Sky and Telescope - January 2017 - 31

SDSS

grown so much over cosmic time through both accretion and
mergers that they essentially suffer from amnesia: they've
forgotten where they came from.
That's not true for the runts. The smallest massive black
holes, and the dwarf galaxies they inhabit, have changed little
since their creation. These galaxies are so small that their
stars ravage them, the stellar winds and supernovae ousting
the cold gas needed to feed the black hole. Thus, black holes
in dwarf galaxies should be about as big today as they were
when they first formed.
Amy Reines (NOAO) and others are ferreting out these
dwarf supermassive black holes. So far, they've turned up
more than 150 of them. Of the couple dozen that they've
weighed, the smallest contains roughly 50,000 solar masses.
Right now the sample is small, and observations aren't sensitive enough to detect black holes much lighter than this.
But if as they keep digging astronomers find that there's a
"plateau" in how low black holes go, the limit could serve as a
paternity test: if the masses peter out around tens of thousands of solar masses, that would favor direct collapse; if they
keep plunging past our observational reach, that would favor
stellar sources.
Jenny Greene (Princeton) wants to go further: she wants
a census of all galaxies, to see how often they contain big
black holes. Direct-collapse seeds are harder to make than
stellar ones, so if supermassive black holes are normally born
through direct collapse, there should be fewer of them -
they'd occupy only about 60% of galaxies whose masses are
at least a billion Suns (comparable to the Small Magellanic
Cloud), she estimates. If massive black holes came from stellar seeds, on the other hand, basically all galaxies of this mass
would have them.
We already know of exceptions to the latter. For example,
there's no sign of a central black hole in the Triangulum
Galaxy (M33), the spiral satellite of the larger Andromeda
Galaxy. "M33 is the whole reason we get to ask whether all
galaxies have black holes," Greene says. "Because we know
that, when you get to a low enough stellar mass, they don't all
have black holes."
Based on observations, she says, the best estimate is that
galaxies with more than 100 million Suns contain a black
hole at least 50% of the time. "We've ruled out below 20%,"
she says. "So it's looking like it's not so uncommon, even
at these relatively low [galaxy] masses, to host a black hole
above 100,000 solar masses." But if this fraction doesn't go
up with more data, it could be a strike against the Population-III scenario.
On the other hand, recent simulations by Volonteri and
her colleagues warn against thinking that building black
holes is ever easy. The team followed black hole formation in
a wide range of galaxies but took a more individual approach
than Di Matteo. For each clump of gas, the team's simulation
took into account its unique conditions and calculated how
massive a black hole could form there, assuming it arose from
Population-III stars or stellar-cluster dynamics.

TINY SUPERMASSIVE HOLE
The dwarf galaxy RGG 118 is
a hundredth as massive as the
Milky Way and contains one
of the smallest central black
holes yet detected: roughly
50,000 solar masses. The disk
galaxy lies about 350 million
light-years away.

The resulting seeds spanned a wide range of masses, but
most had on the order of a thousand Suns - within an order
of magnitude of the teeniest known massive black hole. Plus,
supernova feedback stunted black hole growth in the smallest
galaxies, and stellar cities with only a tenth of the Magellanic
Clouds' mass were unlikely to form a central beast. So even
with a stellar seed, massive black holes might be fairly rare.

LIGO and Beyond
Dwarf galaxies are the most promising lead for answering the
genesis question, but they're not the only one. Kormendy also
points to gravitational wave research. "It's really interesting
and a little surprising that the first binary black hole detected
by LIGO didn't involve 5-solar-mass black holes," he says. "It
involved 30-solar-mass black holes. I really perked up when I
saw that." If gravitational wave observatories regularly turn
up black holes of several tens of solar masses, that would
favor Population-III seeds, he says (see sidebar below).
Future observations of the early universe may also shed
some light on the matter, pushing to within a billion years
of the Big Bang. NASA's proposed WFIRST spacecraft should
detect 10,000 supermassive black holes this early, Di Matteo
estimates. Projects such as NASA's James Webb Space Telescope
and ESA's Athena X-ray observatory (both upcoming) should
also be able to detect accreting black holes with millions
of solar masses in this era. So it's feasible that, in the next
decade, we'll be able to answer for black holes what my mother
answered for me, and definitively dismiss the cosmic stork.

¢ Sky & Telescope's Science Editor CAMILLE M. CARLISLE
thinks black holes are adorable.

eLISA's

Gravitational Waves

◗ Slated for launch in 2035, the European space mission
eLISA will "listen" for gravitational waves from merging
black holes and other astrophysical phenomena. The
experiment will be able to detect spacetime ripples from
black holes with masses of 10,000 to 10 million solar
masses from across the universe. If black holes with
such masses existed, and merged, even just a few million years after the Big Bang, eLISA will find them.

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

* JANUARY 2017

31


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Sky and Telescope - January 2017

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