Discussion

The debate on whether dwarf irregulars (and BCDs) could be progenitors of dwarf ellipticals is still of actuality. There are arguments in favor, against and some linking only two of the three types (dE, dI, BCD). The data presented here are not sufficient to bring an answer to these questions, but the fact is that Figs.  through indicate that there are few or no possible progenitors of bright dEs among the bright dIs, if fading is the mechanism of transformation (although spirals, which are not considered here, could perhaps form some bright dEs and S0s in clusters by harassment and tidal stripping). Indeed, the central surface brightnesses of the dEs are higher than those of most dIs of equivalent magnitude, and if one fades the irregulars, there are only very few bright candidates that eventually could become faint dEs. Looking at the effective radii one has that cluster early types and late types have similar radii, whereas the group&field late types are systematically larger than the early types in the same environment. Considering the exponential scale-length, the early types in clusters are marginally larger on average than the late types, whereas in the group&field the early types are clearly smaller, i.e. the late types are larger: the late types can only fade into the very faintest early types according to their sizes in the clusters, whereas in the group&field only a few individual late types might be 'faded' into very faint early types, see Figs.  and . The combined central surface brightness versus exponential scale length diagrams also illustrate the difficulty in fading group&field late types into early types. Considering Fig. , and keeping in mind that fading moves a galaxy horizontally from left to right, it is evident that the group&field late types in Fig.  cannot move to the location of the group&field early types in Fig. , which are below and to the left of the late types in those plots. The only region of these plots attainable by fading the late types is the region of faint (224#224) galaxies, where early type group&field galaxies are also found.

There are mechanisms that may change the scale lengths and surface brightnesses of galaxies, like tidal interactions, gas loss or gas stripping, if dark matter does not dominate the gravitational potentials of the galaxies. But such effects are not yet well understood and produce a number of effects that may be at odds with each other, depending on the details of the interactions and the galaxies studied.

As the galaxies in this work belong to two different environments that have very different characteristics as to the tidal interactions and ICM-ISM interactions, the differences in physical parameters among the galaxies noted in the previous section may have their origins in these environmental differences. The physical conditions and processes at work in the cluster and group&field environments can be briefly summarized as follows:

<<36>> > 225#225<<37>>

The cluster environment

is a relatively gas-rich environment as can be seen in X-ray studies or by the Sunyaev-Zeldovich effect. This gas can have relatively high electron densities, at 226#226 to 227#227 and temperatures of the order 228#228 to 229#229 and is observed within the central couple of Mpc. The high relative velocities of dwarf galaxies in this gaseous medium, of the order of 230#230, can induce shock fronts at the interface between the galaxies ISM and the external ICM, eventually triggering star formation, ram-strip the gas from the galaxy, or viscously remove the interstellar gas, or there can be a combination of such effects at work, depending on the exact parameters of the galaxies considered. The tidal interactions in clusters are not individually significant, as the encounters in a cluster are high speed encounters. A sum of such encounters, however, can lead to effects such as galaxy 'harassment', which have been shown to be able to drive morphological transformations from late types (spirals) to early types (spheroids).

The group&field environment

is of low gas density, except in the very vicinity of giant galaxies or in compact groups. The gas in groups has probably been torn from the group galaxies during their interactions. This globally low-density environment can therefore not be the cause of ram pressure stripping, or large ISM-ICM interface shocks, except for very close encounters. The tidal effects are also globally low, but close encounters between individual galaxies at low relative velocities are much more effective at perturbing the galaxies involved than the low-level interactions in clusters.It has been shown that high surface brightness (HSB) and low surface brightness (LSB) disk galaxies that are satellites of a massive parent galaxy might become the bright dEs and the low surface brightness dEs (sometimes called dwarf spheroidals dSph), respectively, through their tidal interactions with the parent galaxy. The structural differences of the HSB and LSB galaxies dictate their different evolution/disruption paths leading to the resulting dE-like objects with exponential stellar density profiles.