WinAce

<a href="http://spacedaily.com/news/020913182837.iqkrjqti.html" target="_blank">Black holes in galaxy clusters call evolution theories into question - SpaceDaily</a>

The booth for placing bets on how long it takes AiG to misrepresent the article, given the title, is now open.

Doubting Didymus

Stellar evolution again.

It is a huge misrepresentaion to claim that stellar evolution has anything to do with biological evolution in the first place. (though I am aware that it is only creationists who claim this in the first place.)

atrahasis

May beS&S

Jesus Tap-Dancin' Christ

I read that and became thoroughly confused. Black holes are super-dense areas of matter that have such extreme gravitational forces that light cannot escape from them. All matter within a certain radius from its center will spiral in there, and after that, it will have to have a certain velocity to orbit or escape. Gravity doesn't "turn on" or "turn off" so what the statement about them suddenly re-starting made absolutely no sense to me.

Mageth

I'm no astrophysicist, but about a black hole "turning on" and "turning off":

I think what they mean by a black hole at the center of a galaxy cluster "turning off" is really that it sweeps clean all matter within its "grasp" so to speak, and so it sits "dormant" at the center of an empty space. IIRC this happens fairly early in the "evolution" of a galaxy.

To "turn on," something happens that causes the black hole to start pulling in matter again.

And didn't Hawking or someone discover that black holes "leak" a bit of matter due to some quantum effect? So a "dormant" black hole may actually get "smaller", so to speak?

[wow, I just reread that; someone call the quotation mark police!]

[ September 18, 2002: Message edited by: Mageth ]</p>

Jesus Tap-Dancin' Christ

That "something" would be simply matter getting within it's graviational field.

I'm heading off to class in ten minutes, and while I'm there, I think I'll take a copy of the article to one of my profs and see what he has to say about that.

Mageth

That "something" would be simply matter getting within it's graviational field.

IMO, the "something" would be the something that causes matter to get within its gravitational field, perhaps a reduction of an opposing force (gravitational, centrifugal...?) pulling matter near the center of the galaxy outward.

[ September 18, 2002: Message edited by: Mageth ]</p>

Skydancer

It's not a matter of a 'reduction' in some force. It's just a matter of new material entering the center of a previously depleted galactic core. Most likely cause is the aftereffects of a galactic collision or near-miss on the gas clouds.

Tim Thompson

Don't let Spacedaily get you too worked up. While the results reported are interesting, they hardly merit any great worry over theories of galaxy & galaxy cluster evolution being somehow fundamentally flawed.

The real science is found in "<a href="http://cul.arXiv.org/abs/astro-ph/0208017" target="_blank">An Unexpectedly High AGN Fraction in Red Cluster Galaxies</a>", Paul Martini et al., Astrophysical Journal Letters 576(2): L109-L112, September 10, 2002.

The "red" cluster galaxies are "red" because they have a low star formation rate. If they had a high star formation rate, they would be "blue" (the colors are relative). Active Galactic Nuclei (AGN) have commonly been associated with "blue" galaxies, where active star formation is going on. This paper reports that they appear unexpectedly in "red" galaxies.

AGN are characterized by X-ray emission from matter falling into the large central black hole. "Red" galaxies are short of interstellar gas, and so short of material to do the falling. So one would not expect to see one there.

But, maybe the galaxies are not doing a lot of star forming for some other reason, and not due to a shortage of gas. "Falling" through the cluster center could trigger star formation. That would wake up an AGN, which would do its thing for maybe 100,000,000 years or so, and then shut off, permanently (the comments about "on" & "off" refer to the X-ray emission). But if the galaxy has never been through the deep part of the cluster before, then it's "red" not because its gas has been used up, but because star formation hasn't been provoked yet.

So the authors conclude that the "old" clusters are actually more dynamic than previously thought, and that galaxies can last longer in a cluster environment before they do that "starburst" thing.

That's about all there is to it.

Mageth: And didn't Hawking or someone discover that black holes "leak" a bit of matter due to some quantum effect? So a "dormant" black hole may actually get "smaller", so to speak?

You are thinking of <a href="http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html" target="_blank">Hawking Radiation</a>, but it's a pretty puny effect. A supermassive galactic black hole will weigh in at something like 10^6 to 10^8 solar masses, which means it would "shine" with an equivalent temperature of about 10^-14 to 10^-16 Kelvins, and last for 10^82 to 10^88 years. If it sucks up one stray milligram of space stuff, that would cancel out any Hawking radiation already produced, and still set the clock back.

KeithHarwood

The normal perturbations as stars pass eaach other in their orbits around the centre is enough to send a few stars in towards the centre and, conversely, out towards the rim.