LATEST RESULTS:

►   Galaxy pre-processing in two z=0.4 CLASH-VLT clusters of galaxies (Olave-Rojas et al. 2018):

The central region of MACSJ0416.1-2403 at z=0.4, one of the clusters in the CLASH-VLT survey that was used for this work.

Imaging and spectroscopic observations of two clusters of galaxies at z=0.4 in the CLASH-VLT survey, MACSJ0416.1-2403 and MACSJ1206.2-0847, are used to uncover their substructures and study the properties of cluster members. The number of substructure found in each cluster seems to correlate with their dynamical state, in the sense that the cluster undergoing merging (MACSJ0416) has a larger number of substrcutures. By using the color distribution of galaxies, we are able to compute galaxy fractions for red (passive), green (intermediate), and blue (star-froming) members. These fractions are obtained as a function of galaxy brightness (stellar mass) and distance from the center of either cluster or substructure. Similar fractions are estimated for a sample of coeval field galaxies. Our results show that, beyond r₂₀₀, substructures have a noticeable effect on color fractions when compared with clusters, suggesting that the quenching of the star formation in galaxies may have started in group-sized halos, consistent with pre-processing.

►   The quenching history of cluster galaxies in the EAGLE simulation (Pallero et al. 2019):

Spatial distribution of galaxies for one of the clusters in the EAGLE simulation. The distribution of galaxies with the star formation supressed outside (pre-processed) and inside (post-processed) the BCG's halo are shown.

In order to study the role played by the local environment in the star-formation quenching of galaxies, we have explored the largest of the EAGLE hydro-dynamical simulations trying to trace the quenching history of galaxies in the 10 most massive clusters in it. We follow the z=0 brightest cluster galaxy (BCG) since z=20 and we use its halo to define pre-processing. We consider galaxies as pre-processed those that (a) suffered the strongest drop of SSFR or (b) reached a SSFR value of 10^-11 yr^-1 in a halo different from that of the BCG at any given redshift. In case (a), we observe that pre-processing occurs at any redshift down to z=0, and that the strongest SSFR drop takes place preferentially in dark matter halos of about 10¹¹-10¹² M_☉. In case (b), galaxies that reach the SSFR=10^-11 yr^-1 level do so preferentially in more massive halos of about 10¹³-10¹⁴ M_☉. We conclude that halo mass does play an important role in the quenching of galaxies since an early epoch of cluster assembly.