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View Full Version : Galaxies are the accretion discs of black holes.



spaceflight101
2007-Nov-12, 04:39 AM
Is it true that Black Hole's are the accretion discs of Black Holes?

http://img233.imageshack.us/img233/7595/blackholeao9.jpg

Galaxies look remarkably like suds going down a drain hole don't they. I thought of this idea many years ago but more recently it has been discovered that every major Galaxy has a Super Black Hole at its center. The question astronomers are still debating is which came first. The Galaxy or the Black Hole.

What I'm wondering is whether everything in the Milky Way will eventually end up sucked down that hole. I'm thinking of moving to one of the minor satellite galaxies. In a few trillion years.

novaderrik
2007-Nov-12, 04:45 AM
altho it might not be correct, i look at the galaxy as we know it as the result of a giant accretion disc. compare it to our solar system-the black hole at the center is like our sun, and the stars and other bodies are like the planets and asteroids and what not.
yeah, i know, overly simplified. but it gets me thru my mundane existence.

spaceflight101
2007-Nov-12, 04:57 AM
Yeah, everything spins. God must have been rather dizzy when he designed the place. The black hole at the center of our galaxy could swallow our sun without so much as a hiccup. I don't think there is anything on a similar scale to it. Red Giants are fairly immense but inconsequential as far as density goes.

astromark
2007-Nov-12, 06:15 AM
Fortunately orbital velocities and all taken into account suggests that No. The Galaxy is not spiraling into the central group of black holes. We will no doubt place warning signs to keep space travelers away from this highly volatile area. There would be no coming back...:(I might report this danger to a government body so they can form a sub comity and evaluate the risks and environmental reports...and sue some one...

Lord Jubjub
2007-Nov-13, 12:59 AM
I wouldn't say that that galaxies are accretion disks (this would imply that the black hole was pre-existing), rather that as the galaxies accreted, mass that didn't achieve orbital velocity fell into the center. A black hole was formed when the central mass reached the limit.

Though I understand that galaxy formation is still not considered nailed down.

loglo
2007-Nov-17, 02:15 AM
Have we been able to measure the effect that central galactic black holes have when two galaxies collide?
I know there are a few observable examples of galaxies coliding. Would it not be possible to confirm the theory of galactic black holes by looking at the gravitational effect of the black hole on the colliding matter? Or is gravity just too weak a force?

The affects of Dark Matter would be more important in galaxy collisions as there is much more mass in the DM than in the SMBH. SMBH masses within and outside the Milky Way have already been confirmed by looking at the movement of stars near them. ie this (http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2966.2005.09713.x?journalCode=mnr) paper or this (http://cfa-www.harvard.edu/~kstanek/astro200/bhmw.pdf) PDF or this (http://www.eurekalert.org/pub_releases/2007-06/su-srh060707.php).

neilzero
2007-Nov-17, 04:04 AM
The acreation disc for a black hole at a typical galaxy center may have a radius of a million kilometers, but items orbiting farther from the center are in little danger of being drawn closer. As far as I know, the average radius of galactic orbits is not decreasing in our part of the galaxy = not being acreated. Neil

EvilEye
2007-Nov-17, 01:45 PM
Our solar system has been wandering in and out of the Cygnus-Norma spiral arm for billenia.

If our galaxy were an accretion disk, then we would have only moved inward.

Spaceman Spiff
2007-Nov-21, 03:31 AM
Let's look at the radius of influence of a galaxy's supermassive black hole (smbh) lying at its center. It is:

r = GM/sigma^2

where G is the usual Newton's Grav. constant, M is the smbh's mass, and sigma is the velocity dispersion of the stellar spheroid of the host galaxy. This works out to be:

r = 35 ly * M_8 / (sigma_200)^2,

where M_8 is the smbh mass in units of 10^8 solar masses (to go on the hefty side), and sigma_200 is the stellar spheroid's velocity dispersion in units of 200 km per second (typical for a ~10^10 solar mass spheroid and scaling something like sqrt(M_sph/r_e), where r_e is some surface brightness weighted effective radius). The vast majority of the galaxy does not know (via gravity) about the existence of the "monster" (smbh) sitting at the bottom of the well. And certainly it would be a longshot (and probably insane) to suggest that the angular momentum of the galaxy knows anything directly about that of the angular momentum of the central accretion structure feeding the smbh.

What was it Huxley said about science slaying a beautiful idea with an ugly fact?

Now - the question of how the supermassive black hole formed and grew at the bottom of the well and how the energy released from the accretion of matter onto that growing black hole affected the development of the forming galaxy is a valid one and undergoing intense scientific scrutiny both theoretically and observationally.

Hope this helps. :)