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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.

R
Sch
= 3 km for the Sun.
• Singularity – central
“point” containing all of the mass. Known laws of physics break down.
• 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.
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Light seems to bend, time slows down.

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Case I : M87
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Large elliptical galaxy.
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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.

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Modern large telescopes can track individual stars at galactic center
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Need infrared (to penetrate dust).
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Need very good resolution.
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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).
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Currently the best case for any supermassive black hole.

Schödel et al.

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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.
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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
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Kormendy
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?
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…