Sky and Telescope - January 2017 - 28

Black Holes, Part 1

Many astronomers - and particularly those who specialize in black holes - are huge fans of this scenario. John
Kormendy (University of Texas at Austin), one of the ﬁrst
astronomers to establish that black holes sit at the centers of
most massive galaxies, describes the Population-III picture as
"the only game in town."
"It's enormously natural," he says. "It's essentially inevitable that by the time the universe was half a billion years old,
quite a lot of these 100-ish-solar-mass black holes would have
merged, and then you've got 1,000-ish-solar-mass black holes,
and that's what we need."
Not everyone agrees with Kormendy's assessment. The problem is that at these masses, the black hole seeds would need to
grow continuously at their maximum feeding limit in order to
become the titan quasars that existed 13 billion years ago.
This limit exists because of the same light pressure problem that curtails stars' masses. As gas falls onto a black hole,
it heats up and glows. If you dump too much gas on, the glow
will be so strong that the pressure of photons radiating out
will actually overcome the gravity pulling in and cut the black
hole's fuel line. This balance point between accretion in and
radiation out is called the Eddington limit.
Keeping up Eddington-level growth is exceptionally
hard. Not only does it mean that the galaxy must have fed
its central black hole with a steady supply of gas for several
hundred million years, but also that the black hole kept its
mouth at the straw. That's a tall order.

Populations I, II, and III
◗ Astronomers talk about three populations of stars,
based on Walter Baade's work in the 1940s studying
the Milky Way. These are:

* Pop I: usually young and in the galaxy's spiral disk.
Metals make up about 2-4% of their total composition.
Includes both the Sun and O- and B-type, bluish massive stars.

* Pop II: older (often 10 billion years or more), in the
galaxy's bulge and halo. Metal content much lower,
down to maybe a thousandth that of the Sun. Only the
least massive ones survive today.

* Pop III: the fabled first stars. Would have formed with
only hydrogen and helium (no metals). None yet conclusively detected - either they've died off or are masked
by heavy-element "pollution" they've picked
up over time.

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

Just Get to the Point
So other astronomers turn to the third scenario, directcollapse black holes. In this scenario, warm, metal-free gas
collapses under its own gravity to form a black hole of tens to
hundreds of thousands of Suns. (The gas might or might not
ﬁrst form a supermassive star.) With masses so much greater
than those created by Population-III stars, direct-collapse
seeds offer an attractive alternative, explains Muhammad
Latif (Paris Institute of Astrophysics).
But if growing a Pop-III seed into a titan is hard, making
direct-collapse black holes can seem almost impossible.

"It's the opposite of Occam's razor," she
says, sighing. "It's such a series of events
to occur all at the same time that I'm like,
'Seriously?' I'm a scientist.'"
To make one of these objects, you need a large inﬂux of
gas. In the standard direct-collapse picture, this gas needs
to stay warm (tens of thousands of kelvin) so that it doesn't
fragment too early. Thus the gas must be pristine, with essentially no metals. The gas also needs to be basking in enough
ultraviolet radiation to stiﬂe the formation of molecular
hydrogen, which also cools it.
Here's where things get complicated. These ultraviolet
photons must come from stars. But the gas forming the black
hole seed can't have been tainted by stellar-made metals. To
avoid contaminating the gas but still bathe it in ultraviolet
rays, the stars would need to lie in a massive protogalaxy
just 10,000 light-years or so away, or less than half the
span between our solar system and the Milky Way's center.
Furthermore, these stars would likely be Population-II stars
(which means they'd contain some metals), because such
stars live longer and are more common, Latif says. So to
protect the black-hole-birthing gas from the stellar winds or
supernovae, either the stars' outﬂows must be precisely timed
with the black hole's formation, or they must all aim away
from where the black hole is forming.
The universe's metal enrichment did proceed patchily, and
observations show that pockets of pristine gas survived even
to the time that we see the titan quasars. But many astronomers have trouble swallowing the complexities of the directcollapse scenario. Although Marta Volonteri (Paris Institute
of Astrophysics) contributed to its theoretical groundwork,
she's become increasingly skeptical of direct collapse in the
last couple of years. "It's the opposite of Occam's razor," she
says, sighing. "It's such a series of events to occur all at the
same time that I'm like, 'Seriously?' I'm a scientist.'"
Despite her reservations, Volonteri remains agnostic. She
and others recently suggested that a clump of bright, pristine
gas in the protogalaxy CR7 might contain a direct-collapse
black hole. Others have also found potential sources in the

Sky and Telescope - January 2017

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