why are galaxies around the same size?

[Martin]

If i remember, most galaxies are within a few orders of magnitude of ours in size

 

a lot of galaxies (most I think) are greater than 1/100 of Milkyway mass and less than 100 times Milkyway mass

 

Why should that be?

 

http://arxiv.org/abs/astro-ph/0407238

 

There also seems to be a tendency for stars to be between 1/10 and 100 solar masses.

 

I dont know of any stars bigger than 100 solar masses.

 

Why are stars that uniform in mass?

 

I know there are some simple answers in the case of stars. Too small and fusion wont start and it wont shine. too big and its own starlight blows it apart. But what about galaxies?

 

Has anyone thought through why galaxies are so similar in size?

I'm not talking globular clusters----they are globular clusters

 

why arent there lots of little spiral galaxies with only a million stars apiece

why arent there lots of galaxies a million times more massive than ours?

 

James Binney an astronomy professor at Oxford has some ideas, in that link I gave just now, maybe some other people do too. I dont. the question just occurred to me to wonder about.

[jordan]

I would've guessed it had to do with how the matter was distributed immeadiatly following the big band. Relatively equal distrabution would result in galaxies of relatively equal size.

[Martin]

I would've guessed it had to do with how the matter was distributed immeadiatly following the big band. Relatively equal distrabution would result in galaxies of relatively equal size.

 

you may be on to something

one could say "what's to explain?" but then one could say that about star sizes too

(and I think with star masses the feeling is that one ought to try to explain why they fall in a fairly narrow range, a few orders of magnitude, instead of varying all over the place)

 

have to think about it

 

 

massive ellipticals form by merger of spirals, what one wants to know is why spirals are pretty much the same size---it might be as you say.

maybe someone here knows

[jordan]

It was just a quick thought. I am really in no way qualified to answer the question but I thought I remember reading about it somewhere. Let me know what you come up with.

[JaKiri]

There will probably be a maximum stable size for a galaxy, I'll see if there's anyone I know left at the Uni Physics dept down the road, they're interested in this kind o'thang.

[Martin]

neat to have a university physics department nearby

 

I'm just speculating but there might be a maximum size for spirals

 

and if so, probably someone has tried to use it as one of the

"standard candles" they use to check the distance scale

 

a physical limit on the size for ellipticals would be less likely, I imagine, because

astronomers see very massive elliptical-type galaxies formed by having

smaller (typically spiral) ones collide and merge----superlarge ellipticals might be very rare because collisions are, but if you waited long enough you might see the big one in, say, Virgo Cluster gobble up all the others and be huge. horrible thought

 

a maximum for all types might be too much to hope for (again I'm speculating) but a maximum for spirals might actually exist and be extremely useful as a check on the distance scale

 

I should stop guessing and wait to hear some expert input. really nice that you have a neighborhood Uni

[JaKiri]

Alas, expert input will be lacking. All of the people I knew (mostly postgrads) have moved on.

 

University of Durham, if you're wondering.