3.4. Star formation in cluster galaxies

The first to notice the small spread of the colours of cluster galaxies was Baade [35] in the 30's. Such a small spread was related to the predominance of ellipticals and S0s among cluster galaxies, and the existence of a tight colour-magnitude relation, discovered by Baum [54] - see Fig. 13 - and de Vaucouleurs [133] around 1960, and refined by Visvanathan & Sandage [487] in 1977. Recently, Stanford et al. [431] confirmed the validity of the colour-magnitude relation also for distant clusters (z 0.9). They also showed that the relation is one between the mass and the metallicity of galaxies. The tightness of the colour-magnitude relation and its mild evolution with redshift indicate that most cluster ellipticals (and S0s) have formed at high redshifts, and they evolve passively through the aging of their (old) stellar populations (see, e.g., DICKINSON, these proceedings).

Figure 13. Intrinsic colour indices of old stellar systems as a function of their absolute magnitudes. The circles represent elliptical galaxies, and the dots globular clusters.

As far as cluster spirals are concerned, it was Erik Holmberg [228], in 1958, the first to notice that Virgo spirals are redder than field spirals. His result was confirmed by Chester & Roberts [104], Davies & Lewis [120] and van den Bergh [478] around 1970, and later interpreted [479, 251] as a decreased star formation rate in cluster spirals.

In 1973, Davies & Lewis [120] analyzed the HI-content of 25 Virgo galaxies and showed it to be 60 % lower than in field galaxies, on average. Three years later, van den Bergh [479] coined the term ``anemic spirals'' to indicate a class of galaxies with intermediate characteristics between normal spirals and S0s. He attributed their anemic appearance to a reduced star formation rate, probably a result of their HI-deficiency. A reduced star formation rate could also naturally explain the redder colours of Virgo spirals, an interpretation later supported by Kennicutt [251].

In following years, Davies & Lewis' result was generalized to other clusters by Sullivan and collaborators [440, 439], Giovanelli et al. [181], Chincarini et al. [106], and Giovanelli & Haynes [180]. These authors also showed that HI-deficient galaxies preferentially occur in high-density regions, i.e. the rich cluster cores. A recent update on this topic can be found in SOLANES (these proceedings).