The mixture of galaxy types in clusters and the field is different in the local Universe. Going to higher redshifts, the galaxy population is also changing within clusters with look-back time. This may be caused by their late assembly epoch pre-dicted by bottom-up scenarios of structure forma-tion or due to cluster-specific interacforma-tion processes.
In this thesis, the main findings of a panoramic spectroscopic campaign are presented. It has been focused in 6 clusters at z ≈0.25 (∼3 Gyr of look-back time). The spectra cover a large wavelength range, allowing to explore the galaxy properties us-ing different indicators. The observations targeted galaxies from the cluster cores to the outskirts, al-lowing to study the galaxy evolution in the interface between cluster and field.
Almost 600 spectra obtained with MOSCA at the 3.5m telescope at Calar Alto Observatory were examined, including those from previous projects.
Approximately 300 were useful for further analy-sis. They are splitted in∼150 cluster galaxies and about the same number for field galaxies. In or-der to equalize the sample only∼90 field and∼120 cluster galaxies were used for direct comparison.
Multicolor photometry has also been used to complement the spectroscopy as well as X-ray analysis from previously published studies.
This work has made use of the automatic and in-teractive algorithms which have permitted to char-acterize better the properties of the sample. State-of-the-art codes have been used to calculate abso-lute magnitudes and stellar masses.
The long wavelength range covered by the spec-tra permitted an unbiased differentiation among galaxy types.
The main findings can be summarized as follow-ing:
1. During the analysis of the redshift and spatial distribution of the galaxies in the studied fields four group candidates were serendipitiously found. One and the largest of them overlaps in its position with the cluster VMF194, resolv-ing thereby the doubts about its redshift. This large group may have contaminated the X-ray measurements of previous authors.
2. According to standard techniques most of the clusters appear to be in a virialized state with the notable exception of XDCS220. Neverthe-less, there are differences among them. Some exhibit a great deal of substructure, either evi-denced by dynamical or photometric analysis.
For example, a X-ray structure, detected by previous studies in the field of one of the clus-ters (VMF73), is likely part of it, as its posi-tion coincides with spectroscopic and photo-metrics structures and has a noticeable effect on the galaxy population of this cluster.
This difference is not only restricted to the dy-namic but also to their galaxy content. The fraction of star-forming galaxies is different among the clusters, but is not possible to es-tablish with the current data whether there is a correlation with global cluster properties.
80 Summary and conclusions
An important fraction (∼45%) of the cluster star-forming galaxies has red colors compat-ible with those typically found in ellipticals.
The abundance of those galaxies is also vary-ing among the clusters.
3. Those red “star-forming” galaxies are not composed of a homogeneous class of objects.
Some are ellipticals with low levels of AGN activity. Others appear to be obscured highly inclined objectes but otherwise normal spi-rals. These two classes are not able to fully explain the abundance of these rare objects, as some are face-one spirals whereas others appear to be bulge dominated objects. They are not preferentially located in any environ-ment. They appear to be in an intermediate stage between “normal” star-forming galaxies and passive ones, as their levels of star forma-tion activity is about one order of magnitude lower than the typical star-forming population, but still much higher than the bulk of passive galaxies.
Some authors have also found those objects at different redshifts. They are often interpreted as galaxies with a large component of old stel-lar populations and dust extinction. How-ever, it is intriguing that otherwise normal star-forming objects have the precise amount of dust and old stellar populations to make them fall into the passive sequence. It is an issue that surely warrant further investigation.
Those galaxies may also exist in the field, but they can not be identified precisely, as the er-rors associated to the measurements are larger than the scatter of the red-sequence which was used to identify them in clusters.
4. It has been found that emission line galaxies in the field and in the clusters have abnormal [O]-to-Hαratios. Possibilities suggested by previous authors were explored. High levels of AGN activity has been discarded as an im-portant cause. It has been found, in fact, that the lower chemical abundances displayed by those galaxies compared to local counterparts are possibly the main source of this peculiar-ity.
The chemical abundances are function of mass
and luminosity and are compatible with the values found by other authors. No differ-ence is detected between field and cluster star-forming galaxies.
The effect of lower metallicity does not appear to have an impact in the derived star-formation rates, although there is marginal evidence that the [O]-derived SFRs are slightly overesti-mated at large values when they are compared with HαSRFs.
5. Analyzing the distribution of star-forming galaxies, it was found that it depends strongly on environment. The star-formation activ-ity is strongly depleted at small clustercentric distances and high projected densities. At R ≈ 3Rvir and Σ5 ≈ 10 Mpc−2, the activity aproaches to typical field values. This can be seen either in the fraction or in the mean equivalent widths (which were found to cor-relate extremely well with specific star forma-tion rates).
Although, it is difficult to make direct compar-isons with published works at z∼0, the star-forming-density appear to be evolving. The trend appears to be stepper as the star-forming activity in the field is higher at z≈0.25 (35%
at z=0 versus∼55% at z≈0.25).
However, once the star-forming population is analyzed, it was found that it is similar in all environments (including the field) and there-fore, the change in the average equivalent widths is only driven by the relative abun-dance of the passive versus active population (the fraction) found in different environments.
This puts strong constraints to the possible processes responsible of the suppression of the star-formation activity, arguing against soft mechanisms of galaxy transformation.
6. Nevertheless, the field, the infall and the clus-ter galaxy populations are different. The frac-tion of star forming galaxies depends on the stellar mass, but this fraction shows a differ-ent behavior for each environmdiffer-ent. Galax-ies with higher masses are strongly suppressed in all environments, but intermediated mass ones display the largest change with few of them forming stars in the inner cluster. Dwarf
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galaxies appear unaffected between the field and the infall region.
Comparing this with published results at sim-ilar redshifts, it was found, that the galaxy population in the infall regions is indeed very
“group-like” indicating that group preprocess-ing may play an important role in the galaxy evolution in clusters. Once that some galax-ies are transformed by this environment, they enter deeper in the cluster core, which further quench the star-formation.
7. Much caution must be taken with the univer-sality of the environmental trends. At least two clusters display a very different environmen-tal distribution. In the light of the evidence, appears that cluster of galaxies are unique ob-jects whose characteristic ,and those of their galaxy populations, may depend of more sub-tle properties rather than purely processes re-lated to the mass, such as their X-ray luminos-ity. It is possible that the cluster assembly his-tory as well as the surrounding environment in large scales play an important role. This may explain the large scatter on galaxy populations cluster-to-cluster reported by several authors.
Several possible scenarios were analyzed in or-der to explain the environmental trends. It was found that starvation is unable to stop the star-formation in the necessary time-scales and unlikely would be able to reproduce the trends. It is possible that this mechanism is at work in groups as a form of preprocessing, but this is beyond of the scope of this work.
The current understanding of ram-pressure strip-ping, as well as the other strong interactions be-tween galaxies and the intracluster media, may be able to explain to a great extent the environmental trends, as their times scales match better. The main constraint for this scheme is the low density envi-ronment where the decrease of the star-formation is detected. It is still possible that ram-pressure is act-ing in filaments and groups as suggested by some authors, although this hypothesis is difficult to test here.
In that case, additional processes may be acting.
In particular, mergers are though to be effective in groups. Possible evidence of this, is found in the
fact that the star-formation activity for low mass galaxies remains unchanged between the field and the infall environment. Those galaxies are though to be sensitive to ram-pressure , but are resilient to merger. It was not possible to study their fate in the inner cluster core.
Galaxies with larger masses display the oppo-site behavior and it was detected that intermediate mass galaxies do experience a change in their star-formation activity between the field and the infall region. They are further processed in the inner core.
Galaxies at the high mass end display low level of star-formation despite their environment.
Therefore, there is still room left for mergers as an important mechanism in “pre-processing” galax-ies before they enter in the inner cluster core.
It is important to stress again that galaxies in each cluster may evolve differently as the processes may have dissimilar importance.