Correlation between star forming regions and their position within a galaxy

[symesie04]

So here i am starring at this amazing image of M106 when it dawns on me that the star forming regions appear to be in the same area of opposing arms within the galaxy. I wondered therefore if there is a realtionship between star forming regions and their position within their host galaxy. Could it simply be another example of the "goldie locks" scenario? Any closer to the centre and the required material is either used up/blown away or to buffeted to be able to condense as required. To far out and there either is simply not enough of the required material, or it does not receive the gravitational boost required from passing stars and nova event shock waves to aid the collapse of material. I may of answered my own question here but more likely not. So can anyone confirm or shed light on my thoughts please. I have attached image for viewing.

[JulianO]

For an area to be star forming it needs access to lots of gas to condense. In the middle most of the gas is used up, blown out by black hole eruptions and other factors.

It's also easier to get gas from outlying regions into the spiral arms rather than all the way into the middle, so its sort of a natural consequence. There are some oddities - I know definitely some S0s - where star formation has happened more recently in the bulge rather than the disk, but it gets a little more complicated to work out how that can happen, and usually it's put down to either mergers or some other form of harassment.

[symesie04]

Interesting Julian. So out of curiosity if SO's mark the boundary between Elliptical and Spiral galaxies is it therefore assumed that they are a result of merger between the two?, or is it thought that given time Elliptical will naturally take the form of Spirals? Also just as a side thought regarding classification, how do they classify edge on galaxies? I assume they would be able to decide if Elliptical or Spiral from that view but how can they decide SBc or SBb etc etc?

[JulianO]

I think the general view is that spirals degenerate into ellipticals through mergers and other processes, and that S0's are probably spirals that have had their gas supply cut off - maybe not paying the bills!

Edge on galaxies are tricky, although you can do some clever stuff with light profiles and fitting to known galaxy light profiles. 

[symesie04]

Really? so entropy is marked out with the direction of time as flowing from tight spirals to loose elliptical? I had never figured the arrow to run in that direction for galaxy structure.

[JulianO]

In spirals the stars are all going around in the same direction and plane (mostly). In ellipticals they are every which way.

Here is a simulation of us & andromeda

http://www.youtube.com/watch?v=gmVkmsACgro

The results looks pretty much like an elliptical.

[symesie04]

So given that then all Ellipticals would appear to be the result of spiral mergers? or at least a merger of large galaxies. Wouldnt given enough time the resulting Elliptical reform a spiral shape?

[JulianO]

There are some suggested processes that may make ellipticals have stars settle out into bands, but its basically like mixing ink and water - you'd need a powerful restoring force to get things back into any sort of disk again. So probably not going to happen.

[symesie04]

O.k im obviously missing something here or just a little dumb  But in that case how do spirals form in the first place? Since the direct relationship between black holes and galaxy formation is still unclear (i think) then what drives that particular formation. I assume all spirals must form from the process of stars forming clusters which then attract together to eventually make galaxies whether around a black hole or not. If so what cause the spiral formation? what is present that creates these shapes that wouldn't be able to recreate it with time after a merger?

[JulianO]

When galaxies form, they form, much like stars, from collapsing clouds of matter. These structures naturally tend to collapse in one dimension preferentially. 

The matter usually rotates, and by conservation of momentum you tend to end up with a disk like structure as stuff gets whirled around. The stuff in the plane of the galaxy stays there because it has angular momentum (centripetal force) countering the gravity, whereas gas outside the plane has much less so tends to collapse into the plane.

The black hole relationship isn't particularly required for galaxies to form as far as I understand currently, its more just something that people have noticed. It probably hints at a deeper understanding we are missing, but we don't need them to explain galaxy formation. It may just be a consequence of the way they form.

The same happens with a star, from a gas cloud you tend to get a disk like thing, with the sun and all planets in the same plane.

However mix two of these stable formations such as one _ and one | then you'll get all sorts of interactions, mixing up the stars orbits. Once they are mixed, its difficult to unmix them. You would need to transfer angular momentum not from thin, sticky gas clouds, but discrete massive stars.

Some galaxies form in more complex environments - at dark matter knots, and in this case you tend to get a much more mixed environment to begin with, with material flowing in from lots of directions, so the spirals don't form.

[symesie04]

O.k the above all makes good sense. However when ever we peek back to the youngest galaxies we seem to see messy blobs presumably where stars are just forming and the gas hasn't yet flattened out. Now i know your probably saying well of course they are just forming but in the model in my head (286 4kb ram) the gas would of flattened before the stars starting forming wouldn't it? Isnt the flattening out a part of the collapse that leads to star formation?

Also i believe it is thought that our own milky way has consumed other galaxies if so then why is it still a spiral? were the other galaxies to small to affect overall structure?

Also if ellipticals are a mark of cannibalism whereby the hallmarks are visible for eternity could they be used in helping to age the universe? By looking at how many galaxies exist, how many are currently in collision and how many elliptical exist could this help with ageing the Universe?

If to many questions just ignore

[SnakeyJ]

Great question and responses here - the current jodcast has quite an interesting item on how supermassive black holes effect star forming during the evolution of a galaxy.   Well worth a listen http://www.jodcast.net/archive/201310/

[symesie04]

Thanks Jake, ill have a listen when i get home.

[JulianO]

O.k the above all makes good sense. However when ever we peek back to the youngest galaxies we seem to see messy blobs presumably where stars are just forming and the gas hasn't yet flattened out. Now i know your probably saying well of course they are just forming but in the model in my head (286 4kb ram) the gas would of flattened before the stars starting forming wouldn't it? Isnt the flattening out a part of the collapse that leads to star formation?

There is a lot of complication - that's why. It takes a long while for things to settle down. The gas needs to cool to form stars in a galaxy, but the act of contraction heats it up. Therefore you have to wait for things to settle down again. Also early galaxies are full of all sorts of odd things. Active black holes - making them into quasars, population III stars which are probably very large and hot. In the jargon it takes time for them to virialise.

Plus they are so far away we get precious little light from them, so the images are very raggedy and difficult to resolve.

Also i believe it is thought that our own milky way has consumed other galaxies if so then why is it still a spiral? were the other galaxies to small to affect overall structure?

Yes- mostly dwarf galaxies and globular clusters. Their passage can be tracked though - there are numerous streams evident in the Milky Way where these galaxies have ploughed through. These are used to help determine the shape of the Milky Way.

Also the stars in the bulge are more like in an elliptical galaxy, moving in irregular directions - probably because of the disturbances of these interlopers.

Also if ellipticals are a mark of cannibalism whereby the hallmarks are visible for eternity could they be used in helping to age the universe? By looking at how many galaxies exist, how many are currently in collision and how many elliptical exist could this help with ageing the Universe?

Believe me, they've thought of every possible way of dating galaxies! Including this one. The peak merger rate is (from memory) around redshift of 2 to 1, so about 6+ billion years ago. There are ways of tracking mergers by looking for telltale stars. Mergers churn things up and give a burst of star formation - so looking at the types of stars still present can be used to determine when this might have happened.

Galaxies are used to date the age of the universe, but more commonly by looking at the distribution of them, which gives clues to the big bang in a similar way to the CMB.