APOD: Stephan's Quintet Plus One (2014 Mar 27)

[APOD Robot]

Stephan's Quintet Plus One

Explanation: The first identified compact galaxy group, Stephan's Quintet is featured in this remarkable image constructed with data drawn from Hubble Legacy Archive and the Subaru Telescope on the summit of Mauna Kea. The galaxies of the quintet are gathered near the center of the field, but really only four of the five are locked in a cosmic dance of repeated close encounters taking place some 300 million light-years away. The odd man out is easy to spot, though. The interacting galaxies, NGC 7319, 7318A, 7318B, and 7317 have a more dominant yellowish cast. They also tend to have distorted loops and tails, grown under the influence of disruptive gravitational tides. The mostly bluish galaxy, NGC 7320, is in the foreground about 40 million light-years distant, and isn't part of the interacting group. Still, captured in this field above and to the left of Stephan's Quintet is another galaxy, NGC 7320C, that is also 300 million light-years distant. Of course, including it would bring the four interacting galaxies back up to quintet status. Stephan's Quintet lies within the boundaries of the high flying constellation Pegasus. At the estimated distance of the quintet's interacting galaxies, this field of view spans over 500,000 light-years.

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[Ann]

This is a superb picture!!!

Ann

[yasgur]

Yes, it is superb. Is NGC 7317 an elliptical galaxy or a severely diminished spiral? Most of these groups seem to involve spirals...or this could be an example of my severely diminished memory.

[Boomer12k]

If 7320 is not apart of the group....then why does it look distorted too? Appears to have a star trail to our left view...

Really great shot!!!
:---[===] *

[Guest]

Almost does not look real. The forces involved must be staggering beyond belief...

[starsurfer]

I find it really interesting how many of these compact groups seem to have a galaxy that isn't a physical member of the group. Also I'm not sure but I think there might have been a compact group identified earlier than Stephan's Quintet, maybe Hickson 44 or Hickson 57 as maybe their member galaxies might have been discovered earlier than the ones in this group.

[starsurfer]

If 7320 is not apart of the group....then why does it look distorted too? Appears to have a star trail to our left view...

Really great shot!!!
:---[===] *

There are a few examples of isolated galaxies that appear to be disturbed or have tidal features. One theory is the interaction with small satellite galaxies and subsequent absorption of them into the disk can lead to morphological peculiarities. This is a great topic of research in the professional realm at the moment with a quite a few projects dedicated to unravelling the mysteries of galaxy assembly.

[CURRAHEE CHRIS]

Excellent picture and excellent narration. Easy to follow and as a newbie, I certainly appreciate that.

Ok, someone square the new guy away- when I read statements like this:

"At the estimated distance of the quintet's interacting galaxies, this field of view spans over 500,000 light-years."

I assume that the distance between the right and left borders of the picture I am looking at span 500,000 light years. Am I correct in this interpretation?

Thanks!!
CC

[Ann]

If 7320 is not apart of the group....then why does it look distorted too? Appears to have a star trail to our left view...

Really great shot!!!
:---[===] *

Good call, Boomer. Yes, NGC 7320 really is distorted - or rather, it really does have a tidal tail. By an amazing coincidence, the tidal tail of NGC 7320 exactly mirrors the shape, the orientation and the apparent length of the tidal tail emanating from NGC 7319.

But wait! Is that really a coincidence? Isn't this the final proof that NGC 7320 is really a physical member of Stephan's Quintet?

No, a scrutiny of the stellar populations of the different galaxies definitely proves that NGC 7320 is a foreground object. The disk of NGC 7320 is very grainy in today's APOD, as if it was sprinkled with blue grains of sand. It seems almost certain that many individual stars are resolved here. The blue parts of the other galaxies are not nearly as grainy. Some of their "blue regions" are completely smooth, meaning that nothing is resolved there. Other parts are grainier, but we are almost certainly looking at clusters there and not individual stars.

But the real clincher is the appearance of the bulges of the galaxies. NGC 7320 has a faint, not really yellow bulge and a tiny, faint nucleus. The other galaxies have brilliantly bright, yellow bulges. Clearly their bulges are very rich in stars, whereas the bulge of NGC 7320 is faint precisely because it is star-poor. But if NGC 7320 was at the same distance from us as the rest of Stephan's Quintet, then the stars of its disk and bulge would be much more "spread out" than the stars of the disks and bulges of the other galaxies. NGC 7320 would simply be much "fluffier" than the other galaxies. It is unthinkable that a "fluffy" galaxy would be unaffected by the titanic collision and extreme tidal forces ripping through the rest of the group.

But if NGC 7320 is a foreground object, then its faint bulge and tiny nucleus is just normal for a galaxy of its class, a quite small galaxy.

Ann

[neufer]

The forces involved must be staggering beyond belief...

The forces may be large but the accelerations are tiny: ~1 mm/hr per hour
(; or about 1% of the sun's acceleration on Sedna at aphelion).

[MargaritaMc]

Ann said (rhetorically!)
But wait! Is that really a coincidence? Isn't this the final proof that NGC 7320 is really a physical member of Stephan's Quintet?

This 2009 post on the Chandra blog discusses just this:
http://chandra.harvard.edu/blog/node/150

The presence of a second tidal tail in optical images of the group is not controversial and is noted in papers going back at least 20 years. Radio observations prove that the tail is unrelated to NGC7320 and is actually connected to the X-ray *mission and the two spiral galaxies on the right (NGC7318a/b). The radio lets us see cold neutral hydrogen gas in the tails and trace the southern tail even where it passes behind NGC7320; there is no sign of disturbance in the gas or a connection to NGC7320, and the hydrogen is at the redshift of the other galaxies.

*I suspect a typo there, and that emission was intended

Margarita

[dmbeaster]

But if NGC 7320 is a foreground object, then its faint bulge and tiny nucleus is just normal for a galaxy of its class, a quite small galaxy.

Ann

This must be so since it is approximately seven times closer, but seems to be part of the group based on relative size. It is larger than the others, but not nearly as large as its relative distance would seem to dictate.

[geckzilla]

NGC7320 is one of those galaxies that wants to have a majestic set of spiral arms but seems to have a hard time since it is actually a dwarf galaxy. It's an interesting juxtaposition of galaxies because NGC7320 is so much smaller than those in the background and yet it looks a little larger from our perspective.

[geckzilla]

I was wondering what NGC 7320 would look like if it were pushed back to be with the other galaxies and calculated that it would be about 20% of its apparent size at that distance. If it were anywhere near the others it would be quickly distorted and probably integrated into its neighbors like a little galaxy snack.

[Anthony Barreiro]

I was wondering what NGC 7320 would look like if it were pushed back to be with the other galaxies and calculated that it would be about 20% of its apparent size at that distance. If it were anywhere near the others it would be quickly distorted and probably integrated into its neighbors like a little galaxy snack.

Sometimes Geckzilla lets the power of photoshop go to her head. What a mean thing to do to a perfectly fine little galaxy!

[retrogalax]

I didn't see those two galaxies at first !
NGC 7318B could be in the alignment also as its arms are not so much distorted.
NGC 7320C looks regular so i don't think of a previous interaction with the group.
Could NGC 7317 have the larger effects on its neighbour's structures (NGC 7318A) ?

[Cousin Ricky]

I was wondering what NGC 7320 would look like if it were pushed back to be with the other galaxies and calculated that it would be about 20% of its apparent size at that distance. If it were anywhere near the others it would be quickly distorted and probably integrated into its neighbors like a little galaxy snack.

Can you red-shift it as well?

[Ann]

Nice picture, geckzilla!

Cousin Ricky wrote:
Can you red-shift it as well?

Click to view full size image

Photo: Gemini Observatory

Nothing much would happen to the appearance of NGC 7320 if it was located at the same distance from us as the other members of Stephan's Quintet (except the fact that it would look so much smaller, as you can see from geckzilla's image). One more thing might happen, though: you might redshift the numerous emission nebulas of NGC 7320 "out of view", so to speak. Check out this picture of Stephan's Quintet, where the emission nebulas of NGC 7320 are very obvious and numerous, but the emission nebulas of the more distant galaxies have been redshifted out of view for the filters that were used for this image.

Oh, and the bulge of NGC 7320 would probably look a little yellower, too.

Ann

[owlice]

He's so cute!

Click to view full size image 1 or image 2

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