Comprobación de la pregunta hipótesis

The classical Butcher–Oemler effect measures the fraction of blue galaxies in clusters. As ar- gued in Sect. 1.6, this may introduce a strong bias towards low-massive, star-forming galaxies. But at masses higher than our mass completeness limit, the blue sequence is rather sparse, and almost as many galaxies are in the green valley. Instead of the blue fraction, we first investigate the red fraction, i.e. the number of galaxies with aged stellar populations.

In Fig. 5.10 we show the fraction of galaxies on the red sequence at masses larger than log(M⋆/M⊙) ≥ 10.2, and within R200, as function of the cluster velocity dispersion. The scatter of the red fraction is quite large, particularly for the less massive systems, which have a small number of members. In the clusters with σ >550km/s, generally a large fraction of the galaxies are quiescent. In EDisCS, this is between 40% and 80% in all but one case. In the low-redshift SDSS sample, most massive clusters have about 80% red galaxies, although also here, some systems have about 50% blue galaxies. This is also demonstrated by the two clusters with the highest velocity dispersion: Abell 2255, which is X-ray luminous, has a “normal” mass-to-light ratio, and little substructure according to the Dressler-Shectman statistic, has a red fraction of 80%, whereas C4 DR3 2035, without strong X-ray emission, a high mass-to-light ratio inferred from the velocity dispersion, and possible substructure has a red fraction of 50%.

In individual lower-mass systems, the fraction is highly variable in the EDisCS sample. In the SDSS sample, the fraction also varies, but the clusters clearly concentrate at fred∼0.8.

The left panel Fig. 5.11 shows the red fraction of the individual clusters as function of redshift. Clearly, the locus of the SDSS clusters is at higher red fractions than the EDisCS

0 500 1000 0 0.2 0.4 0.6 0.8 1 0 500 1000 0 0.2 0.4 0.6 0.8 1

Figure 5.10: The fraction of red galaxies, at masses greater than log(M⋆/M⊙)≥10.2, and within

R200, as a function of cluster velocity dispersion σv in the EDisCS sample (left) and in the SDSS

comparison sample (right). Clusters with σ > 550km/s are shown as red symbols, groups with

σ <550km/s as blue symbols. In the EDisCS sample, the shape of the symbols indicates the redshift: systems atz <0.6 are shown as circles, those atz >0.6 as triangles. For the SDSS sample, individual error bars are shown only for the two clusters with the highest velocity dispersion; the median error bars for the other clusters are shown in the lower left (for the σ <550km/s clusters) and lower right corner(for the σ >550km/s clusters) .

clusters, for both velocity dispersion ranges. For the more massive clusters, the redshift depen- dence continues within the EDisCS sample: the higher the redshift, the lower the red fraction. We split the EDisCS galaxies into four subsamples: for each of the two redshift samples (with the division at z = 0.605), we split by the velocity dispersion of the cluster, with the division at σ > 550km/s. The SDSS galaxies are split into two samples according to the cluster velocity dispersion. For each sample, we can consider the composite cluster as the average cluster. In the right panel of Fig. 5.11 we show the red fractions of these composite clusters, and also the red fractions of the EDisCS poor groups and field samples. For the SDSS galaxies, we define the field sample to be those galaxies at distances larger than 10 Mpc from the BCG.

Despite the large scatter in the red fraction of the individual low velocity dispersion sys- tems, their average red fraction is quite similar to that of the more massive clusters, at all redshifts. The composite red fraction is largest in the SDSS clusters (about 75%), but also the EDisCS clusters have high red fractions, particularly within 0.5R200. In the SDSS sample, the field red fraction is 50%, and thus substantially lower than in the clusters. This demon- strates that the color–density relation is well in place at low redshifts. The red fraction in the EDisCS poor groups is significantly lower than in the cluster environment, as would be expected from the color-density relation. Puzzlingly, the red fractions we find in the EDisCS field sample are counter-intuitive to this expectation: since it should be the lowest density environment we probe, it should have the lowest red fraction. Instead, the red fraction in the

5.2 An alternative CMD: 4000˚A break vs. stellar mass 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 1

Figure 5.11: The fraction of red galaxies, at masses greater than log(M⋆/M⊙)≥10.2, and within

R200, as a function of redshift. In the left panel, individual clusters are shown; in the right panel,

the red fractions of the composite clusters, the field, and the poor groups are shown. Clusters with σ >550km/s are shown as red symbols, groups withσ <550km/s as blue symbols. The shape of the symbols indicates the redshift: EDisCS systems at z <0.6 are shown as circles, those atz >0.6 as triangles. SDSS systems are shown as small circles in the left panel, and as squares in the right panel. The error bars in the right panel are Poissonian.

high-z field subsample is comparable to that in clusters. At mid-z, it is identical to the poor groups. As can be seen from Fig. 5.8, many blue field galaxies lie just beyond our mass limit. The high red fraction for the high-z field may thus just be a statistical fluke. Without the distinction into mid-z and high-z samples, the field red fraction is 60%, which is still higher than in the SDSS sample.

If we rephrase the Butcher-Oemler effect such that high-redshift clusters contain fewer quiescent galaxies, then Fig. 5.11 confirms such a trend even in a mass-limited sample. At least partially, this is due to a dearth of low-mass red galaxies, as also suggested by De Lucia et al. (2007) and Stott et al. (2007) (but see Andreon 2007, for a different result, i.e. no decrease of red sequence galaxies with redshift).