BURSTEIN : Many of your E galaxy luminosity profiles show deviations
from an r1/4 law at faint surface brightnesses. Can
that such deviations cannot be due to sky subtraction? Sky
subtraction is a problem we have to deal with at the 1-2% level in
the data on E galaxies being taken for our own survey. It is a
straightforward test to make.
SCHOMBERT : Of course I cannot prove that the deviations at faint surface photometry levels are indeed real. The behavior of elliptical profiles at the 27 to 28 mag arcsec-2 level is in the realm of 0.5% of sky brightness and are very sensitive to proper sky subtraction as you noted. However, there is a great deal of overlap in surface photometry in the literature; i.e. same galaxy from different telescopes and observers. A comparison shows that there is very good agreement down to 28 mag arcsec-2. In other words, faint surface photometry is repeatable. I believe everyone in this field has be very careful and very honest about their techniques. The burden of proof here is for someone to show that there is a legitimate reason to believe that everyone has been systematically wrong in their sky subtraction.
SURMA : At Heidelberg Observatory we have performed a study of deep surface photometry of E's, where we also looked at the galaxies in Kormendy's `tidal class' galaxy sample. What we have found is that the occurrence of so-called `tidal halos' is strongly coupled with the problem of accurate sky subtraction. M. Cappaccioli has been emphasizing the importance of sky subtraction in order to reach the faint outer regions of E's, SO's and also cD's. It is evident that you need a sky subtraction accurate to << 1% if you want to reach brightness levels of 24 mag arcsec-2, and this is the region you need to reach in order to see cD envelopes! I wonder how much of the discussion about cD's is influenced by these problems.
SCH0MBERT : First, I strongly disagree with your statement that better than 1% sky is need to obtain the 24 arcsec-2 photometry level. A sky of 1% (in the Johnson V band) corresponds to 26 not 24 mag arcsec-2. The exact shape of the outer halos of ellipticals is certainly open to debate and new work; however, CCD's have achieved 0.1% flattening, on average, for many years now. And many observers, with different telescopes and detectors, using different techniques and software, have time and time again proven the repeatability of faint surface photometry. I cannot accept the claim that everyone has performed their sky subtraction incorrectly for 20 years. cD envelopes have been known for 15 years and cannot be the result of sky errors.
KENT : D and gE galaxies have identically shaped profiles (i.e. r1/4). Hence, they can only differ in the value of µ at re. Is your "diffuse" characteristic of D galaxies another way of saying that they have lower µe?
SCHOMBERT : No, D galaxies are noted by their shallower r1/4 fits. This places them larger and brighter in the µe-re diagram (see Figure 6). I prefer to interpret this as enlarged re as predicted by N-body simulations.
BUTA : What are the three-dimensional shapes of cD's, particularly the envelopes?
SCHOMBERT : Alan Porter's thesis (Caltech 1988) addresses this problem more fully. His answer is that cD isophotes become more flattened with radius, presumably shifting from the galaxy potential to the cluster potential. However, it is an excellent future project, with the advent of reimaging cameras, to study the behavior of cD envelopes with respect to twisting and central shifts.
D JORGOVSKI : Two more bits of information come from Porter's thesis. First, BCM's show systematic ellipticity gradients, in the sense of becoming flatter outwards, whereas the normal ellipticals show no such trend. Second, the envelopes tend to be well aligned with the parent clusters (the Binggeli effect). To me, this also suggests that the origin of envelopes is at the epoch of cluster formation. The BCM galaxy and the cD envelope may be independent entities, sharing the same special location (i.e. the bottom of the cluster potential well).
KING : I didn't understand your argument that the extended envelopes must be primordial. Can you distinguish against the case of a merger of galaxies that already consisted only of old stars? In answering, please note that the merging process favors massive galaxies, which have colors like those of cD envelopes. Also, w'th regard to Djorgovski's arguments about the alignment of cD envelopes with the major axis of their cluster, this could be simply due to the greater velocities of galaxies in this direction.
SCHOMBERT : If tidal stripping is the origin of cD envelopes, then a majority of their luminosity must be taken from low luminosity members of the cluster. The mass-metallicity relation predicts that this material will be bluer than the stellar population of the cD parent body. Although only a handful of cD envelopes have been studied by myself (in Johnson B and V plus a Stromgren narrow band system) all show very flat color profiles. The merging of bright ellipticals can maintain a red cD interior, but the outer envelope is going to be influenced by lower luminosity E's and, of course, any disk galaxies in the cluster. I agree that the elongation of the cD envelope could be due to the orbits of galaxies being stripped if that is the mechanism of formation. However, a more likely explanation is that the cD envelope is reflecting the shape of the cluster potential in which it was born which, if galaxies are on strongly radial orbits, is the same statement.
MELNICK : I disagree with your statement that there is no correlation between cluster parameters and cD envelopes. I see in your plots a rather strong correlation which, of course, eventually disappears if one eliminates critical data (such as the brightest envelopes). My question is, is there any physical or statistical arguments to eliminate the points that give a strong correlation?
SCHOMBERT : I agree that the extreme points in these diagrams are the "best" cD galaxies, but a simple correlation test gives only marginal significance. My statement is not that the correlations do not exist, but that they are surprising weak, and, therefore, have been partially erased during cluster collapse.
MERRITT : Is there any evidence that the presence of a cD galaxy correlates with the dynamical properties of a cluster, such as its velocity dispersion?
SCHOMBERT : A comparison of cD envelope properties with cluster dynamical properties (velocity dispersion, Bautz-Morgan type, etc... ) is an exploration into the world of bad statistics. The correlations exist, but they are only marginally significant, strongly weighted by the very few brightest cD's. I argue that this merely reflects the early cluster state into the present epoch, and that cD envelopes are linked to the local conditions (i.e. its original subcluster) at birth as you predicted in 1955.
MERRITT : Does this fact argue for a primordial origin of the cD's, as well as for the diffuse envelopes?
SCHOMBERT : There is only a weak correlation between the underlying galaxy luminosity and the cD envelope luminosity. Therefore, I don't believe we can conclude that the merging that produced the parent galaxy was only primordial. In fact, bound populations exist today with dynamical friction timescales of only 5 × 109 yrs, so the process of dynamical evolution must still be ongoing. My interpretation of the parent to envelope correlations is that parallel dynamical formation processes are being echoed from early epochs (i.e. mergers versus tidal stripping)
NIETO : Can you comment on the flat central profiles of BCM's? Are they intrinsically flat or is your material resolution limited?
SCHOMBERT : Due to the large variety of surface photometry for the sample; some from CCD's, others from large scale photographic plates, I cut the inner limit at 2 kpc and, therefore, can say very little on the core properties of BCM's.