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ToSeek
2004-Sep-08, 04:22 PM
Spitzer Arrives at Scene of Galactic Collision (http://www.spitzer.caltech.edu/Media/releases/ssc2004-14/release.shtml)


NASA's Spitzer Space Telescope has set its infrared sight on a major galactic collision and witnessed not death, but a teeming nest of life.

Normandy6644
2004-Sep-08, 06:41 PM
Yeah, this seems to be the current theory in galactic collisions. For some reason (that I don't know about for sure, Harvestar where are you on this one? :D ), colliding galaxies give rise to new areas of star formation. They even seem to form in the tails of the galaxies as they get stretched out away from the main concentration of stars. It's cool that Spitzer is looking at this kind of thing.

ngc3314
2004-Sep-08, 07:26 PM
Yeah, this seems to be the current theory in galactic collisions. For some reason (that I don't know about for sure, Harvestar where are you on this one? :D ), colliding galaxies give rise to new areas of star formation. They even seem to form in the tails of the galaxies as they get stretched out away from the main concentration of stars. It's cool that Spitzer is looking at this kind of thing.

Nobody knows for sure. There are some assorted plausible ideas for what might trigger the star formation, of which more than one probably works. Collisions between gas clouds (but you see the enhancement even in galaxies where the gaseous disks have yet to contact each other), pressure changes in the hot interstellar medium collapsing denser clouds, changes in the dynamics making interstellar clouds that used to be stable (but just barely) go over the edge... It is very interesting that we see star-forming regions in gas that has left the galaxies a long time back, so the massive stars have to have formed out there (meaning the clouds stayed together gravitationally without dissipating). Some such clouds are apparently gravitationally bound and may stay together to become "tidal dwarf" galaxies, which have been a cottage industry for some groups.

While the mechanism is unclear, the evidence is unmistakeable that galaxy encounters provoke star formation. I have some class notes summarizing the evidence at a wide range of wavelengths at http://www.astr.ua.edu/keel/galaxies/mergers.html which I'll just point to and avoid retyping or cut/pasting.

IRAS showed that far-IR is the place to look to find these star-forming regions wholesale, and Spitzer is turning up wonders here as in many other areas.

Spaceman Spiff
2004-Sep-08, 07:54 PM
Yeah, this seems to be the current theory in galactic collisions. For some reason (that I don't know about for sure, Harvestar where are you on this one? :D ), colliding galaxies give rise to new areas of star formation. They even seem to form in the tails of the galaxies as they get stretched out away from the main concentration of stars. It's cool that Spitzer is looking at this kind of thing.

These two galaxies were spirals before they collided, and I would guess Sc or Sbc types (maybe barred?), judging from their bulges. Spirals and late Hubble type spiral especially have significant amounts of gas and a good fraction of it is in the form of giant molecular clouds. All they need is a nudge from the outside to become gravitationally unstable to begin producing stars. Normally, only a small portion of the cloud undergoes star formation because the typical nudges are relatively localized. However, colliding GMCs in head-on colliding spiral galaxies are quite another matter and the result is obvious (even if the detailed mechanisms are not, as per the post above) if not stupendous.

Only galaxies with significant supplies of cold gas can do what you see here, depending upon the nature of the collision between the two galaxies. Two colliding ellipticals or S0 galaxies would not result in major bursts of star formation, such as you see here.

Harvestar
2004-Sep-08, 10:12 PM
Thanks for the head's up, Normandy! :)

It's a great picture! I just called the people in my office over and we were ogling the image. The discussion was over the fact that the Antennae is often pointed to as the quintessential example of how most of the star formation is obscured and we're underestimating star formation elsewhere. The flip side is that the Antennae seems to be different from other places (or at least one of a small class of objects). So that's currently an ongoing debate.

And as the press release said, it's not surprising that they observed this (it's been seen before in NICMOS observations and in groundbased IR observations), but the detail at these longer wavelengths gives more insight into the amount of star formation going on in the obscured areas.

I'm also interested in the few "star cluster-like" objects in the outer regions of the IRAC image, though they may just be foreground stars. I'll have to look at them more closely.

And yes, I'm going to agree with my collegues above. :) Though I will note that NGC 3921 may show what happens when an S0 and Sb (or so, a gas poor and a gas rich) collide.
http://www.cv.nrao.edu/~jhibbard/n3921/n3921.html