Sky and Telescope - February 2017 - 22

Black Holes, Part II

Black hole mass (solar masses)

1010
Ellipticals
Classic bulges
Pseudobulges
109

108

107

106

100

200

300

400

Range of stellar velocities in bulge
("sigma") (km/s)
S M-SIGMA As astronomers gathered more observations, it became
clear that the masses of all galaxies' black holes do not tightly trend
with the galaxies' other properties. In their 2013 review John Kormendy
and Luis Ho divvied up galaxies based on their shapes and concluded
that, although elliptical galaxies and those with classical bulges still
follow the trend (black line), galaxies with pseudobulges do not. Not
all astronomers are convinced by this distinction, but this result helped
astronomers recognize that all black holes and galaxies don't grow in
lockstep. This figure only includes black holes for which astronomers
have direct mass measurements, not indirect ones (see sidebar, p. 20).

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

S BULGES M87 (above left) is the classic example of a massive elliptical
galaxy. Distinguishing between classical bulges like the one in M81 (center)
and pseudobulges like that of M77 (right) is tougher - knowing the stars'
orbital paths helps, because those in classical bulges are less orderly.

in 2012. "People often say that black holes have to grow by
accretion and that feedback must happen. To that I usually
respond by saying, 'All those things could happen but still
have no bearing on a correlation.'"
That's because, although the black hole is no doubt partly
responsible for heating galaxies' halos of gas, the thermostat
effect would preserve a correlation that was already there - it
wouldn't necessarily create it. And AGN feedback is too weak
to control growth in most galaxies. There are plenty of active
galaxies that don't follow the trends, Kormendy says, including the two spirals NGC 1068 and NGC 4151, whose black
holes astronomers have studied for decades.
"It's still thought that the black hole has an inﬂuence on
the galaxy, but it may not be as dominant as was thought,"
Gültekin sums up. Nowadays the question is what kind of
feedback is important - maybe jets, or winds driven out by
the accreting black hole's glow (called quasar feedback). Generally jets get the attention, says Gültekin.
"I'm actually probably in the minority now in thinking
that quasar feedback is still an important component," he
says. "But there's been no vote on this, this is just my informally taking the temperature of the community."

Option B: Mergers
Peng was among the ﬁrst to say that the apparent connection
between galaxies and their black holes could simply be a matter of math. Take any two galaxies and merge them, then do
it again and again and again, and the correlations will arise
naturally, no feedback needed. It's an inevitable outcome of
adding big numbers together.
But when Peng ﬁrst suggested the idea, feedback was "a
super-hot topic," he says. "You can imagine the excitement,
and fear, I had to potentially start a controversy," he says,

M 87: CHRIS MIHOS (CASE WESTER N RESERV E UNIV.), ESO; S&T: LE A H TISCIONE / SOURCE:
J. KOR MENDY A ND L. HO / ANNUAL R E VIE W OF ASTR ONOMY AND ASTR OPHYSICS 2013

tating clouds (S&T: Nov. 2016, p. 11), but nothing like the
downpour that should exist if the gas were left to cool on its
own. Something is heating it.
One solution is black hole feedback. Dribble some gas on
the beast, and it'll rouse like an angry dragon, shooting jets
and inﬂating gigantic bubbles as it powers an active galactic
nucleus, or AGN. The rising bubbles drag surrounding gas
with them, creating eddies and turbulence. These motions
heat the gas, preventing star formation.
Observers see signs of jet activity in more than 70% of the
galaxies in clusters' centers - generally the biggest, brightest
galaxies with the most hot gas. Many have cavities in their
X-ray-emitting gas, too. Black holes thus serve as "cosmic
thermostats," as one 2014 review article put it, modulating
gas temperatures and closely regulating starbirth and, therefore, galaxy growth.
"I don't think anyone disagrees with that," Kormendy says.
"I would be very surprised if we were barking up the wrong
tree."
Spurred on by its utility in heating gas, feedback became
the thing in astronomy. Many suspected that the hole forced
its host to adhere to its own growth rate as the beast haphazardly chomped on gas, explaining the trends. "At some black
hole conferences you'll still see people show the M-sigma
correlation and say, 'This is evidence for AGN feedback,'" says
Chien Peng (Giant Magellan Telescope Organization) told me

Sky and Telescope - February 2017

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