Quote:
Originally Posted by marco
IC 1459 is a seldom imaged elliptical S0 galaxy (i.e. large bulge with a minimal or faint disk) about 70 IC 1459 shows a prominent halo on deep exposures.
Clear skies
Marco
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Hello Marco,
The 'halo' is better shown than I have ever seen it before!! (I have been looking for a good image of the outer parts of this galaxy, for a long long time......)
There are shells visible in your superb image, which are evidence of the accretion of smaller galaxies; it would seem that a remarkably large percentage of elliptical galaxies do have 'shells' in their envelopes, though they are always at very very very low contrast. Cannibalization of large numbers of small galaxies by big ellipticals is , these days, regarded as being virtually proven for big elliptical galaxies. (Maybe
all big spheroidal galaxies have swallowed multiple small galaxies)
You have characterized the morphology of this galaxy very well, in your post:
As you have indicated, the "two component" structure of this galaxy image is typical of an S0 galaxy image.
The outer extremely-faint 'halo' has been modelled, in the literature, with an exponential radial falloff of light, which can be (though does not have to be!) the signature of an actual planar disk structure.
Thus, the
apparent (on inspection) Hubble class of this galaxy is "very mild S0", and also, if we adopt the classification precepts of Allan Sandage in the Carnegie Atlas of galaxies, the mixed Hubble type E/S0
(= dominant spheroid, plus a very very very faint disk),
but the outer very-faint and very-extended structure may or may not actually be a real disk in three dimensions.
Hubble's original galaxy classification system only considered the structure of the
two-dimensional image of an S0 or spiral galaxy, with a rapidly falling off component in the light (
presumably a spheroid) and also a second component that has a more shallow falloff of light;
but more recent galaxy classification systems consider the various
three-dimensional structures that each galaxy is composed of (this is an example of a
physical classification system).
If the very outermost isophotes of this galaxy are somewhat blocky and rectilinear (= boxy) rather than slightly pointy (disky), then this is evidence that the very-extended outermost structure is
not an actual disk structure.
Best regards,
Robert Lang
P.S.
In further cogitations, I am now wondering if the "shells" may or may not look more like arms.