Q1We present a new sample of galaxy groups identified in the Sloan Digital Sky Survey Data Release 3. Following previous works, we use the well-tested friends-of-friends algorithm developed by Huchra and Geller, which takes into account the number density variation due to the apparent magnitude limit of the galaxy catalog. To improve the identification, we implement a procedure to avoid the artificial merging of small systems in high-density regions and then apply an iterative method to recompute the group centers position. As a result, we obtain a new catalog with 10,864 galaxy groups with at least four members. The final group sample has a mean redshift of 0.1 and a median velocity dispersion of 230 km s À1 .

We construct a galaxy groups catalogue from the public 100-K data release of the 2dF Galaxy Redshift Survey. The group identification is carried out using a slightly modified version of the group-finding algorithm developed by Huchra & Geller. Several tests using mock catalogues allow us to find the optimal conditions to increase the reliability of the final group sample. A minimum number of four members, an outer number density enhancement of 80 and a linking radial cut-off of 200 km s −1 are the best obtained values from the analysis. Using these parameters, approximately 90 per cent of groups identified in real space have a redshift space counterpart. On the other hand, the level of contamination in redshift space reaches 30 per cent, including ∼6 per cent of artificial groups and ∼24 per cent of groups associated with binaries or triplets in real space. The final sample comprises 2209 galaxy groups covering the sky region described by Colless et al. spanning over the redshift range of 0.003 z 0.25 with a mean redshift of 0.1.

Using galaxy groups identified in the Fourth Data Release of the Sloan Digital Sky Survey (SDSS), we compute the luminosity function for several subsamples of galaxies in groups. In all cases, the luminosity functions are well described by Schechter functions, down to the faintest magnitudes we probe, M0.1 r − 5 log(h) ∼ −16. For the general luminosity function of galaxies in groups in the five SDSS bands, we observe that the characteristic magnitude is brighter in ∼ 0.5 magnitudes compared to those obtained for field galaxies by Blanton et al. (2003b). Even when the observed faint end slope is steeper in galaxy groups, it is statistically comparable with the field value. We analyze the dependence of the galaxy luminosity function with system masses finding two clear trends: a continuous brightening of the characteristic magnitude and a steepening of the faint end slope as mass increases. The results in 0.1 g, 0.1 r, 0.1 i and 0.1 z bands show the same behavior. Using the u − r color to split the galaxy sample into red and blue galaxies, we show that the changes observed as a function of the system mass are mainly seen in the red, passively evolving, galaxy population, while the luminosities of blue galaxies remain almost unchanged with mass. Finally, we observe that groups having an important luminosity difference between the two brightest galaxies of a system show a steeper faint end slope than the other groups. Our results can be interpreted in terms of galaxy mergers as the main driving force behind galaxy evolution in groups.

We perform an exhaustive analysis of the luminosities of galaxies in groups identified in the Sloan Digital Sky Survey (SDSS) Data Release 7. Our main purpose is to perform a detailed study of the Schechter luminosity function parameters: the characteristic absolute magnitude and the faint‐end slope, as a function of group virial mass in order to quantify their dependence on environment. We analyse the trends of the Schechter parameters as a function of group mass for different photometric bands, galaxy populations, galaxy positions within the groups and the group surrounding large‐scale density. We find that the characteristic magnitude brightens and the faint‐end slope becomes steeper as a function of mass in all SDSS photometric bands, in agreement with previous results. From the analysis of different galaxy populations, we observe that different methods to split galaxy populations, based on the concentration index or the colour–magnitude diagram, produce quite different behaviours in the luminosity trends, mainly for the faint‐end slope. These discrepancies and the trends with mass mentioned above are explained when analysing the luminosity function of galaxies classified simultaneously according to their concentrations and colours. We find that only the red spheroids have a luminosity function that strongly depends on group mass. Late‐type galaxies, whether blue or red, have luminosity functions that do not depend on group mass. The intrinsic change in the luminosity function of spheroids and the varying number contributions of the different types explain the overall trend of the luminosity function with group mass. On the other hand, dividing the galaxy members in the inner and outer regions of the groups do not introduce a significant difference in the Schechter parameter trends, except for the characteristic absolute magnitude in the high group virial mass range () which is an indication of luminosity segregation in massive groups. Finally, we also analyse the possible influence of the large‐scale surrounding environment on the luminosity function of group galaxies. We find that galaxies inhabiting groups at low‐density regions experience more pronounced variations on the Schechter parameters as a function of groups mass, while galaxies in groups at high‐density regions show an almost constant behaviour. We discuss the possible implications of our findings in the galaxy evolution scenario.

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 estimate the fraction of star‐forming galaxies in a catalogue of groups, constructed from the 2dF Galaxy Redshift Survey by Merchán & Zandivarez. We use the η spectral type parameter of galaxies and subdivide the sample of galaxies in groups into four types depending on the values of the η parameter following Madgwick et al. We obtain a strong correlation between the relative fraction of galaxies with high star formation and the parent group virial mass. We find that even in the environment of groups with low virial mass M∼1013 M⊙ the star formation of their member galaxies is significantly suppressed. The relation between the fraction of early‐type galaxies and the group virial mass obeys a simple power law spanning over three orders of magnitude in virial mass. Our results show quantitatively the way that the presence of galaxies with high star formation rates is inhibited in massive galaxy systems.

Several statistics are applied to groups and galaxies in groups in the 2 • Field Galaxy Redshift Survey. First, we estimate the luminosity functions for different subsets of galaxies in groups. The results are well fitted by a Schechter function with parameters M * − 5 log(h) = −19.90 ± 0.03 and α = −1.13 ± 0.02 for all galaxies in groups, which is quite consistent with the results of Norberg et al. for field galaxies. When considering the four different spectral types defined by Madgwick et al. we find that the characteristic magnitude is typically brighter than in the field. We also observe a steeper value, α = −0.76 ± 0.03, of the faint end slope for low star-forming galaxies when compared with the corresponding field value. This steepening is more conspicuous, α = −1.10 ± 0.06, for those galaxies in more massive groups (M 10 14 h −1 M ) than that obtained in the lower-mass subset, α = −0.71 ± 0.04 (M < 10 14 h −1 M ).Secondly, we compute group total luminosities using the prescriptions of Moore, Frenk & White. We define a flux-limited group sample using a new statistical tool developed by Rauzy. The resulting group sample is used to determine the group luminosity function and we find a good agreement with previous determinations and semi-analytical models.Finally, the group mass function for the flux-limited sample is derived. An excellent agreement is obtained when comparing our determination with analytical predictions over two orders of magnitude in mass.

Aims. In this work we present an algorithm to identify compact groups (CGs) that closely follows Hickson's original aim and that improves the completeness of the samples of compact groups obtained from redshift surveys. Methods. Instead of identifying CGs in projection first and then checking a velocity concordance criterion, we identify them directly in redshift space using Hickson-like criteria. The methodology was tested on a mock lightcone of galaxies built from the outputs of a recent semi-analytic model of galaxy formation run on top of the Millennium Simulation I after scaling to represent the first-year Planck cosmology. Results. The new algorithm identifies nearly twice as many CGs, no longer missing CGs that failed the isolation criterion because of velocity outliers lying in the isolation annulus. The new CG sample picks up lower surface brightness groups, which are both looser and with fainter brightest galaxies, missed by the classic method. A new catalogue of compact groups from the Sloan Digital Sky Survey is the natural corollary of this study. The publicly available sample comprises 462 observational groups with four or more galaxy members, of which 406 clearly fulfil all the compact group requirements: compactness, isolation, and velocity concordance of all of their members. The remaining 56 groups need further redshift information of potentially contaminating sources. This constitutes the largest sample of groups that strictly satisfy all the Hickson's criteria in a survey with available spectroscopic information.