Class 24 : Supermassive black holes

Recap: What is a black hole? Case studies:

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.

• Event horizon – point of “no return”. Everything within this radius is dragged to the singularity by enormous gravity.

RSch = 3 km for the Sun.

Both space and time are strongly distorted in the vicinity of a black hole.

Light seems to bend, time slows down.

II : Evidence for supermassive black holes – three case studies Case I : M87

Large elliptical galaxy. Black hole suspected

due to presence of prominent jet.

Target of early study by Hubble Space Telescope.

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!

Case II : M106 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

Case III : MCG-6-30-15 “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…

Modern large telescopes can track individual stars at galactic center 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.

The central object is… 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 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… 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

But, even better correlation with stellar velocity in bulge…

Kormendy

Correlations crucially important! 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.

Currently at the forefront of research…