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.
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!
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 : :
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 : :
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 : :
Cool galaxies Ray. Nice field of view and sharp detail. Well done.
Hi Paul - thanks for that.
Quote:
Originally Posted by FranckiM06
Great shot Ray, I like this image.
Franck
Thanks Franck - glad you like it
Quote:
Originally Posted by codemonkey
Great shot mate!
thanks Lee.
Quote:
Originally Posted by RickS
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
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.
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!
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!
...it's only the second best galaxy image for ages..... 
