Quote:
Originally Posted by Weltevreden SA
I see that 4329 is variously identified as a lenticular (S0) and an elliptical (SIMBAD merely IDs it s ‘Es D’. I’m not sure what ‘Es D’ means, either.) Robert has been writing quite a bit about S0 lenticulars lately and I think he’s right in two ways about this pair: (a) 4329 looks like a small-bulge, large-disc S0 and not an E, and (b) the pair are closer than their stated 59 Mpc. It’s interesting than a hobbyist looking through an eyepiece can spot a likely misidentification the pros do not. The few relevant IC 4329 distance studies are old (1995-98) and based on CCD photometry rather than enhanced hi-rez .01 mag-capable spectrography. Spectrographic signal-
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The "D" simply refers to an elliptical which has a very extended and extremely-faint envelope around it.
(but real "D" galaxies, however, have outermost halos so faint that they can be invisible in standard imaging. These halos have a tendency to merge eventually with the intracluster light (which is composed of stars that float in between the cluster galaxies)
The old definition of an S0 galaxy is simply that the apparent two-dimensional
image of a galaxy (necessarily it is a galaxy with very-smooth light) has two components; the central component which has a fast falloff of surface brightness with increasing radius , and the outer component which has a shallow falloff of surface brightness with increasing radius.
Thus, this traditional definition of an S0 galaxy says nothing at all about what an actual galaxy is, in
three dimensions.
In other words, we might
assume that the central component is a spheroidal bulge and we might
assume that the outer component is a rotationally-flattened & planar disk; but this does not have to be the case.
In the new systems of
physical galaxy classification, we focus more on what is actually going on within the three-dimensional 'real' space of each galaxy;
so an S0 Galaxy is today redefined as a galaxy having a bulge component plus a disk component, plus it must also have a Star Formation Rate and a Cold Gas Content
intermediate between what we find in elliptical galaxies and what we find in Sa galaxies.
In any galaxy with two
apparent components, the
radial falloff of surface brightness and the
shapes of the isophotes of both the apparent "bulge component" and the apparent "disk component" are amenable to numerical
measurement; so by making these measurements we can better assess whether or not a galaxy with
two apparent components really does have a genuine disk component and a genuine bulge component.
(Disks and bulges are today
very strictly defined in terms of shape, surface brightness falloff, stellar orbits, etc.)
IC 4329 is an "apparent" S0, as are good numbers of galaxies often classified as ellipticals. But whether or not the outer component is a
real disk is something that cannot be decided from mere inspection of a two-dimensional image.
Isophotes (= the elliptical lines of equal surface brightness) within an elliptical galaxy can change their shapes in a complex way, with progressively changing (increasing or decreasing) galactocentric radius;
sometimes the isophotal ellipses get a bit pointy, which is often a sign of a low-surface-brightness disk component, while at other times the isophotal ellipses look markedly "blocky" (somewhat rectilinear) which is more likely to be the sign of a slowly-rotating component with little angular momentum and with stellar orbits in many different orientations.
cheers,
The V.Bad Galaxy Man
I have plenty of imaging material about IC 4329 so I may present an analysis of this, at some time.