Sky and Telescope - January 2017 - 29

A

CANDELS/GOODS-S survey. Astronomers are still arguing
over whether any of these sources could indeed be a directcollapse seed.
Some claim that the direct-collapse process is not as
complicated as we think. Even if it is, Latif says, his team's
simulations show that sufficient seeds would form to explain
the titan quasars we see in the infant universe.

Black hole's
radiation expels

B

Black ho
hole's
le
e's
gravity pu
pulls
llss in

S&T ILLUSTR ATION: LE A H TISCIONE, SOURCE: M A RTA VOLON TERI; SIM UL ATION: TIZIA N A DI M AT TEO E T A L. / AR XIV.OR G 2016

Overcoming Eddington
But even a 10,000-solar-mass seed is a far cry from the 2 billion solar masses of the black hole powering the quasar ULAS
J1120+0641, whose light left it 12.9 billion years ago. To reach
that size in less than a billion years, the black hole must have
accreted gas at its Eddington limit for roughly 60% of the
time if it began as a direct-collapse seed. If the seed was a
stellar one of 300 solar masses, then it had to eat at the limit
for all of those 900 million years. Either way, somehow the
galaxy shoves tens of thousands to tens of millions of solar
masses' worth of gas down the black hole's throat, all within
a few hundred million years.
It's at this point that the accretion ceiling begins to feel
more like a straitjacket than physics. "I'm not a believer in
the Eddington limit," Volonteri said flatly in a recent talk at
the Harvard-Smithsonian Center for Astrophysics. "If the
accretion rate can be higher, then building the biggest black
holes is not crazy."
But overcoming the Eddington limit is no easy task, she
admits. The super-Eddington inflow of gas must trap the
gas's glow well enough that the radiation doesn't launch the
gas away. Nor can the black hole shoot material away in jets,
unless the jets reach far enough away from it that the plasma
doesn't heat the nearby accretion flow and stifle the downpour
onto the hole. Galactic centers are also hotbeds of starbirth,
and the stars mustn't do anything to disrupt the pipeline - no
cataclysmic antics allowed. Not to mention that the violent
glare from the accreting black hole can "damage" the galaxy,
heating up ambient gas and preventing it from funneling into
the core and feeding the beast. "Aaaah," Volonteri groans.
"There are so many variables that they're driving me nuts."

Gas
clouds

Jet
et
Radiation
Accretion disk
d k

Jet
J

p SUPER-EDDINGTON ACCRETION There are two conditions to consider when talking about whether a black hole can accrete faster than it's
theoretically capable of doing. One is the balance between the gravitational
pull inward and the pressure outwards, caused by the glow from the accreting black hole (red arrows). The radiation can push and shove away gas
clouds (A) that would otherwise fall toward the black hole (B). The other
condition is what happens inside the accretion disk itself - the disk's own
glow and internal pressure work against it, limiting how much material can
drain down onto the black hole (black arrow). On the other hand, when the
black hole is fed at rates much higher than the theoretical limit, the disk can
become so dense and thick that some of the radiation becomes trapped
inside, suppressing the push on nearby gas. Regardless of the theoretical
limit, the hot and powerful radiation from an accreting black hole, as well
as the jets it sometimes has, would eventually stifle accretion and stymie
growth by pushing or shredding the surrounding gas clouds.
q SIGN OF THE TIDES (below) These snapshots from the BlueTides
simulation show the gas environment of one of the most massive black
holes about 650 million years after the Big Bang. Hotter gas appears
redder. Large crosses mark the positions of black holes, and their sizes
are proportional to each hole's mass. The inset in the rightmost panel is a
zoom 10 times closer than the panel it's in and shows the host galaxy.

s k y a n d t e l e s c o p e .c o m

* JANUARY 2017

29


http://www.ARXIV.ORG http://www.skyandtelescope.com

Sky and Telescope - January 2017

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