Galaxy Merger (2009 November 9 APOD)

[neufer]

Interacting galaxies (Colliding galaxies)
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Satellite interacting with its primary:
http://en.wikipedia.org/wiki/Whirlpool_Galaxy
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galaxies going through the first phase in galactic collision
http://antwrp.gsfc.nasa.gov/apod/ap080420.html
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galaxies going through the second phase in galactic collision
http://antwrp.gsfc.nasa.gov/apod/ap090426.html
http://antwrp.gsfc.nasa.gov/apod/ap090911.html
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galaxies going through the third phase in galactic collision
http://antwrp.gsfc.nasa.gov/apod/ap061024.html
http://en.wikipedia.org/wiki/NGC_520
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<<Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centres of the original galaxy pair now merged into one nucleus, but stretching out from the centre are two tidal tails of young stars, a strong indicator that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails.

The prominent lower tail is richly populated with bright star clusters - 100 of them have been found in these observations. These star clusters may have formed as part of a loop of stretched material associated with the northern tail, or they may have formed from debris falling back onto the nucleus. In addition to this active star-forming region, both galactic arms harbour very young stars in the early stages of their evolutionary journey.>>

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http://en.wikipedia.org/wiki/Centaurus_A
http://antwrp.gsfc.nasa.gov/apod/ap060704.html

[GaryR]

Will the stars and other matter in the tidal tails be falling back into the merged galaxy or will they be dispersed into the intergalactic space?

Gary

[neufer]

Will the stars and other matter in the tidal tails be falling back into the merged galaxy or will they be dispersed into the intergalactic space?

My guess is that they are the result of a temporary null gravitational point that existed between the two galaxies before merger.
Now that the two galaxies have fully merged they will all fall back to the center.

[Chris Peterson]

Will the stars and other matter in the tidal tails be falling back into the merged galaxy or will they be dispersed into the intergalactic space?

Most of the material doesn't reach escape velocity, so it will remain part of one or the other galaxy (or the merged galaxy). Only a relatively small number of stars are actually ejected in collisions like this.

[john_crompton]

I have a supplementary question on this topic... If the universe is expanding as we believe, surely objects as large as galaxies are all so far apart and all moving away from each other at such a rate that a collision shouldn't happen???

Help me out here someone!

[neufer]

I have a supplementary question on this topic... If the universe is expanding as we believe, surely objects as large as galaxies are all so far apart and all moving away from each other at such a rate that a collision shouldn't happen???

Help me out here someone!

With the universe expanding as we believe, objects as large as galactic clusters are all so far apart and all moving away from each other at such a rate that a galactic cluster collision shouldn't happen. This situation hasn't happened with galaxies quite yet and astronomers even predict a Andromeda-Milky Way collision in about 3 billion years.

<<The Andromeda-Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 billion years' time between the two largest galaxies in the Local Group – the Andromeda Galaxy and the Milky Way which contains Earth.

It is often used as an example of the kind of phenomena associated with such collisions in simulations. As with all such collisions, it is unlikely that objects such as stars contained within each galaxy will actually collide, as galaxies are in fact very diffuse—the nearest star to the Sun is in fact almost thirty million solar diameters away from the Earth. (If the sun were scaled to the size of an American quarter, 24.26 mm (0.955 in), the next closest quarter/star would be 700 km [475 miles] away.) If the theory is correct, the stars and gas contained in Andromeda will be visible to a naked-eye viewer in approximately 2 billion years. If the collision occurs, the galaxies will likely merge into one larger galaxy.

There is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/s, there is no way to tell whether it is going to collide or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/s. This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. A future European Space Agency spacecraft, the Gaia mission, expected to launch in the Spring of 2012, is intended to measure the positions of stars in the Andromeda galaxy with sufficient precision to pin down the transverse velocity.

Such collisions are relatively common however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past.>>

[Chris Peterson]

I have a supplementary question on this topic... If the universe is expanding as we believe, surely objects as large as galaxies are all so far apart and all moving away from each other at such a rate that a collision shouldn't happen???

Many galaxies are components of galaxy clusters, where the strength of gravity easily keeps the galaxies in orbit around each other against the much weaker "force" of the expansion of space. In other words, within clusters, galaxies are not moving away from each other (except transiently), and collisions are fairly common.

[jerbil]

My opinion, which might not be worth much, is that galactic clusters seem to be gravitationally coherent, so that almost all objects of, say, stellar size are confined therein (barring the odd sling shot event.)

The presumed existence of dark matter and/or dark energy inevitably complicates our discussion.

However, I agree with Chris Peterson that the size of even the largest galactic cluster so far detected is too small for the general expansion of the Universe to be relevant.