Upon examining luminosity distributions, it is found that BCM's are
generally distributed into three morphological types; gE, D and cD.
A normal elliptical is represented by a smooth manifold of surface
photometric profiles. On the low mass end of the sequence these profiles
are exponential in shape
(Binggeli, Sandage and
Tarenghi 1984).
On the high mass end these profiles become approximately
r1/4 shaped.
The transition from exponential to power-law is smooth and an interested
reader can view template profiles for ellipticals in Figure 1 of
Schombert (1987).
At the extreme end in luminosity of the normal elliptical
sequence are, by my nomenclature, giant ellipticals. They are a natural
extension of the elliptical sequence, being roughly
r1/4 in appearance, and lie on the same fundamental
plane of structural and kinematic properties
(Djorgovski and Davis
1987)
as normal ellipticals. I simply delineate them from other ellipticals by
their size (R 
75 to 100 kpc) and magnitude (Mv < -22). On the other
hand, the D class galaxies,
similar in luminosity, size and mean surface brightness as gE's, display
special properties with respect to normal ellipticals on the fundamental
plane. For example, their surface brightness profiles are
more extended at a characteristic radius than normal ellipticals
causing a shallower profile slope (see below). The cD class is recognized
by D-like interiors plus a large, low surface brightness
(
= 26
to 27 mag arcsec-2) envelope
(Oemler 1976).
All three types are shown in Figure 2 along
with a intermediate luminosity elliptical. The manifold of profiles
for gE and D type galaxies overlap; however, the brightest D galaxies
are more luminous than the brightest gE types.
 |
Figure 2. Surface brightness profiles for the four classes of BCM's, normal elliptical, giant elliptical, D and cD. The dotted line indicates the region of a profile most strongly weighted in visual classification. |
Individual examples of surface brightness profiles of all these
morphological classes can be found in Figure 2 of
Schombert (1987).
Most notable of these examples are some "classical" cD galaxies,
such as the BCM in A2029, which, although being a very large D galaxy,
does not have the extended envelopes that I associate with the cD
class. Extreme cD examples are seen with the BCM's in A1413
(Lenv = 7 × 1011
L
) and
the southern cluster Shapley 8 (Lenv = 2 ×
1012
L).
Homology merger theory
(Hausman and Ostriker
1978)
predicts that cD galaxies should have depressed central surface
brightnesses; however, as noted by
Oemler (1976),
most D and cD galaxies have high central
surface brightnesses (central refers to the inner 2 kpc, not a core
value). Nonetheless, there does exist a sub-sample of cD galaxies with
depressed central surface brightness (e.g., A85 and NGC 6166 in
Schombert 1987).
These objects are rare, but it is interesting to note that most are
associated with emission lines, IRAS emission and other evidence of
recent star formation in their cores.
Several characteristics are common to BCM's evidenced through comparison of their surface brightness types. The first is that almost all BCM's are of the D or cD class. This is true for poor clusters (Thuan and Romanishin 1981) as well as rich clusters (Malumuth 1983, Schombert 1987). A second point is that cD envelopes are not detected by eye (most being below 1% of sky brightness). The "diffuseness" seen for BCM's is not the extended cD envelope, but rather the slope of the profile around the 24 V mag arcsec-2 level. This is an important distinction since I reserve the cD class for objects with extended envelopes only, a matter for deep surface photometry, and that the "diffuseness" is a characteristic independent of the existence of a cD envelope. This is seen in Figure 3, where the morphological type by surface photometry is compared to the morphological type as determined by visual classification of Struble and Rood (1984). What Struble and Rood would call a cD galaxy is equally divided into the D and cD classes meaning their definition of cD had nothing to do with extended envelopes, but rather depended on profile slope. Fortunately, as I will prove in the following section, this shallow profile is a merger signature and, therefore, the cD classification by Struble and Rood still has merit with respect to the galaxy type. Also, very few gE's are called cD by Struble and Rood. Sometimes, as in NGC 6034, cD galaxies are misidentified as SO galaxies, probably due to a high elongation combined with a shallow profile which simulates the appearance of a disk. Lastly, not all D or cD galaxies are BCM's; however, no D or cD type galaxies have been found in the field despite an extended search of bright, field ellipticals from redshift surveys (Schombert 1987) and all are positioned on local cluster density enhancements (Beers and Geller 1983).
 |
Figure 3. Comparison of morphology from Struble and Rood versus classification based on surface photometry profiles as shown in Figure 1. |