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Old 30-08-2014, 08:19 PM
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Shiraz (Ray)
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NGC6872 - the biggest spiral of them all?

Hi

this group of galaxies was interesting to image - it's pretty low down and decent resolution was not easily available (average for lum was 2.2 arcsec FWHM). The major spiral - NGC6872 - is apparently a giant, about 5x further across than the milky way (ie about 500,000 light years) with possibly as many as 2 trillion stars. It has probably been disrupted by collision with IC4970. The affected arm has been replenished and now hosts new blue star forming regions and a faint throw-off ball of stars that may be a new dwarf galaxy in the making. The colour has not been unduly fiddled with - this seems to be roughly what it would look like through hypersensitive eyes (eg see the much higher res reference image). The other galaxies are probably also worth a look.

full res (0.91 arcsec) cropped:
http://www.astrobin.com/full/117049/D/?real=&mod=


Thanks for looking. Regards Ray

ref see http://en.wikipedia.org/wiki/File:NG...nd_IC_4970.jpg.
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Last edited by Shiraz; 31-08-2014 at 08:53 AM.
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  #2  
Old 31-08-2014, 03:15 PM
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Cool galaxies Ray. Nice field of view and sharp detail. Well done.
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  #3  
Old 31-08-2014, 03:25 PM
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Great shot Ray, I like this image.
Franck
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  #4  
Old 31-08-2014, 03:34 PM
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Great shot mate!
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  #5  
Old 31-08-2014, 05:57 PM
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Great image, Ray! A fascinating galaxy group.

I was going to have a go at this one at a dark sky escapade over the weekend but it was too far down in the murky light dome from Brisbane 100+ kms away so I picked another target!

Cheers,
Rick.
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  #6  
Old 31-08-2014, 06:49 PM
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Unusual structures in NGC 6876

G'day there Ray,

Your shot is nice and deep, and the angular resolution looks good. In fact, it has dragged me away from my studies of single-celled animals and lobe-finned fishes!

Yeah, the giant spiral is very pretty and somewhat distorted, but what really interests me is the unusual "layered" (onionlike) structure of the big elliptical NGC 6876, with various position angles and ellipticities of its outline that are dependent on the surface brightness to which its image is displayed. This big elliptical galaxy also has a remarkably extended extremely-extremely-faint outermost halo, which I seem to pick up when I try to stretch your .jpg

Here is the outer halo, from one of David Malin's "co-added UK Schmidt plates" images : :
Click image for larger version

Name:	N6876_UKS_veryDeep_V.V.ExtensiveHalo_(differentPa)_Malin.jpg
Views:	49
Size:	71.9 KB
ID:	169098
As you can see, the true size of NGC 6876 is absolutely colossal!

The isophotes of NGC 6876 have at least three different shapes and position angles, depending upon how far out you are from its centre. [ While elliptical galaxies are - at face value - a bit bland in appearance, some of them have very complex isophotal structure in the outline of the galaxy at various radii.]

As I see it, this galaxy has at least three different shapes, depending upon how far out from its centre we display its outline:
- the innermost region is very elongated, and there is some further evidence of other fine structure near the very centre of this galaxy.
- outside of this inner elongated region, the isophotes of this galaxy become very round
- outside of this round zone, there is a vast halo which is noticeably elongated

Moreover, there are hints that many of the elliptical isophotes are not entirely regular.

In the below image, I superpose, in a crude way, lines of equal surface brightness (= isophotes) onto an image of this galaxy from the Chart 32 telescope : :
Click image for larger version

Name:	_________________N6876_chart 32.jpg
Views:	37
Size:	160.3 KB
ID:	169099

The general structures that are seen here are also visible in several other images that I have analyzed. I am interested, in particular, in the possibility that the outermost isophotes of this galaxy are not regular ellipses.

The innermost few arcseconds of this galaxy are also peculiar, as shown in HST imagery; there is a very unusual two-lobed structure near to the centre of this galaxy.
For instance, here are the isophotes of the central 4 by 4 arsecond region, from an F814W image taken with the HST & WFPC2 : :
Click image for larger version

Name:	N6876_F814W isophotes__(HST WFPC2)_(2002_AJ_124_1975).gif
Views:	54
Size:	133.0 KB
ID:	169100

cheers, Robert
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  #7  
Old 31-08-2014, 07:25 PM
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Quote:
Originally Posted by Paul Haese View Post
Cool galaxies Ray. Nice field of view and sharp detail. Well done.
Hi Paul - thanks for that.

Quote:
Originally Posted by FranckiM06 View Post
Great shot Ray, I like this image.
Franck
Thanks Franck - glad you like it

Quote:
Originally Posted by codemonkey View Post
Great shot mate!
thanks Lee.

Quote:
Originally Posted by RickS View Post
Great image, Ray! A fascinating galaxy group.

I was going to have a go at this one at a dark sky escapade over the weekend but it was too far down in the murky light dome from Brisbane 100+ kms away so I picked another target!

Cheers,
Rick.
Thanks Rick - yes, needed fairly dark sky to get at it.

Quote:
Originally Posted by madbadgalaxyman View Post
G'day there Ray,

Your shot is nice and deep, and the angular resolution looks good. In fact, it has dragged me away from my studies of single-celled animals and lobe-finned fishes!

Yeah, the giant spiral is very pretty and somewhat distorted, but what really interests me is the unusual "layered" (onionlike) structure of the big elliptical NGC 6876, with various position angles and ellipticities of its outline that are dependent on the surface brightness to which its image is displayed. This big elliptical galaxy also has a remarkably extended extremely-extremely-faint outermost halo, which I seem to pick up when I try to stretch your .jpg

Here is the outer halo, from one of David Malin's "co-added UK Schmidt plates" images : :
Attachment 169098
As you can see, the true size of NGC 6876 is absolutely colossal!

The isophotes of NGC 6876 have at least three different shapes and position angles, depending upon how far out you are from its centre. [ While elliptical galaxies are - at face value - a bit bland in appearance, some of them have very complex isophotal structure in the outline of the galaxy at various radii.]

As I see it, this galaxy has at least three different shapes, depending upon how far out from its centre we display its outline:
- the innermost region is very elongated, and there is some further evidence of other fine structure near the very centre of this galaxy.
- outside of this inner elongated region, the isophotes of this galaxy become very round
- outside of this round zone, there is a vast halo which is noticeably elongated

Moreover, there are hints that many of the elliptical isophotes are not entirely regular.

In the below image, I superpose, in a crude way, lines of equal surface brightness (= isophotes) onto an image of this galaxy from the Chart 32 telescope : :
Attachment 169099

The general structures that are seen here are also visible in several other images that I have analyzed. I am interested, in particular, in the possibility that the outermost isophotes of this galaxy are not regular ellipses.

The innermost few arcseconds of this galaxy are also peculiar, as shown in HST imagery; there is a very unusual two-lobed structure near to the centre of this galaxy.
For instance, here are the isophotes of the central 4 by 4 arsecond region, from an F814W image taken with the HST & WFPC2 : :
Attachment 169100

cheers, Robert
Hi Robert - thanks for the info. FWIW, attached a "sort of" isophote representation from the current image that might be of interest - it sure is a big galaxy.

Regards Ray
Attached Thumbnails
Click for full-size image (aaangc6872isophotes.jpg)
180.3 KB27 views

Last edited by Shiraz; 31-08-2014 at 09:26 PM.
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  #8  
Old 31-08-2014, 08:03 PM
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Wow, what a stunner of an image! The best image for ages. you smashed that!

Greg.
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  #9  
Old 01-09-2014, 10:20 AM
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Quote:
Originally Posted by gregbradley View Post
Wow, what a stunner of an image! The best image for ages. you smashed that!

Greg.
thanks Greg - very generous comment. regards Ray
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  #10  
Old 01-09-2014, 10:42 AM
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strongmanmike (Michael)
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Originally Posted by gregbradley View Post
Wow, what a stunner of an image! The best image for ages. you smashed that!

Greg.
Oh Greg, don't be so rediculous ...it's only the second best galaxy image for ages..... THIS one is clearly the best one

Great work Ray

Mike
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  #11  
Old 01-09-2014, 12:10 PM
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graham.hobart (Graham stevens)
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Ngc6872

Ray that is just gorgeous !
Graz
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  #12  
Old 01-09-2014, 05:13 PM
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multiweb (Marc)
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Great shot Ray. Beaute field too.
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  #13  
Old 01-09-2014, 05:42 PM
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Oh Greg, don't be so rediculous ...it's only the second best galaxy image for ages..... THIS one is clearly the best one

Great work Ray

Mike
Heheh, yes true that is a stunner also.

Greg.
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  #14  
Old 01-09-2014, 06:02 PM
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Nicely done Ray! I imaged this one a while back and it's probably one of the best targets in the southern sky.

Cheers

Steve
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  #15  
Old 01-09-2014, 09:44 PM
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Shiraz (Ray)
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Quote:
Originally Posted by strongmanmike View Post
Oh Greg, don't be so rediculous ...it's only the second best galaxy image for ages..... THIS one is clearly the best one

Great work Ray

Mike
Thanks very much Mike

Quote:
Originally Posted by graham.hobart View Post
Ray that is just gorgeous !
Graz
Hi Graham - thank you
Quote:
Originally Posted by multiweb View Post
Great shot Ray. Beaute field too.
Thanks Marc. Yep, it is a great combination of objects isn't it

Quote:
Originally Posted by gregbradley View Post
Heheh, yes true that is a stunner also.

Greg.
thanks again Greg

Quote:
Originally Posted by Stevec35 View Post
Nicely done Ray! I imaged this one a while back and it's probably one of the best targets in the southern sky.

Cheers

Steve
Thanks Steve. your image is a real beauty. Agree that it is a wonderful target - has to be one of the most interesting available to a smallish scope.

regards Ray
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  #16  
Old 04-09-2014, 09:35 AM
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Quote:
Originally Posted by Shiraz View Post
Hi Robert - thanks for the info. FWIW, attached a "sort of" isophote representation from the current image that might be of interest - it sure is a big galaxy.
Ray
Thanks very much for that, Ray,

The details in your image confirm some of the subtle internal structures I have been noting in other images of NGC 6876

This is my second most favourite elliptical galaxy (after PGC 48896) on account of the fact that it has very significant internal structure that is easily evident.

For instance, the outermost isophotes of NGC 6876, in your image, and in many other images of this galaxy, are a long way from being regular ellipses.

In the literature, we do find some speculations that the noticeably unusual overall morphology of this galaxy could be due to a merger between galaxies. However, the extremely-extended outer envelope of NGC 6876 has had very very little study, so far, in the professional literature.

Oddly enough, "6876" might be the least known bright elliptical galaxy in the sky, despite its obvious strong abnormalities! (This undoubtedly has to do with its far southern declination, which puts it outside of the "mental map" of the majority of professional astronomers)

Normally, with the isophotes of the vast majority of elliptical galaxies, the deviations in isophotal shape from a perfect ellipse are very subtle;
so the strong deviations from regularity in the outer parts of this galaxy represent a very significant abnormality.

It is remarkable how little the outermost regions of elliptical galaxies are understood by professional astronomers;
for example, even the 'familiar' M86 ,which looks to be a relatively normal elliptical in its inner parts, has a highly peculiar outer envelope which is neither that of an S0 galaxy or that of an elliptical with a significant excess outer light.
(in other words, M86 cannot be assigned to a single unique Hubble type!!).

The outer parts of elliptical galaxies show large variations in their morphologies. So it is perhaps not unexpected that spectroscopic and kinematic studies have shown that the stellar orbits within elliptical galaxies and also the orbits of their globular clusters , also vary a lot between various elliptical galaxies.

For example, this recent study shows that the distributions, the rotation, the orbits, and the ages, of the systems of globular clusters that trace the outermost regions of elliptical galaxies, vary a lot from galaxy to galaxy:
http://researchbank.swinburne.edu.au...ular&y=21&x=11

So there is a lot of diversity in the population of elliptical galaxies, it is just not so obvious as in those "pretty" spirals!

Cheers, Robert
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  #17  
Old 04-09-2014, 10:42 AM
PeterEde (Peter)
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Wow
Incredible detail.
Well done
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  #18  
Old 05-09-2014, 12:33 PM
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Quote:
Originally Posted by madbadgalaxyman View Post
Thanks very much for that, Ray,

The details in your image confirm some of the subtle internal structures I have been noting in other images of NGC 6876

This is my second most favourite elliptical galaxy (after PGC 48896) on account of the fact that it has very significant internal structure that is easily evident.

For instance, the outermost isophotes of NGC 6876, in your image, and in many other images of this galaxy, are a long way from being regular ellipses.

In the literature, we do find some speculations that the noticeably unusual overall morphology of this galaxy could be due to a merger between galaxies. However, the extremely-extended outer envelope of NGC 6876 has had very very little study, so far, in the professional literature.

Oddly enough, "6876" might be the least known bright elliptical galaxy in the sky, despite its obvious strong abnormalities! (This undoubtedly has to do with its far southern declination, which puts it outside of the "mental map" of the majority of professional astronomers)

Normally, with the isophotes of the vast majority of elliptical galaxies, the deviations in isophotal shape from a perfect ellipse are very subtle;
so the strong deviations from regularity in the outer parts of this galaxy represent a very significant abnormality.

It is remarkable how little the outermost regions of elliptical galaxies are understood by professional astronomers;
for example, even the 'familiar' M86 ,which looks to be a relatively normal elliptical in its inner parts, has a highly peculiar outer envelope which is neither that of an S0 galaxy or that of an elliptical with a significant excess outer light.
(in other words, M86 cannot be assigned to a single unique Hubble type!!).

The outer parts of elliptical galaxies show large variations in their morphologies. So it is perhaps not unexpected that spectroscopic and kinematic studies have shown that the stellar orbits within elliptical galaxies and also the orbits of their globular clusters , also vary a lot between various elliptical galaxies.

For example, this recent study shows that the distributions, the rotation, the orbits, and the ages, of the systems of globular clusters that trace the outermost regions of elliptical galaxies, vary a lot from galaxy to galaxy:
http://researchbank.swinburne.edu.au...ular&y=21&x=11

So there is a lot of diversity in the population of elliptical galaxies, it is just not so obvious as in those "pretty" spirals!

Cheers, Robert
thanks Robert - very interesting info.

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Originally Posted by PeterEde View Post
Wow
Incredible detail.

Well done
thanks very much Peter.

regards ray
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  #19  
Old 08-09-2014, 02:12 AM
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Beautiful Ray! My only remark is on the background a tad dark but still fine. Love the resolution you got, this is an amazing field of view!

Clear skies
Marco
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  #20  
Old 08-09-2014, 09:02 AM
Ross G
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An amazing photo Ray.

So many galaxies!

Ross.
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