CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

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CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by bystander » Wed Feb 07, 2018 4:33 pm

Widespread Galactic Cannibalism in Stephan's Quintet Revealed by CFHT
Canada-France-Hawaii Telescope | 2018 Feb 06

An extremely deep multi-band optical image from the Canada-France-Hawaii Telescope (CFHT, Hawaii. USA) casts a new light on the formation process of the famous group of 5 colliding galaxies. The image reveals structures undetected thus far, in particular a very extended red halo composed of old stars, and centered on an elliptical galaxy, NGC 7317, which had been ignored in previous studies on the dynamics of the global collision. ...
[c][attachment=0]cfht_stephan_quintet.jpg[/attachment][/c][hr][/hr]
The wide field image captured with the 380 megapixel camera called MegaCam is focused on the nearby galaxy NGC 7331. The image exhibits several galactic and extragalactic features, some very extended and dim, including filaments of interstellar dust in the foreground (galactic cirrus). The scientists' attention was however captured by the condensation of galaxies in the field, much further beyond NGC 7331: the famous Stephan's Quintet named after the French astronomer Édouard Stephan who was the first to observe it in 1878. ...

Due to its unique features, Stephan's Quintet has been widely observed across the entire electromagnetic spectrum, and has been the subject of many complex numerical simulations. The team detected a red halo composed of old stars centered on a galaxy, NGC 7317. NGC 7317 was thought to be in a stable state or recently arrived near the group. The detection of red stars implies the contrary, that this galaxy has been interacting for a very long time with the other members of the group. Interactions such as the one seen in these observations are called galactic cannibalism. Galactic cannibalism occurs when the gravitational forces from a larger galaxy or group of galaxies slowly tear apart a smaller galaxy. Characteristic features of galactic cannibalism are streams or halos of stars orbiting the larger galaxy, like the halo of red stars seen around NGC 7317. A first implication is that Stephan's Quintet is far older than currently admitted. The models of formation and evolution of this emblematic system will have to be revised. This global case of galactic cannibalism should eventually lead to the formation of a giant elliptical galaxy. ...

Revisiting Stephan's Quintet with Deep Optical Images - Pierre-Alain Duc, Jean-Charles Cuillandre, Florent Renaud
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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by Ann » Thu Feb 08, 2018 12:39 am

That's a very interesting picture, and I can understand that the red halo around NGC 7317 is important, since it demonstrates that this seemingly "quiet" galaxy has been interacting with the other members of Stephan's Quintet for a long time. Therefore, Stephan's Quintet is also an older group than has been generally assumed.
Image
Stephan's Quintet. Photo: Bob Franke.
Stephan's Quintet. Photo: Hubble.
















What I find most interesting, however, is the blue-green star stream apparently emanating from NGC 7320, which is actually not a member of Stephan's Quintet at all, but a foreground dwarf galaxy which we accidentally see along the line of sight to the compact group of galaxies far behind it. The shape of the star stream that seems to emanate from NGC 7320 parallels the shape and direction of the even more prominent tidal tail from NGC 7319 "above" it. You can see both star streams, although faintly, in the picture by Bob Franke.

To me, the tidal tail apparently emanating from NGC 7320 makes it look as if NGC 7320 was a true member of the interacting group of galaxies. But that is not so, as is clear from the Hubble picture of Stephan's Quintet. Note how "grainy" the disk of NGC 7320 is, how faint and barely yellow its small bulge is, and how tiny its nucleus is. It is clear that these properties of NGC 7320 are quite different than the properties of the galaxies it appears to interact with, and that is only possible if NGC 7320 is smaller and more nearby than the other galaxies.

But still, that tidal tail that seems to emanate from NGC 7320 and which parallels the shape and size of the tidal tail of NGC 7319 really makes it look as if NGC 7320 a true member of Stephan's Quintet, although that is impossible. Can anyone explain?

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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by rstevenson » Sat Feb 10, 2018 3:41 pm

Ann wrote:...But still, that tidal tail that seems to emanate from NGC 7320 and which parallels the shape and size of the tidal tail of NGC 7319 really makes it look as if NGC 7320 a true member of Stephan's Quintet, although that is impossible. Can anyone explain?
I would assume that 7320 had a drive-by interaction with another galaxy that is not visible in the frame of these photos. I don't know how else it would get a tail that long. As for the direction of the tail paralleling that of 7319, coincidence, surely.

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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by Ann » Sat Feb 10, 2018 9:06 pm

rstevenson wrote:
Ann wrote:...But still, that tidal tail that seems to emanate from NGC 7320 and which parallels the shape and size of the tidal tail of NGC 7319 really makes it look as if NGC 7320 a true member of Stephan's Quintet, although that is impossible. Can anyone explain?
I would assume that 7320 had a drive-by interaction with another galaxy that is not visible in the frame of these photos. I don't know how else it would get a tail that long. As for the direction of the tail paralleling that of 7319, coincidence, surely.

Rob
Thanks for your reply, Rob! Yes, that sounds plausible. However... both tidal tails, one from NGC 7319 and one seemingly from NGC 7320, point at a small spiral galaxy which can be seen at far left in the picture from CFHT. The small spiral galaxy is NGC 7320C, and it is fairly generally assumed to be a true member of Stephan's Quintet. The tidal tail from NGC 7319 to NGC 7320C is seen as proof that these two galaxies have been interacting.

Yes, but what about the tidal tail that seems to emanate from NGC 7320? That tidal tail seems to point "even more directly" at NGC 7320C, which is particularly obvious when you look at the picture from CFHT. The idea that a true tidal tail from NGC 7320 would just happen to point straight at a background galaxy that is truly interacting with Stephan's Quintet seems like too much of a coincidence to me.

What I'm wondering is if the tidal tail that seems to belong to NGC 7320 really comes from one of the true members of Stephan's Quintet, but the foreground object NGC 7320 blocks the "beginning" of this tidal tail and prevents us from seeing where it comes from. If this is correct, there would be two tidal tails from the classic members of Stephan's Quintet, both curving in the same way and pointing straight at NGC 7320C, but the foreground object NGC 7320 hides vital parts of one of the tidal tails.

How about that hypothesis? Is it possible?

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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by Chris Peterson » Sat Feb 10, 2018 9:48 pm

Ann wrote:What I'm wondering is if the tidal tail that seems to belong to NGC 7320 really comes from one of the true members of Stephan's Quintet, but the foreground object NGC 7320 blocks the "beginning" of this tidal tail and prevents us from seeing where it comes from.
Not just possible, but known to be the case. When imaged with radio telescopes, the tail is seen behind NGC 7320, connected with NGC 7318a/b. And the redshift of the hydrogen in the tail has been measured and agrees with the much more distant galaxies. (And I believe, it is not actually a tidal tail, which would contain stars, but rather, a shock structure of some sort.)
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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by Ann » Sun Feb 11, 2018 6:45 pm

Chris Peterson wrote:
Ann wrote:What I'm wondering is if the tidal tail that seems to belong to NGC 7320 really comes from one of the true members of Stephan's Quintet, but the foreground object NGC 7320 blocks the "beginning" of this tidal tail and prevents us from seeing where it comes from.
Not just possible, but known to be the case. When imaged with radio telescopes, the tail is seen behind NGC 7320, connected with NGC 7318a/b. And the redshift of the hydrogen in the tail has been measured and agrees with the much more distant galaxies. (And I believe, it is not actually a tidal tail, which would contain stars, but rather, a shock structure of some sort.)
Thanks, Chris! I have really been wondering.

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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by neufer » Sun Feb 11, 2018 7:23 pm

Chris Peterson wrote:
Ann wrote:
What I'm wondering is if the tidal tail that seems to belong to NGC 7320 really comes from one of the true members of Stephan's Quintet, but the foreground object NGC 7320 blocks the "beginning" of this tidal tail and prevents us from seeing where it comes from.
Not just possible, but known to be the case. When imaged with radio telescopes, the tail is seen behind NGC 7320, connected with NGC 7318a/b. And the redshift of the hydrogen in the tail has been measured and agrees with the much more distant galaxies. (And I believe, it is not actually a tidal tail, which would contain stars, but rather, a shock structure of some sort.)
  • The tail is seen behind NGC 7320, connected with NGC 7319 :
http://iopscience.iop.org/article/10.1088/0004-637X/724/1/80 wrote:

<<Optical images show two large tidal arms, both extending from NGC 7319 to the southern region of the group (blue in Figure 1). The outer one, very diffuse, passes behind the foreground galaxy (NGC 7320). Another arm, more concentrated, is almost parallel to the first one and seems to point to the small NGC 7320c galaxy. The differences in length and luminosity of these two structures have been explained by two separate interaction steps: the outer arm is produced by a first passage of an intruder close to NGC 7319 while the inner plume formed in a more recent event (Moles et al. 1997). This would explain that the second tail is still very young and not quite extended (regarding to the other one). Xu et al. (2005) pointed out that it took >5 × 108 yr, after the close encounter with NGC 7319, for NGC 7320c to move to its current position. They argued that it is likely that the new tail is triggered by a close encounter between NGC 7319 and NGC 7318a, which is ~3 times closer to NGC 7319 than NGC 7320c is.>>
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Re: CFHT: Widespread Galactic Cannibalism in Stephan's Quintet

Post by Chris Peterson » Sun Feb 11, 2018 8:06 pm

neufer wrote:
Chris Peterson wrote:
Ann wrote: What I'm wondering is if the tidal tail that seems to belong to NGC 7320 really comes from one of the true members of Stephan's Quintet, but the foreground object NGC 7320 blocks the "beginning" of this tidal tail and prevents us from seeing where it comes from.
Not just possible, but known to be the case. When imaged with radio telescopes, the tail is seen behind NGC 7320, connected with NGC 7318a/b. And the redshift of the hydrogen in the tail has been measured and agrees with the much more distant galaxies. (And I believe, it is not actually a tidal tail, which would contain stars, but rather, a shock structure of some sort.)
  • The tail is seen behind NGC 7320, connected with NGC 7319 :
A later paper I found placed its connection to NGC 7318a and/or 7318b. So perhaps this remains uncertain. What isn't uncertain (in terms of Ann's question) is that the structure is part of whatever is going on in Stephan's Quintet, and has no association with NGC 7320.
Chris

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