The behaviour of the relative fraction of galaxies with different spectral types in groups is analysed as a function of projected local galaxy density and the group‐centric distance. The group sample was taken from the 2dF Group Galaxy Calatogue constructed by Merchán & Zandivarez. Our group sample was constrained to have a homogeneous virial mass distribution with redshift. Galaxies belonging to this group sample were selected in order to minimize possible biases, such as preferential selection of high‐luminosity objects. We find a clear distinction between high virial mass groups (MV≳ 1013.5 M⊙) and the less massive ones. While the massive groups show a significant dependence of the relative fraction of low star formation galaxies on local galaxy density and group‐centric radius, groups with lower masses show no significant trends. We also cross‐correlate our group subsample with the previously identified clusters, finding that this sample shows a very similar behaviour to that observed in the high virial mass group subsample.

We study the relative fraction of galaxy morphological types in clusters, as a function of the projected local galaxy density and different global parameters: cluster projected gas density, cluster projected total mass density , and reduced clustercentric distance. Since local and global densities are correlated, we have considered different tests to search for the parameters to which segregation show the strongest dependence. Also, we have explored the results of our analysis applied to the central regions of the clusters and their outskirts. We consider a sample of clusters of galaxies with temperature estimates to derive the projected mass density profile and the 500 density contrast radius (r 500 ) using the NFW model and the scaling relation respectively. The X-ray surface brightness profiles are used to obtain the projected gas density assuming the hydrostatic equilibrium model. Our results suggest that the morphological segregation in clusters is controlled by the local galaxy density in the outskirts. On the other hand, the global projected mass density, shows the strongest correlation with the fraction of morphological types in the central high density region, with a marginal dependence on the local galaxy density.

We analyze photometric data in SDSS-DR7 to infer statistical properties of faint satellites associated to isolated bright galaxies (M r < −20.5) in the redshift range 0.03 < z < 0.1. The mean projected radial number density profile shows an excess of companions in the photometric sample around the primaries, with approximately a power law shape that extends up to ≃ 700 kpc. Given this overdensity signal, a suitable background subtraction method is used to study the statistical properties of the population of bound satellites, down to magnitude M r = −14.5, in the projected radial distance range 100 < r p /kpc < 3 < R vir >. The maximum projected distance corresponds is in the range 470 − 660 kpc for the different samples. We have also considered a colour cut consistent with the observed colours of spectroscopic satellites in nearby galaxies so that distant redshifted galaxies do not dominate the statistics. We have tested the implementation of this background subtraction procedure using a mock catalogue derived from the Millenium simulation SAM galaxy catalogue based on a ΛCDM model. We find that the method is effective in reproducing the true projected radial satellite number density profile and luminosity distributions, providing confidence in the results derived from SDSS data. We find that the spatial extent of satellite systems is larger for bright, red primaries. Also, we find a larger spatial distribution of blue satellites. For the different samples analyzed, we derive the average number of satellites and their luminosity distributions down to M r = −14.5. The mean number of satellites depends very strongly on host luminosity. Bright primaries (M r < −21.5) host on average ∼ 6 satellites with M r < −14.5. This number is reduced for primaries with lower luminosities (−21.5 < M r < −20.5) which have less than 1 satellites per host. We provide Schechter function fits to the luminosity distributions of satellite galaxies where the resulting faint end slopes equal to 1.3 ± 0.2, consistent with the universal value. This shows that satellites of bright primaries lack an excess population of faint objects, in agreement with the results in the Milky Way and nearby galaxies.