What is an "inactive" black hole?

Question

In the context of the news article below about NASA and a sky survey that they believe has identified all of the galaxies in about 400 million lightyears from us that have super-massive black holes in their center, how would some of these be inactive? Supposedly our own galaxy has an inactive blackhole. First, I know the idea of inactivity because all of the matter in the local space has been sucked out already, but we are talking galactic centers, the gravitational pivot on which the pinwheel (in our case) spins. So, how would something that is at the center of all that action, stars packed closer than icebergs on the north Atlantic, be inactive?
http://news.yahoo.com/s/space/20061005/sc_space/nasacompletessurveyofnearbysupermassiveblackholes

I'm tempted to give it to Vitamin C and these links indicates my similar feeling. Gene Smith's summary of the galaxy nucleus shows a very massive site and also "that the Galactic Center region is an unusually crowded place": http://casswww.ucsd.edu/public/tutorial/MW.html
http://rsd-www.nrl.navy.mil/7213/lazio/GC/
http://science.nasa.gov/headlines/y2002/21feb_mwbh.htm
It seems far too "busy" to have built some static zone where it has eaten everything in reach already.

Answer 1

look. i have been researching about astronomy and i've never encountered a word or term INACTIVE BLACKHOLE. there is no such thing. there are only blackholes and supermassive blackholes

Answer 2

It's not that easy to feed gas into the center of a galaxy, and the processes that do so tend to be sporadic.

As you point out, there are a lot of stars near the black hole in a galactic center. But all the stars that can fall into the hole (or, more specifically, into the region near the hole where they are tidally destroyed) have already done so. Even at the stellar densities found in the center of galaxies, ten million stars per cubic parsec, there are no significant two-body (star-star) gravitational interactions that would change a star's orbit enough to send it toward the black hole.

There is an irreducible, but small, flow of gas that comes from gas evolving off of stars that are near the center (within, say, 200 parsecs). This gas has low angular momentum, and will flow into the black hole accretion disk unimpeded. The flow rate from this process is, however, only a few tenths of a solar mass per year, not enough to make much difference to a multi-million solar mass black hole, and the conversion of a fraction of this matter into energy is not going to produce much power.

Gas from stars further out, a thousand parsecs or so from the center, will tend to get trapped in a ring around the hole. Basically, the gas can find itself on an orbit where the "frictional" losses due to shocks are small, and it will smooth out into a ring a few hundred parsecs in diameter, where it can happily remain in orbit for tens of millions of years. This ring should not be confused with the black hole accretion disk---it is a hundred million times larger. It is only when enough material accumulates in such a ring that it will become unstable, coagulate into a cloud, and fall into the black hole. Bang!

The activity of the black hole can also be stimulated by galaxy collisions. This destabilizes the orbits of stars and gas, and provides a lot more gas. Such events precipitate the most powerful periods of activity, feeding thousands of solar masses per year into the black hole accretion disk.

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Additional comments:

The idea that a galactic center is so crowded that the black hole cannot be starved is incorrect. First of all, the black hole is not that big. The diameter of a black hole is proportional to mass, and one solar mass corresponds to only 3 kilometeres. A three million solar mass black hole is therefore about 9 million kilometers. Compare this to the orbit of Mercury around the Sun, at 58 million kilometers. The tidal break-up distance for stars is about 1 astronomical unit. This is a small target, even with 10 million stars per cubic parsec. A cubic parsec, after all, is 8 x 10^15 cubic astronomical units, so each star gets 9 billion cubic astronomical unit to itself. Once the black hole forms, all the stars on orbits that intersect are already eaten, so there are no stars left with orbits that intersect. If a star's orbital energy and angular momentum (in the gravitational potential of the hole plus galaxy) forbid it from intersecting the break-up radius, it simply won't intersect until the energy and angular momentum of that star are signficantly changed, and that almost never happens---there is no mechanism to make it happen. Similarly, if the inner few hundred parsecs of a galaxy are empty of gas, there is nothing to feed the black hole and it will remain inactive until an instability or galaxy collision funnel new gas down the potential well.

This is manifestly possible, because the Milky Way definitely containts a 3 million solar mass black hole, and it is definitely inactive.

There is supposed to be a Nova TV show on this topic on Halloween.

Answer 3

Active black holes are that way when some event causes matter to fall into them. That's really common in young galaxies in the process of forming: lots of material still falling in toward the center. In older galaxies most stuff than could fall in already has. Orbits of stars and large gas clouds are fairly stable. So while matter certainly still falls into the supermassive black holes at the center of older galaxies, it doesn't do so in the large quantities it did when they were younger.

It could be that an "inactive" black hole could become active again if something starts feeding it again, say a collision between two galaxies.

Answer 4

Stops emmiting X rays (or emits minimal X rays) thus making it "quiet" or "dormant" or "inactive".

Answer 5

I have no idea. You should e-mail a science magazine or something. My guess is no one who puruses Yahoo Answers will know the answer to this one.

Answer 6

it is a non active black hole