that other Ultramassive Black
Hole we spoke about earlier, Holmberg 15A, within a giant elliptical galaxy
700 million light years from Earth. The black hole at the heart of this
galaxy is another whose size has been historically beefed up by overly optimistic
predictions, with studies in the early 2000s proposing its mass to exceed three-hundred
billion suns. But in 2019, this figure was substantially reigned in using our most detailed
observations of the galaxy’s nucleus to date, to a more modest 40 billion Solar Mass
figure, in line with the aforementioned. Something that may be a fraction larger is
the unnamed Stupendously Large Black Hole designated 4C+74.13; an Active Galactic
Nucleus at the heart of a BCG around 2.6 billion light years from Earth, in
the constellation Camelopardalis. In 2005, this galaxy was found to have
experienced an enormous outburst, which churned out the equivalent of hundreds of millions
of Gamma-Ray Bursts, over a hundred million years, also giving rise to a pair of X-Ray cavities, both
measuring 600,000 light years in diameter. Such an extreme, far-reaching eruption resolves to an
Ultramassive Black Hole growing at an alarming rate as it devours a progenitor galaxy- at least
15 billion Solar Masses, but perhaps as much as 51 billion- where it teeters precariously
on the limit of our theoretical model. However, with so many unpredictable, unseen
variable factors, it is difficult to tie down the ejecta of eruptions to precise
estimates of a black hole’s mass. There’s a lot of ambiguity, and it will take many
more years of observations before we are able to definitively establish the masses of the largest
galactic nuclei. In the meantime, however, we are forced to conclude that no known supermassive
or ultramassive black hole that we’ve found yet defies our theoretical limit of 50 or 60 billion
Solar Masses, and that includes Phoenix A*. As it stands, TON-618 is still our best shot for
the largest black hole in the known universe, not least because of its extraordinary depth. Explaining how a black hole could grow so massive
is one thing… but so quickly, is another question entirely. It truly bends the mind. But what is
even crazier to consider is how large that black hole must’ve grown today, in the 10 billion years
since its light we are observing was emitted. It must’ve cannibalised billions of extra
Solar Masses in that time, which would render it by far the largest-known black hole
to humanity. For now, though, we can never know. If you liked what you just saw, then please
consider subscribing to help the channel. But otherwise, thanks for watching,
and I’ll see you in the next episode.
