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Stonecut
2007-Jul-17, 11:33 AM
According to the Chandrasekhar limit, a star that is 8+ solar masses cannot shed enough mass and will eventually form a neutron star or blackhole.

Since we are confident that there are Super massive Black holes at the center of galaxies, I have a question. What is the largest known diameter of a galaxy? Is the maximum galaxy size an indication of the maximum black hole size at its center? (I understand there is a complication in that outer regions of a galaxy are mysteriously held together by a proposed dark matter.)

My question is: "Is there a limit to the size of a black hole.. maybe 10^10 solar masses before it is unable to remain in this universe?:

Kullat Nunu
2007-Jul-17, 12:05 PM
What is the largest known diameter of a galaxy?

Largest known galaxies are so-called giant diffuse galaxies (dE), which are located in centers of large galaxy clusters. They are, well, diffuse and it is hard to say exactly how large they are, but they are about the size of the Local Group (2-3 million ly).


Is the maximum galaxy size an indication of the maximum black hole size at its center? (I understand there is a complication in that outer regions of a galaxy are mysteriously held together by a proposed dark matter.)

The size of a central black hole depends on the mass of central bulge of the galaxy (in the case of ellipticals the whole galaxy is a bulge), but I have no idea how large black holes dEs can have.


My question is: "Is there a limit to the size of a black hole.. maybe 10^10 solar masses before it is unable to remain in this universe?:

Most likely the only upper limit of a galaxy or black hole mass is set by available time and material. The universe is still relatively young, and gigantic galaxies and black holes are still growing.

Certainly a 10 billion solar mass black hole could exist. IIRC the giant elliptical galaxy Messier 87 has a 2 billion solar mass black hole. There are certainly larger black holes elsewhere, given that M87 and the Virgo Cluster are rather small compared to really gigantic clusters of galaxies.

antoniseb
2007-Jul-17, 12:37 PM
My question is: "Is there a limit to the size of a black hole.. maybe 10^10 solar masses before it is unable to remain in this universe?:

We get this question every so often. Paraphrasing, you are not asking about mechanisms that result in a black hole, but rather whether there is some limit where a black hole will cease to exist in our universe if it gets too massive.

The answer is that general relativity gives no upper limit. If all matter in the universe except the two of us in our spacesuits were put into a giant black hole, that black hole would still be in our universe.

You can speculate that maybe this would be a Heisenberg foam issue, and maybe in the realm below quantum physics in M theory, and therefore not governed by GR entirely. If you want to speculate that way, you may, but it is not on ground understood by physics right now. Even then, I think you'd be hard pressed to find a way to make the mass of the black hole go away.

hhEb09'1
2007-Jul-17, 12:50 PM
According to the Chandrasekhar limit, a star that is 8+ solar masses cannot shed enough mass and will eventually form a neutron star or blackhole. Just a nit. The Chandrasekhar limit is 1.4 solar masses, but there is a 8 solor mass limit as you say.

Kullat Nunu
2007-Jul-17, 02:19 PM
Just a nit. The Chandrasekhar limit is 1.4 solar masses, but there is a 8 solor mass limit as you say.

And a clarification: Chandrasekhar limit is for white dwarfs. Some white dwarfs are located in close binaries where the dwarf accretes material from its companion. If its mass crosses the Chandrasekhar limit, it explodes as a type Ia supernova.

ngc3314
2007-Jul-17, 02:50 PM
Largest known galaxies are so-called giant diffuse galaxies (dE), which are located in centers of large galaxy clusters. They are, well, diffuse and it is hard to say exactly how large they are, but they are about the size of the Local Group (2-3 million ly).



The size of a central black hole depends on the mass of central bulge of the galaxy (in the case of ellipticals the whole galaxy is a bulge), but I have no idea how large black holes dEs can have.



Most likely the only upper limit of a galaxy or black hole mass is set by available time and material. The universe is still relatively young, and gigantic galaxies and black holes are still growing.
Certainly a 10 billion solar mass black hole could exist. IIRC the giant elliptical galaxy Messier 87 has a 2 billion solar mass black hole. There are certainly larger black holes elsewhere, given that M87 and the Virgo Cluster are rather small compared to really gigantic clusters of galaxies.

Nit - these are known as cD galaxies (dE is a somewhat informal abbreviation for dwarf ellipticals). This name is almost the only remaining wide useage from W.W. Morgan's Yerkes galaxy classification system (http://www.astr.ua.edu/keel/galaxies/classify.html) (the c is patterned after the traditional nomenclature for spectroscopic sharpness of lines from supergiant stars).

There are clearly central black holes up to about 5 billion solar masses. We don't know much about how high this goes - a Hubble program targeting stellar motions in the cores of the most luminous local galaxies had been approved when an electronics failure rendered STIS (the spectrograph most effective at such work) inoperative. Since the correlations with central black-hole mass are either with total (bulge) luminosity or stellar velocity dispersion, the most massive ones would be expected in the most luminous galaxies, which are not always the ones we can trace to the greatest radius at some given surface brightness level. Above a characteristic luminosity, the space density of galaxies undergoes an exponential falloff, so there are very few galaxies luminous enough that we would expect the usual scaling relation to give more than 10 billion solar masses. (Oh, is that all???)

There are pretty good reasons for the growth of a black hole to virtually stop once it reaches 0.5-1.0% of the mass of the surrounding stars. At that mass, it dominates a large region gravitationally and changes the typical orbits of stars, reducing the number of stars on "death orbits" which will eventually come close enbough to be tidally shredded and on some of whose gas the black hole can feed. To my mind, the more interesting puzzle lies in how they managed to grow so fast in the early Universe. It makes a big difference if one could start with thousands of solar masses rather than a few.

Stonecut
2007-Jul-17, 03:10 PM
Paraphrasing, you are not asking about mechanisms that result in a black hole, but rather whether there is some limit where a black hole will cease to exist in our universe if it gets too massive.


Exactly Anton.
__

It is interesting to me that nature tends to set limits (other than the universe itself?) It sounds like there is no limit to size of a black hole other than the amount of matter it can chomp on.

Thanks for the input. Bautforum is great place to ask wise, interesting and knowledgeable people questions about abstract ideas.

Cougar
2007-Jul-17, 05:22 PM
Well, obviously merging galaxies are going to create ever-larger SMBHs. But the frequency of such mergers is not as great as in the past, so the simple passage of time would seem to be a constraint.

Absent merger activity, there is apparently a feedback mechanism at work that cuts off significant further accretion, and there is a lot of research looking into this. See, for example, this pdf article (http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0411108), which rather flatly states:



Once an accreting supermassive black hole has grown to a critical size, feedback terminates its further growth, and expels gas from the central region in a powerful quasar-driven wind.

Cougar
2007-Jul-17, 05:24 PM
My question is: "Is there a limit to the size of a black hole.. maybe 10^10 solar masses before it is unable to remain in this universe?
Where is it going to go? :think:

Kullat Nunu
2007-Jul-17, 05:41 PM
Nit - these are known as cD galaxies (dE is a somewhat informal abbreviation for dwarf ellipticals). This name is almost the only remaining wide useage from W.W. Morgan's Yerkes galaxy classification system (http://www.astr.ua.edu/keel/galaxies/classify.html) (the c is patterned after the traditional nomenclature for spectroscopic sharpness of lines from supergiant stars).

Argh, this is what you get when you resort only on your memory and don't check your facts...:doh:

Stonecut
2007-Jul-17, 08:42 PM
Where is it going to go? :think:


Good question Cougar. My thought is that the black hole would begin to dissipate in a more rapid fashion..maybe the dissipation would be some other primordial particles...or a form of dark matter...or into the multiverse...grrr

Ever since I read up on fractals and the repeat of tree branches similarity to a leopard spots... I look for commonalities in all of nature.
:rolleyes:

COUGAR, thank you for that article. This is "feedback" concept is very interesting. I am going off to a quiet corner to read this article.

Nicias
2007-Jul-18, 01:56 PM
There is no upper limit. As a black hole gets larger the gravitational gradient at the event horizon gets smaller, this means that all it has less Hawking radiation, and that things fall into it more easily, (also due to larger surface area.) Thus it decays more slowly and also has less trouble forming a large accretion disk.

sirius0
2007-Jul-18, 09:58 PM
Less Hawking radiation per unit area perhaps, but there will be more area. I don't know enough yet to put this mathematically. I mean does it have more Hawking overall, the same, or less?

antoniseb
2007-Jul-18, 10:01 PM
Less Hawking radiation per unit area perhaps, but there will be more area. I don't know enough yet to put this mathematically. I mean does it have more Hawking overall, the same, or less?
More Hawking Radiation, but also a much longer period of time before the Hawking Radiation will have depleted the mass of the object.

Nicias
2007-Jul-19, 12:34 PM
The temperature of a black hole (in terms of its Hawking radiation) goes as 1/M. The rate of energy release of a black body goes as A*T^4. The area of a black hole goes as M^2. Thus the rate of energy release goes as M^(-2). Hence, less energy per unit area, and less energy overall.