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View Full Version : Size of accretion disks around supermassive black holes



jonaanderson
2009-Apr-02, 06:23 PM
How large are the accresion disks that we have observed around supermassive black holes in our own galaxy and other similar galaxies? How large do they get in active galaxies and quasars?

antoniseb
2009-Apr-02, 06:34 PM
Ours doesn't have one so far as I know.

stutefish
2009-Apr-02, 08:41 PM
Wouldn't the accretion disk's size be a function of how much stuff there was floating around nearby, and how many other influences might be in effect?

ETA: I mean, and therefore isn't each disk going to be a unique case?

parejkoj
2009-Apr-07, 07:34 PM
It is as yet unclear whether our own supermassive black hole (Sgr A*) possesses an accretion disk or not. It probably is undergoing what is termed, radiatively inefficient accretion, where material doesn't form a disk, but just falls in roughly spherically. But for quasars and other strongly accreting systems, the accretion disk ranges from ~6 Schwarzschild radii (RS) to hundreds or thousands of RS.

The size is dependent on how big the black hole is (that's why it is written in terms of RS, which is about 1AU for a 10^8 solar mass black hole), and on how much material can fall in. If there isn't enough material falling in, an accretion disk won't form, like Sgr A*. But if there is plenty of material, the radiation pressure will tend to create a similar disk structure for every system.

chornedsnorkack
2009-Apr-07, 08:17 PM
It is as yet unclear whether our own supermassive black hole (Sgr A*) possesses an accretion disk or not. It probably is undergoing what is termed, radiatively inefficient accretion, where material doesn't form a disk, but just falls in roughly spherically. But for quasars and other strongly accreting systems, the accretion disk ranges from ~6 Schwarzschild radii (RS) to hundreds or thousands of RS.

The size is dependent on how big the black hole is (that's why it is written in terms of RS, which is about 1AU for a 10^8 solar mass black hole), and on how much material can fall in. If there isn't enough material falling in, an accretion disk won't form, like Sgr A*. But if there is plenty of material, the radiation pressure will tend to create a similar disk structure for every system.

Radiation pressure cannot create angular momentum! If the infalling matter is not carrying angular momentum then surely it is impossible to form a disc no matter how much matter is arriving...

parejkoj
2009-Apr-07, 10:26 PM
Radiation pressure cannot create angular momentum! If the infalling matter is not carrying angular momentum then surely it is impossible to form a disc no matter how much matter is arriving...

Except that in galaxies, most of the matter near the central blackhole will have significant angular momentum. Spherical accretion only works in specific circumstances (such as very rapid, or very slow infall rates).