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Emission lines are the most widely used indicators of ongoing star formation in the optical, because their flux is roughly proportional to the current star formation rate. Surprisingly, the evolution with redshift of the fraction of cluster galaxies with emission lines has not been properly quantified so far, although there is a trend for higher emission line fractions at high z (e.g. [30, 62]). I try to show this graphically in Fig. 1, where I have plotted the SFR per unit L* luminosity (as derived from the [OII]3727 line) at z = 0.5 and at z = 0 for field galaxies (solid line) and cluster galaxies (dotted line). At z = 0.5 this is based on the MORPHS cluster and field samples, and at z = 0 on the Dressler & Shectman spectroscopic database (Dressler et al. in prep.). The most striking result of this figure is the fact that the evolution in the clusters appears to be accelerated with respect to the field, in a similar way as found by [4] on the basis of photometric data.

Figure 1

Figure 1. Evolution of the SFR per unit L*B in the field (solid line) and cluster (dotted line).

Most works have instead focused on the clustercentric radial behaviour of the emission line properties: the mean EW([OII]) is known to decrease with radius (e.g. [63]). This mean EW is calculated including all galaxies in the clusters (also early-type galaxies) and in principle could be simply explained by the morphology-density relation. However, even for a given morphological type, the EW([OII]) distribution appears to be skewed towards lower EWs in the cluster than in the field [56, 64].

A still open and debated question is whether, before quenching star formation, the cluster environment produces also a star formation enhancement in the infalling galaxies. Evidence for this enhancement arises from the strong k+a cases, whose spectra can only be explained as post-starburst galaxies. Considering that these strong-lined case must be the youngest (observed relatively soon after truncation), and that soon they will evolve into k+a's with more moderate line strength, the fraction of post-starburst galaxies among k+a spectra is necessarily high. In principle, the starbursting progenitors of these post-starburst galaxies could simply be field starburst galaxies that have infallen into the clusters and had their star formation terminated. Whether the starbursts in the field population are sufficient to account for the k+a population observed in distant clusters is an issue requiring further study (Dressler et al. in prep.). Examples of cluster-induced starbursts have been observed in nearby clusters, where it is easier to study the star formation signatures in great detail.

No need for a star formation enhancement in clusters is found instead by looking at the [OII] or Halpha equivalent width distributions in clusters versus field and as a function of radius: no excess of emission line galaxies (as fractions of total number of galaxies) is observed, as well as no tail at high EWs (e.g. [63, 41]). However, eventual cluster-induced starbursts might have gone undetected by this kind of analysis for at least three reasons: a) a possibly short burst timescale; b) if the end-effect of the cluster is to quench star formation, the fraction of emission-line galaxies detected does not say anything about the amount of star formation ongoing in the (still) currently starforming galaxies, and c) dust could moderate the line emission, and it is expected to do so especially in those galaxies with the highest star formation rates.

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