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A comparison of spectral indices for cluster members shows large differences in the galaxy populations of the two systems. RXCJ0014.3-3022 shows a larger fraction of actively star-forming systems, with more extreme regimes (i.e., higher values of W0[OII]), while RXCJ2308.3-0211 shows a population typical of well evolved clusters,

with higher values ofD4000 and an overall passively evolving population.

3.6 Discussion

regime around R200 along its filamentary structure. This increase is concentrated

in a very thin region acrossR200 and confirms our previous results from photometry

(see Braglia et al. (2007)). Along the filaments and just inside R200, we find a pop-

ulation of relatively young systems that are no more forming stars. This suggests that these galaxies could be what has been classified as passive spirals, i.e. stellar disks largely depleted of gas. These could become the S0’s found in cluster cores, in agreement with our infall interpretation. Indeed, as galaxies approach the cluster, a sudden increase in their intrinsic star formation rate can be detected via their [OII] emission. As these galaxies enter the cluster, interaction with the denser ICM can lead to gas loss via ram-pressure stripping, leaving these galaxies with an intermedi- ate age population of stars without the fuel needed to trigger newer star formation. This is also supported by the radial trend of spectral indices, that confirms in this cluster a rapid transformation in the characteristics of galaxies across R200. While

below this scale cluster galaxies are quite typical of a cluster environment, beyond it a sudden lack of passive systems and a rapid increase in the star formation activity is noticed.

Part of the UV-detected objects show significant star formation activity (i.e., large values of W0[OII]) and accordingly very blue (FUV-V) colour. Also, many objects,

both with UV detection and without, lie in a region of the diagram expected to be populated mainly by truncated systems, i.e. galaxies where the star formation was abruptly stopped on short timescales at an intermediate age (less than 7 Gyrs, marked in Fig. 3.12 with a dashed line; cfr. Moran et al. (2007)). These objects still present relatively smallD4000 values (<1.3), but no more significant UV emission, and have been classified as passive spirals. Conversely, the upper right part of the diagram, i.e. the region where passive galaxies are expected to lie, is not heavily populated. This is the region where galaxies end while following a natural, undisturbed evolution, with their gas being progressively consumed in timescales of several Gyrs. These galaxies follow the starvation (dotted) track shown in Fig. 3.12, from the lower left part of the diagram (where blue galaxies with large [OII] emission are found) to the upper right, where galaxies have turned red and have strong D4000 absorptions. All this suggests that the galaxy population of RXCJ0014.3-3022 is still relatively young, or is being rejuvenated.

This can be explained considering the effect of the ongoing merger and of the large- scale structure of this cluster. It compares well with other complementary findings. In particular, Couch et al. ((1998)) found a population of distorted, star-forming galaxies in the central region of RXCJ0014.3-3022: their results suggests that the strong tidal interactions in the cluster core can increase the star formation activity, thus rejuvenating the galaxy stellar population. At the same time, our interpretation of an increased star formation activity along the infall directions provides a natural way to extend the rejuvenation to larger cluster-centric radii.

additional population of galaxies (see Fig. 3.10). Specifically, while the largest part of galaxies follows a well defined track in the D4000 vs. (B-R) plane (with redder galaxies also having higher values ofD4000), some very red galaxies (with colour even redder than the red sequence) can be noticed with low (<1.2) values of D4000. In- deed, in the last years, mid-infrared observations withSpitzerhave revealed strongly obscured star-forming systems in clusters (see Saintonge et al. (2008) and references therein). It has been shown that Butcher-Oemler clusters and merging or interacting clusters are likely to hide an even larger fraction of luminous infrared galaxies, i.e. dusty star-forming systems. This is directly connected with the evolutionary state of the cluster, as these galaxies are mainly found in clusters assembling from the cosmic web (Tran et al. (2005); Loh et al. (2008)).

On the other hand, RXCJ2308.3-0211 appears as an undisturbed cluster. Its popu- lation of passive galaxies is rich and well established in both bright and faint systems. No remarkable interaction with the cosmic web is found, but a bridge of red galaxies connecting to two groups. This bridge is almost depleted of blue galaxies, and also the few sampled spectra from this region show no signs of active star formation.

The radial behavior also shows a well populated cluster with a dominating fraction of passively evolving galaxies and only a few systems still actively forming stars. Remarkably, while inside R200 the galaxy population is well mixed with respect to

luminosity, beyond R200 a luminosity segregation is seen in the radial behavior (see

Fig. 3.13). This suggests that in this field, massive galaxies are already following a passive evolution even outside the cluster. This, together with the cluster relaxed morphology, suggests an indeed old system, whose galaxy population could have formed earlier than RXCJ0014.3-3022 and evolved relatively undisturbed.

The (FUV-V) vs. D4000 diagram for RXCJ2308.3-0211 (see Fig. 3.12) also reveals an older system. Not many galaxies are detected in UV, neither show intense star formation activity from W0([OII]). The few detected objects are located in the

outer regions of the cluster and are loosely scattered, not being directly related to the few blue overdensities found in the density maps. The largest part of the galaxies, both detected and undetected in UV, lie in regions of the diagram that suggest an undisturbed evolution towards passively evolving systems. The upper right part of the diagram is indeed largely populated, while the lower left, where young and active systems are found, is almost completely empty. Still, a non negligible fraction of objects lies well separated in the upper left region, where truncated systems are found. Although only one object is actually detected in this region, it suggests that also in this cluster a population of truncated systems is found. Overall, this cluster shows an older galactic population, with no detectable blue objects and lots of passively evolving systems. RXCJ2308.3-0211 is a cool core cluster, which suggests that the cluster was globally undisturbed in the last Gyrs: such a kind of system is indeed expected to be mainly populated by passive galaxies, whose star formation history was relatively undisturbed.