As the transformation picture from late-type dwarfs into dEs is still not completely understood and only qualitative, in sect. 3 we tried to shed light on the expected witnesses of differences in system parameters which allow to distinguish between two different populations of dEs. That almost all DGs can be associated with morphological types and that only a few exceptions show morphological transformations, implies that the act of mutation seems to happen rapidly and thus to be observable with only low probability.
During the approach to galaxy clusters, ram pressure should act on
dIrrs already at group conditions or in the outskirts of clusters
[56,
85].
When this process pushes the gas out of dIrrs, wouldn't one expect to
observe many head-tail structures of stellar body vs. stripped-off ISM?
Several candidates exist which, however, are characterized as BCDs with
decentered bright star-forming knots (see e.g.
[68]).
The recently detected best candidate in the rapid phase of RPS
in the Virgo cluster is VCC1217/IC4318
[21]:
A main almost old stellar body of 3 × 108
M
leaves behind a bundle of
H
and near-UV
emitting knots. Nevertheless, the gas distribution and SF progression
within some blobs are not fully understandable.
It is still, in general, unclear to what extent the ram pressure
can trigger SF by compression of the ISM.
Other BCDs are observed although their gas is already exhausted [14]. Peebles et al. [72] find a number of dIrrs with excessively high metallicity, what they interpret as the last stage of gas consumption before they reach the dE state.
On the other hand, rejuvenation of a fraction of cluster dEs seems also to occur which are found to harbour central warm gas [64]. Whether this fact is indicative of a possible re-transformation from gas-free to gaseous DGs by gas accretion is a matter of debate, since it seems impossible within cluster environments, but plausible in less dense regions where gas infall enhances SF or even triggers starbursts (see sect. 2) and should not only affect dIrrs.