We present the X-ray and optical properties of the galaxy groups selected in the Chandra X-Boötes survey. We used follow-up Chandra observations to better define the group sample and their X-ray properties. Group redshifts were measured from the AGES spectroscopic data. We used photometric data from the NOAO Deep Wide Field Survey (NDWFS) to estimate the group richness (N gals ) and the optical luminosity (L opt ). Our final sample comprises 32 systems at z < 1.75, with 14 below z = 0.35. For these 14 systems we estimate velocity dispersions (σ gr ) and perform a virial analysis to obtain the radii (R 200 and R 500 ) and total masses (M 200 and M 500 ) for groups with at least five galaxy members. We use the Chandra X-ray observations to derive the X-ray luminosity (L X ). We examine the performance of the group properties σ gr , L opt and L X , as proxies for the group mass. Understanding how well these observables measure the total mass is important to estimate how precisely the cluster/group mass function is determined. Exploring the scaling relations built with the X-Boötes sample and comparing these with samples from the literature, we find a break in the L X -M 500 relation at approximately. Thus, the mass-luminosity relation for galaxy groups cannot be described by the same power law as galaxy clusters. A possible explanation for this break is the dynamical friction, tidal interactions and projection effects which reduce the velocity dispersion values of the galaxy groups. By extending the cluster luminosity function to the group regime, we predict the number of groups that new X-ray surveys, particularly eROSITA, will detect. Based on our cluster/group luminosity function estimates, eROSITA will identify ∼1800 groups (L X = 10 41 − 10 43 ergs s −1 ) within a distance of 200 Mpc. Since groups lie in large scale filaments, this group sample will map the large scale structure of the local universe.

We investigated the typical environment and physical properties of "red discs" and "blue bulges", comparing those to the "normal" objects in the blue cloud and red sequence. Our sample is composed of cluster members and field galaxies at z ≤ 0.1, so that we can assess the impact of the local and global environment. We find that disc galaxies display a strong dependence on environment, becoming redder for higher densities. This effect is more pronounced for objects within the virial radius, being also strong related to the stellar mass. We find that local and global environment affect galaxy properties, but the most effective parameter is stellar mass. We find evidence for a scenario where "blue discs" are transformed into "red discs" as they grow in mass and move to the inner parts of clusters. From the metallicity differences of red and blue discs, and the analysis of their star formation histories, we suggest the quenching process is slow. We estimate a quenching time scale of ∼ 2−3 Gyr. We also find from the sSFR−M * plane that "red discs" gradually change as they move into clusters. The "blue bulges" have many similar properties than "blue discs", but some of the former show strong signs of asymmetry. The high asymmetry "blue bulges" display enhanced recent star formation compared to their regular counterparts. That indicates some of these systems may have increased their star formation due to mergers. Nonetheless, there may not be a single evolutionary path for these blue early-type objects.