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Old 04-22-2012, 11:02 AM
 
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Quote:
Originally Posted by Brian.Pearson View Post
I just hypothesized that some could've shifted from one galaxy to another during the last collision. Same thing could've happened the previous time.
I don't know if the Milky Way shows any signs of any collisions, although there may have been early on. The Milky Way is thought to be nearly as old as the universe itself. The Andromeda galaxy, on the other hand, does show signs of past collisions, including one (as I recall) being a near bulls eye.

Dark Matter has been thought to play a major role throughout the interior of our galaxy. Evidently that's not the case. One explanation is that there's enough dust and gas providing gravitational attraction in the galaxy that Dark Matter isn't as essential as had been thought. A larger survey of stars in our galaxy should (hopefully) give a better idea about the distribution of Dark Matter in the galaxy. That too could provide some interesting surprises.
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Old 04-23-2012, 10:22 AM
 
Location: Wasilla, Alaska
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Quote:
Originally Posted by NightBazaar View Post
How would that explain the lack of Dark Matter?
Keep in mind that it is the gravitational effects they are looking for within a 13,000 light year radius of Sol. Who said Dark Matter had to be evenly distributed throughout the Milky Way? The matter that we see certainly is not evenly distributed. Therefore, I suspect that it is possible that Dark Matter clumps may have formed, with sufficient gravity to effect the rotation of galaxies, but also with Dark Matter voids. Considering the size of the Milky Way galaxy, a 13,000 light year radius sphere is not that big.

One thing is perfectly clear, the stars on the outer edge of the Milky Way (and other galaxies) cannot rotate as fast as we have observed without additional gravity being present. Something, that we have not yet detected, is exerting enough gravitational force to keep stars rotating within their galaxy.

I will agree with their other conclusion. If they are not detecting the gravitational presence of Dark Matter within a 13,000 light year radius of Sol, then it is highly unlikely that Earth-based sensors will be able to detect Dark Matter.
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Old 04-23-2012, 10:39 AM
 
Location: Wasilla, Alaska
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Quote:
Originally Posted by NightBazaar View Post
I don't know if the Milky Way shows any signs of any collisions, although there may have been early on. The Milky Way is thought to be nearly as old as the universe itself. The Andromeda galaxy, on the other hand, does show signs of past collisions, including one (as I recall) being a near bulls eye.

Dark Matter has been thought to play a major role throughout the interior of our galaxy. Evidently that's not the case. One explanation is that there's enough dust and gas providing gravitational attraction in the galaxy that Dark Matter isn't as essential as had been thought. A larger survey of stars in our galaxy should (hopefully) give a better idea about the distribution of Dark Matter in the galaxy. That too could provide some interesting surprises.
The Milky Way is currently colliding with three other galaxies. The biggest of the three is the Canis Major Dwarf galaxy. Since the Milky Way is considered a barred spiral galaxy, I would be willing to bet that the bar at the center of the galaxy is also the result of a prior collision with another galaxy.

It is obvious that Dark Matter does play a significant role, but I am not sure why it has to play a major role on the interior of galaxies. From what I understand, the matter that we see is at the center of Dark Matter formations. Which would put the bulk of the Dark Matter outside of galaxies.
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Old 04-23-2012, 11:01 AM
 
Location: Texas
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A bit like a peeling of an apple?
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Old 04-23-2012, 03:59 PM
 
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Quote:
Originally Posted by Glitch View Post
The Milky Way is currently colliding with three other galaxies. The biggest of the three is the Canis Major Dwarf galaxy. Since the Milky Way is considered a barred spiral galaxy, I would be willing to bet that the bar at the center of the galaxy is also the result of a prior collision with another galaxy.
Collision? Or could they they be gradually absorbed into the Milky Way by gravitational attraction. It depends on where they are in relation to the galaxy. If they're near the fringe of the disk, they'd most likely be absorbed.

I'm not sure that the Milky Way's bar is the result of a collision. The reason I say that is because it's thought the Andromeda galaxy, which is a spiral galaxy, was a barred galaxy until a collision with Triangulum galaxy. Ir tht's true, then it's pretty unlikely the Milky Way's bar is the result of a collision. I'd be willing to bet that early on in the Milky Way's history, the bar may be the result of merged black holes. Regardless, when the Andromeda galaxy and the Milky Way collide some 4.5 billion or so years from now, it's going to be pretty messy for quite a while.

Canis Major is believed to being pulled apart by tidal forces of the Milky Way's enormous gravitational field. It has a long trail of stars behind it as it orbits the Milky Way. That suggests Canis Major is being absorbed rather than a collision.
Canis Major Dwarf Galaxy - Wikipedia, the free encyclopedia


Quote:
It is obvious that Dark Matter does play a significant role, but I am not sure why it has to play a major role on the interior of galaxies. From what I understand, the matter that we see is at the center of Dark Matter formations. Which would put the bulk of the Dark Matter outside of galaxies.
I have the same view that Dark Matter might well reside outside of the galaxy rather than inside. However, I still think it's going to require a much larger survey of perhaps millions of stars before drawing a conclusion.
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Old 04-23-2012, 04:45 PM
 
Location: Wasilla, Alaska
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Quote:
Originally Posted by NightBazaar View Post
Collision? Or could they they be gradually absorbed into the Milky Way by gravitational attraction. It depends on where they are in relation to the galaxy. If they're near the fringe of the disk, they'd most likely be absorbed.
Not all collisions are catastrophic. This NASA image below shows the Canis Major Dwarf being tidally ripped apart with each successive pass through the Milky Way.



Source: APOD: 2007 November 4 - The Closest Galaxy: Canis Major Dwarf

Quote:
Originally Posted by NightBazaar View Post
I'm not sure that the Milky Way's bar is the result of a collision. The reason I say that is because it's thought the Andromeda galaxy, which is a spiral galaxy, was a barred galaxy until a collision with Triangulum galaxy. Ir tht's true, then it's pretty unlikely the Milky Way's bar is the result of a collision. I'd be willing to bet that early on in the Milky Way's history, the bar may be the result of merged black holes. Regardless, when the Andromeda galaxy and the Milky Way collide some 4.5 billion or so years from now, it's going to be pretty messy for quite a while.
The reason why I think barred galaxies are the result of a collision is because if there was just one super-massive black hole at the center of a galaxy it would not have a bar. The bar must be the result of two super-massive black holes that have not yet combined into a single super-massive black hole.

Quote:
Originally Posted by NightBazaar View Post
Canis Major is believed to being pulled apart by tidal forces of the Milky Way's enormous gravitational field. It has a long trail of stars behind it as it orbits the Milky Way. That suggests Canis Major is being absorbed rather than a collision.
Canis Major Dwarf Galaxy - Wikipedia, the free encyclopedia
Canis Major Dwarf is certainly being ripped apart by the gravitational forces of the Milky Way, but if that is not the result of a collision then we have a problem explaining why the clusters orbiting the fringe of the Milky Way have also not been tidally ripped apart like the Canis Major Dwarf.

Below is a nice simulation of what a collision between the Milky Way and Andromeda may look like:


Milky Way and Andromeda Galaxy collision simulation - YouTube

Very messy indeed.
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Old 04-23-2012, 08:42 PM
 
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Quote:
Originally Posted by Glitch View Post
Not all collisions are catastrophic. This NASA image below shows the Canis Major Dwarf being tidally ripped apart with each successive pass through the Milky Way.


Source: APOD: 2007 November 4 - The Closest Galaxy: Canis Major Dwarf
Thanks for the image and the link. If Canis Major is a dwarf galaxy that has made successive passes through the Milky Way, there should be signs of damage to Milky Way's structure, somewhat like holes or ripples similar to those of the Andromeda galaxy, although those may have resulted from a collision with the Triangulum galaxy. From the APOD image, it's hard to tell if CM has actually passed through the galaxy or just made near passes. The text appears to have stirred some controversy.

Did you click on the link on the page with the image? It looks like there's been some dispute over the galaxy designation of Canis Major, that it may be an interesting hypothesis rather than a confirmed object. I had to click my way through a number of reference links. The APOD explanation seems to be less than satisfactory. The whole issue is pretty confusing.
Thoughts on the Canis Major controversy | Galaxy Map

Starship Asterisk* • View topic - APOD: WISE Infrared Andromeda (2010 Feb 19)


Quote:
The reason why I think barred galaxies are the result of a collision is because if there was just one super-massive black hole at the center of a galaxy it would not have a bar. The bar must be the result of two super-massive black holes that have not yet combined into a single super-massive black hole.
The big question about supermassive black holes is whether they all began like that or what? More likely they became supermassive by pulling in loads of matter as well as mergers with other black holes. I agree that a pair of black holes can certainly dance together. Some may merge, and some (the less massive of the pair) can potentially be booted out.

I'm not so certain that a a bar is necessarily created by a pair of supermassive black holes though. The reason I say that is because M-31 (Andromeda) is a spiral galaxy but also has two supermassive black holes. I'm not sure why some galaxies are barred and some are spiral.
NASA's Chandra finds nearest pair of supermassive black holes


Quote:
Canis Major Dwarf is certainly being ripped apart by the gravitational forces of the Milky Way, but if that is not the result of a collision then we have a problem explaining why the clusters orbiting the fringe of the Milky Way have also not been tidally ripped apart like the Canis Major Dwarf.
If Csnis Major is a dwarf galaxy (and there seems to be some doubt about that), then the presumption is that Canis Major is closer to the galaxy, and not necessarily orbiting the edge of the galactic disk. If it actually punches through the galaxy, it would certainly lose much of its population of stars to the mass of the Milky Way. Of galaxies orbiting the edge of the disk, my guess is that they'd eventually be stretched, pulled apart and absorbed. It really depends on how close the dwarf galaxies are to the Milky Way, the direction and angle of their approach and how fast they're traveling. If the orbit is eccentric, they're likely to create some damage to the galactic structure if they happen pass through the galaxy.


Quote:
Below is a nice simulation of what a collision between the Milky Way and Andromeda may look like:


Milky Way and Andromeda Galaxy collision simulation - YouTube

Very messy indeed.
I've seen that simulation before. It's an impressive view. Good thing it's about 4.5 billion years in the future. LOL! I saw one that included the amount of time it would take from the collision to when it begins settling down. It's a very long period of time.
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Old 04-24-2012, 12:36 AM
 
Location: Wasilla, Alaska
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Quote:
Originally Posted by NightBazaar View Post
Thanks for the image and the link. If Canis Major is a dwarf galaxy that has made successive passes through the Milky Way, there should be signs of damage to Milky Way's structure, somewhat like holes or ripples similar to those of the Andromeda galaxy, although those may have resulted from a collision with the Triangulum galaxy. From the APOD image, it's hard to tell if CM has actually passed through the galaxy or just made near passes. The text appears to have stirred some controversy.
I agree. Even though the galaxy designation may be in dispute, and even if it is not in dispute, it is still only a fraction of the mass of the Milky Way. Nevertheless, there should be evidence of perturbation within the Milky Way, as you say.

Quote:
Originally Posted by NightBazaar View Post
Did you click on the link on the page with the image? It looks like there's been some dispute over the galaxy designation of Canis Major, that it may be an interesting hypothesis rather than a confirmed object. I had to click my way through a number of reference links. The APOD explanation seems to be less than satisfactory. The whole issue is pretty confusing.
Thoughts on the Canis Major controversy | Galaxy Map

Starship Asterisk* • View topic - APOD: WISE Infrared Andromeda (2010 Feb 19)
No, I did not follow the links when I posted the image, but I have now. I am content to leave it at the "Canis Major Dwarf Hypothesis." At least until the controversy is resolved one way or the other.

Quote:
Originally Posted by NightBazaar View Post
The big question about supermassive black holes is whether they all began like that or what? More likely they became supermassive by pulling in loads of matter as well as mergers with other black holes. I agree that a pair of black holes can certainly dance together. Some may merge, and some (the less massive of the pair) can potentially be booted out.
I agree that it is more likely that super massive black holes start out as ordinary stellar black holes, and then bulk up on the matter (and other smaller black holes) in their vicinity. I cannot conceive of another way to obtain the immense pressure required to form a black hole, but that could just be a lack of imagination on my part.

Quote:
Originally Posted by NightBazaar View Post
I'm not so certain that a a bar is necessarily created by a pair of supermassive black holes though. The reason I say that is because M-31 (Andromeda) is a spiral galaxy but also has two supermassive black holes. I'm not sure why some galaxies are barred and some are spiral.
NASA's Chandra finds nearest pair of supermassive black holes
I would suggest that the pair of super massive black holes in NGC 3393 have not formed a bar because of their proximity to each other and the difference in size. Although they do not mention the size of each of these super massive black holes in NGC 3393, they do describe it as a "minor" collision or merger and that these two super massive black holes are only 490 light years apart. If one super massive black hole is substantially smaller than another, it is possible for the larger super massive black hole to absorb the smaller without being disturbed. I suspect barred spiral galaxies are formed when two galaxies with super massive black holes of roughly equal mass collide or merge. Particularly since these two roughly equally massed black holes will be rotating around a common barycenter.

Quote:
Originally Posted by NightBazaar View Post
If Csnis Major is a dwarf galaxy (and there seems to be some doubt about that), then the presumption is that Canis Major is closer to the galaxy, and not necessarily orbiting the edge of the galactic disk. If it actually punches through the galaxy, it would certainly lose much of its population of stars to the mass of the Milky Way. Of galaxies orbiting the edge of the disk, my guess is that they'd eventually be stretched, pulled apart and absorbed. It really depends on how close the dwarf galaxies are to the Milky Way, the direction and angle of their approach and how fast they're traveling. If the orbit is eccentric, they're likely to create some damage to the galactic structure if they happen pass through the galaxy.
If Canis Major is not a dwarf galaxy, then the other dwarf galaxy that is currently colliding with the Milky Way is the Sagittarius Dwarf elliptical galaxy. It also is being ripped apart by tidal forces and being absorbed by the Milky Way.

Source: Sagittarius Dwarf Elliptical Galaxy / SagDEG

Quote:
Originally Posted by NightBazaar View Post
I've seen that simulation before. It's an impressive view. Good thing it's about 4.5 billion years in the future. LOL! I saw one that included the amount of time it would take from the collision to when it begins settling down. It's a very long period of time.
I am looking forward to the event.
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Old 04-24-2012, 01:30 AM
 
Location: Texas
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It would really be something if both Cansises and Andromeda all collided along with the Milky Way.
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Old 04-24-2012, 01:50 AM
 
Location: Wasilla, Alaska
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Originally Posted by Brian.Pearson View Post
It would really be something if both Cansises and Andromeda all collided along with the Milky Way.
It will happen, eventually.

There are roughly three dozen galaxies in our local group, and our local group of galaxies is part of the Virgo Cluster which consists of well over one thousand galaxies.

Eventually we will end up looking like the Shapley Super Cluster, which has the largest contentration of matter in the "nearby" universe. It is often referred to as being in our "local" universe, but that should not be confused with our local group of galaxies. The Shapley Super Cluster is 652 million light years away, whereas the Virgo Cluster is only 53 million light years away.

Source: http://www.eso.org/sci/publications/...o124-30-31.pdf
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