Sky and Telescope - February 2017 - 24

Black Holes, Part II

galaxy is much bigger than the black hole, and it serves as the
fuel reservoir for both star and black hole growth.
That might explain why astronomers sometimes find weak
echoes of a trend across other segments of the galactic population. Marta Volonteri (Paris Institute of Astrophysics) and
Amy Reines (NOAO) recently looked at 341 nearby galaxies,
262 of which contained an actively accreting black hole. The

"Even if there's no real coevolution going on,
it would be a little surprising if there wasn't a
crummy correlation."
duo found that the black hole's mass did increase with the
galaxy's total stellar mass. But for a given galactic weight, the
accreting beasts were roughly one-tenth as massive as those
that weren't feasting on gas.
Volonteri and Reines couldn't see the shape of the galaxies
they studied, because the accreting black hole acts as a floodlight, blinding telescopes to the stellar metropolis that contains
it. But today's active galaxies are usually spirals, with small
bulges and smaller black holes than ellipticals have. If these
AGN are spirals, then the result hints that there is some sort of
trend governing black hole masses in these galaxies.

Active galactic nuclei
Elliptical galaxies
Disk galaxies with classical bugles
Disk galaxies with pseudobulges
Kormendy & Ho 2013, scaled
Kormendy & Ho 2013

11

9

Finding the Culprit
8

7

6

5

McConnell & Ma 2013
Haring & Rix 2004

4
8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

Stellar mass of galaxy (log solar masses)
S GALAXIES AND THEIR BLACK HOLES This graph shows the relationship between galaxies' masses in stars and the masses of their central black holes. The straight lines are possible relations between black
hole mass and the mass of the galaxy's bulge (ellipticals are essentially
all bulge). The active galactic nuclei (AGN) shown have had their masses
measured in a more indirect way (so they're more uncertain) and include
many dwarf galaxies. For many AGN shown here, it's unclear what shape
the host galaxies have. But today's AGN usually appear in spiral galaxies,
so if most of these galaxies are spirals, then this graph confirms that
spirals generally have less massive black holes than ellipticals do.

24

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

Astronomers do not agree on which of these scenarios is
true. They might all be, to some extent. Part of the problem is
that we don't know when the correlations arose or what they
looked like early on. Studying galaxies from the universe's
first couple billion years requires valiant struggle. The systems
are far away, so they're small and faint, plus cosmic expansion shifts their light to longer wavelengths, making them
harder to study. Not to mention the universe was smaller and
more crowded, and things were rowdy; a big galaxy back then
was much smaller - or in pieces - compared with now, with
gas pouring in from the cosmic web to boot. "Life back then
was really seriously messy," Kormendy says.
Astronomers see hints that the M-sigma relation is looser
in the early universe, with bigger black holes for a given range
of star speeds. But they're wary of trusting that impression.
The AGN detectable in that cosmic era are the brightest ones,
and they might be the basketball players of the population,
far larger than the norm, Volonteri cautions.
Kormendy agrees. "I wouldn't want to stick my neck out
terribly far on our understanding" of what was happening 11
or 12 billion years ago, he admits.

S&T: LE A H TISCIONE, CH A RT SOURCE: A . REINES & M. VOLON TERI / ASTR OPHYSICAL JOUR NAL 2015 (813:2:82)

Black hole mass (log solar masses)

10

That makes sense, Kormendy says. "Even if there's no real
coevolution going on, it would be a little surprising if there
wasn't a crummy correlation between the gas reservoir and
how much you could feed a black hole," he says. "It would be
completely unnatural if there weren't."
Perhaps all the correlations are crummy. All of the
astronomers interviewed for this article noted that, because
feedback and lockstep evolution were such popular ideas,
they're now entrenched far more deeply than the data justify.
"I tend to be a little more cynical," Greene says. "I was a
kid when the M-sigma relation got everyone excited," so she
doesn't have a lifetime of work riding on its defense.
Volonteri also doesn't believe in lockstep growth. "I
strongly believe that there is a coevolution, but the way I
mean coevolution is very different," she says. "To me, coevolution means simply big black holes, big galaxies; small black
holes, small galaxies."
Both galaxy and black hole depend on something much
bigger, though: the cosmic gas supply. Recent simulations by
Tiziana Di Matteo (Carnegie Mellon University) and others
follow the growth of galaxies and their black holes across the
early eons of cosmic time. The researchers found that the
biggest black holes tend to grow in spheroidal galaxies, not
those dominated by disks - which matches what astronomers
see observationally. But, Di Matteo's team explains, this is
because such galaxies form at the nodes of several filaments
in the cosmic web, where cold gas pours straight in instead of
coming in at an angle, as it does for disk galaxies. That could
explain why a galaxy's shape is connected to its black hole's
mass. The simulations also suggest that whether a galaxy is a
ball or a disk depends on where it's born and how the cosmic
web feeds it, not on whether it merges with something else.



Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

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Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
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