Sky and Telescope - January 2017 - 30

Black Holes, Part I

mum rate pretty much since the time they are seeded," she
explains. Such fast growth is "virtually unstoppable."
Locations where this kind of infall is possible are few and
far between. "In the simulations we see hundreds of thousands of black holes - in every newly forming galaxy in the
early universe," she says. "But the fast growth is induced in
only a handful of regions." That's in keeping with observations, she explains: the several dozen early quasars detected
are a drop in the bucket of the nearly 1 billion galaxies
uncovered by surveys. Other teams' recent simulations also
support this conclusion.

Delving into Dwarfs
Simulations cannot yet tell us what kind of seed was planted
in these rare, dense quasar regions: Di Matteo's code plops a
50,000-solar-mass black hole into place whenever the mass
of the growing galaxy surpasses about a hundredth that of
the Milky Way. But given the BlueTides results, the fantastic
growth rate should make the seed mass irrelevant, she says.
"You can pretty much start from anything, and still end up
close to what you need at a redshift of 7," she says. (A redshift
of 7 corresponds to 13 billion years ago; this era is when
astronomers see the enigmatically large black holes.)
Unfortunately, that means that the titan quasars tell us
nothing about what type of seed they formed from. Nor do
most supermassive black holes seen today. These objects have

p EMPTY NEST The Triangulum Galaxy (M33) is the third largest galaxy in the Local Group, roughly a tenth as massive as the Milky Way and Andromeda.
Yet this spiral contains no supermassive black hole in its core, and astronomers aren't sure why. (There's no sign of it having been ejected.)

30

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

S&T: SE A N WA LK ER

It's more natural for black holes to grow in spurts. In
some recent simulations of growing protogalaxies, the black
hole seemed to be "breathing," its accretion rate rising and
falling as feedback from nearby exploding stars turned on
and off, Volonteri says. But once the galaxy reached maybe a
hundredth the Milky Way's mass, there was enough gas coming in that material built up and shoved into the black hole.
Growth then took off like a rocket.
The key is to shovel enough gas into one place, says Tiziana
Di Matteo (Carnegie Mellon University). For several years
her team has been crafting computer simulations to explore
how galaxies and black holes grow together. Their most recent
simulation, BlueTides, follows the formation of galaxies and
their black holes in a volume 1.3 billion light-years on a side.
That's roughly 300 times larger than the largest observational
survey to date of the universe in this early cosmic era. Such a
sweeping simulated view enables astronomers to make testable predictions about what we should and shouldn't see in
the real universe.
It's this work that has solved the mystery of how the titan
quasars formed, Di Matteo declares. The problem was that
previous simulations didn't have the resolution or volume
coverage to probe the rarest, highest density regions of the
universe. In some of these regions, the gas can plunge inward
directly instead of settling into disks and draining in more
slowly. In these places, "black holes can grow at their maxi-



Sky and Telescope - January 2017

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Sky and Telescope - January 2017 - Cover3
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Sky and Telescope - January 2017 - SGA17RAL
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