Jan 27/28, 1979
103aO + GG385
NGC 2902 is a prototype of the S0_1_
sub-type. The image on photographic plates,
inspected with the non-linear human eye, appears
to contain three distinct concentric luminosity
zones separated by apparent "edges" between the
zones. Particularly strong examples are NGC
3056 (S0_1/2_ panels 39, 53) and NGC 1553
(S_1/2_; panel 39). The inner, high-surface-brightness
region resembles a normal E0 galaxy.
A fainter but still high-surface-brightness
"intermediate" envelope comprises the middle third of
the image, well seen on the print here. This
deviation of the profile from that of a normal E
galaxy generally appears as a sharp apparent
edge. It is the principal classification feature
common to the S0 class.
A third outer luminosity zone of very low
surface brightness is present in NGC 2902. This
extended envelope can be traced on the plate to
about twice the radius of the middle zone; the
outer zone is not visible on this print.
The impression of three luminosity zones
that appear on visual inspection to have nearly
discrete edges is characteristic of the S0 class.
This feature separates the E and the S0 class.
Although the apparent discontinuity in the
luminosity gradient is quite definite to the eye,
the differences are minute in measured profiles.
From this it has been supposed (van den Bergh
1989) that the S0 class is a mixture of generally
misclassified E galaxies, and that the S0 class is
That the S0 class is in fact well defined is,
however, demonstrated by the continuity
throughout the S0 classification set out in the
next 24 panels here. The differences between the
images of E and S0 galaxies are generally highly
apparent on the prints. Among the prototype
examples is NGC 2902, here. Other examples
include NGC 5193 (S0_1_; shown here), NGC
5011 (S0_1_; shown here), NGC 4379 (S0_1_; panel
31), NGC 4024 (S0_1_; panel 31), and NGC 3998
(S0_1_ panels 31, 53).
Apparently, the visual impression
emphasizes changes of gradients (the eye takes the
first derivative) rather than the gradient itself.
Gradient changes appear as edges to the eye. It
is this property that makes separation of the E
and S0 classes by the classical morphologists
(using the non-linear photographic plates) more
powerful than photometric tracings, giving rise to
the false animadversions.
NGC 2902 is a pronounced case of deviation
of an E galaxy profile, as recognized also in
the RC1 by the comment "similar to NGC 1553"
(S0_1/2_; panel 39).
Star much fainter than galaxy.
vdB - E0
deV - SA(s:)0^o^
Very bright, diffuse nucleus in a bright lens: 0.6 arcmin x 0.45 arcmin.
Similar to NGC 1553.