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Class 24 : Supermassive
black holes
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Recap: What is a black hole?
Case studies:
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M87.
M106.
MCG-6-30-15.
What’s at the center of the Milky Way?
The demographics of black holes.
I : Recap of black holes
• Singularity – central
“point” containing all of the
mass. Known laws of
physics break down.
RSch = 3 km for the Sun.
• Event horizon – point of
“no return”. Everything
within this radius is
dragged to the singularity
by enormous gravity.
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Both space and time
are strongly distorted
in the vicinity of a
black hole.
Light seems to bend,
time slows down.
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II : Evidence for supermassive
black holes – three case studies
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Case I : M87
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Large elliptical galaxy.
Black hole suspected
due to presence of
prominent jet.
Target of early study
by Hubble Space
Telescope.
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HST found…
Rotating gas disk at
galactic center.
Measured rotation
implied a central
object of 3 billion
solar masses!
Mass cannot be due to
normal stars at
center… not enough
light is seen.
Good evidence for 3
billion solar mass
black hole!
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Case II : M106
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Contains central gas disk.
Disk produces naturally
occurring MASER* emission.
Radio telescopes can
measure position & velocity
of MASERs to great
accuracy.
Velocity changes with radius
precisely as expected if all
mass is concentrated at
center!
30 million solar mass black
hole…
*MASER:
Microwave Amplification by
Stimulated Emission of Radiation.
MCG-6-30-15
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Case III : MCG-6-30-15
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“Active galactic nucleus.”
Bright X-ray source.
Find signature of a gas
disk in X-ray spectrum.
This disk is orbiting
something at 30% speed
of light!
Also see strong
“gravitational redshifts.”
Strong evidence for a
black hole in this object.
III : The center of our galaxy
There’s something strange at
the center of our galaxy…
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Modern large
telescopes can track
individual stars at
galactic center
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Need infrared (to
penetrate dust).
Need very good
resolution.
We have been
observing for past 10
years…
Keck, 2 m
Ghez, et al.
Motions of stars
consistent with
large, dark
mass located at
Sgr A*…
Ghez, et al.
Schödel et al.
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The central object is…
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Very dark.
Very massive (3 million solar masses).
Must be very compact (star S0-2 gets
within 17 light hours of the center).
Currently the best case for any
supermassive black hole.
Schödel et al.
IV : Demography of black holes
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Black holes exist in centers of some galaxies…
But how widespread are they?
Does every galaxy have a supermassive
central black hole?
Several teams set out to answer that
question…
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Use best resources (HST, large telescopes on ground
etc.) to gather lots of data on many nearby galaxies.
Systematic search for black holes.
They found them, and discovered interesting
patterns…
Correlation between size of black hole and the
brightness of the galaxy’s bulge (but not the disk)…
Color code: MASERs, gas, stars
Kormendy
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But, even better correlation with stellar
velocity in bulge…
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Correlations crucially important!
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Bigger bulges  bigger black holes.
 More fuel?
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Faster stars  bigger black holes.
 Connection between the formation of the galaxy
(bulge) and the supermassive black hole.
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Currently at the forefront of research…