Sky and Telescope - February 2017 - 25

TIZIA N A DI M AT TEO E T A L. / AR XIV.OR G 2016

S COSMIC WEB These snapshots from the BlueTides simulation show the gas environments of the most massive disk galaxy that formed in the
simulation (top row) compared with the most massive black hole and host galaxy. These stills are from about 650 million years after the Big Bang; the
rightmost, circular inset in each row is a zoom in on the host galaxy. Hotter gas appears redder. Large crosses mark the positions of black holes, and
their sizes are proportional to each hole's mass. Although massive disk galaxies did grow big black holes in the simulation, they generally didn't contain the most massive ones. The simulation also confirms that spheroidal (elliptical) galaxies grow at the crossroads of many filaments in the cosmic
web, whereas disk galaxies form along more isolated filaments. The results suggest that a galaxy's shape depends on where it forms in the cosmic
web, not on whether it merges with another galaxy.

Dwarf galaxies may help. Greene and others have been
tracking down the black holes at the centers of these galactic
runts because dwarfs are the primordial crumbs of galaxy
formation, left more or less unscathed since they ﬁrst formed.
"There's something interesting to learn by looking speciﬁcally
at galaxies that don't really have mergers and didn't really
undergo that kind of growth," she says. If they and their black
holes follow any trends today, it will be because they were
born with them.
Gültekin agrees that galaxies shirking M-sigma, whether
barely or ﬂagrantly, are an important place to look. "Things
that don't follow the relation are important clues for telling
us what's not important for driving the relationship," he says.
His team is investigating gas outﬂows from a range of
galaxies. If researchers can pin down how fast this material
moves, then they can determine how much energy must be
launching the ﬂows and whether the black hole is to blame.
What astronomers really need is a census, Greene says.
What is the full distribution of black hole masses across the

entire galaxy population? ALMA and the next-gen superscopes should enable observers to peer out to 60 or 70 million
light-years, detecting all types of galaxies and measuring black
holes down to a million solar masses, the same size as the
Milky Way's (relatively puny) supermassive beast, she predicts.
"Then we'll be able to see the full range of black hole
masses," she says. "We'll be able to slice and dice the sample
into things that are round and things that are ﬂatter and
things that evolved slowly and things that are more like
bulges and ask whether they're different."
At that point, astronomers will ﬁnally discover just what
the connection between black holes and galaxies is. Maybe
they'll exonerate the poor little beasts of cosmic guilt. Or
maybe the objects will prove just as ruthless as the sci-ﬁ
stories claim.
¢ Every time she writes a story about these spacetime potholes, Science Editor CAMILLE M. CARLISLE resists the urge
to cry out, "BWAHAHAHA BLACK HOLES!"
s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

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Sky and Telescope - February 2017

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