Aims. We study galaxy pair samples selected from the Sloan Digital Sky Survey (SDSS-DR7) and we perform an analysis of minor and major mergers with the aim of investigating the dependence of galaxy properties on interactions. Methods. We build a galaxy pair catalog requiring r p < 25 kpc h −1 and ΔV < 350 km s −1 within redshift z < 0.1. By visual inspection of SDSS images we remove false identifications and we classify the interactions into three categories: pairs undergoing merging, M; pairs with evident tidal features, T ; and non disturbed, N. We also divide the pair sample into minor and major interactions according to the luminosity ratio of the galaxy members. We study star formation activity through colors, the 4000 Å break, and star formation rates. Results. We find that ∼10% of the pairs are classified as M. These systems show an excess of young stellar populations as inferred from the D n (4000) spectral index, colors, and star formation rates of the member galaxies, an effect which we argue is directly related to the ongoing merging process. We find ∼30% of the pairs exhibiting tidal features (T pairs) with member galaxies showing evidence of old stellar populations. This can be associated either to the disruptive effect of some tidal interactions, or to the longer time-scale of morphological disturbance with respect to the bursts of the tidal induced star formation. Regardless of the color distribution, we find a prominent blue peak in the strongest mergers, while pairs with tidal signs under a minor merger show a strong red peak. Therefore, our results show that galaxy interactions are important in driving the evolution of galaxy bimodality. By adding stellar masses and star formation rates of the two members of the pairs, we explore the global efficiency of star formation of the pairs as a whole. We find that, at a given total stellar mass, major mergers are significantly more efficient (a factor ≈2) in forming new stars, with respect to both minor mergers or a control sample of non-interacting galaxies. We conclude that the characteristics of the interactions and the ratio of luminosity galaxy pair members involved in a merger are important parameters in setting galaxy properties.

We present a catalogue of galaxy photometric redshifts and k‐corrections for the Sloan Digital Sky Survey Data Release 7 (SDSS‐DR7), available on the World Wide Web. The photometric redshifts were estimated with an artificial neural network using five ugriz bands, concentration indices and Petrosian radii in the g and r bands. We have explored our redshift estimates with different training sets, thus concluding that the best choice for improving redshift accuracy comprises the main galaxy sample (MGS), the luminous red galaxies and the galaxies of active galactic nuclei covering the redshift range 0 < z≤ 0.3. For the MGS, the photometric redshift estimates agree with the spectroscopic values within rms = 0.0227. The distribution of photometric redshifts derived in the range 0 < zphot≤ 0.6 agrees well with the model predictions. k‐corrections were derived by calibration of the k‐correct_v4.2 code results for the MGS with the reference‐frame (z= 0.1) (g−r) colours. We adopt a linear dependence of k‐corrections on redshift and (g−r) colours that provide suitable distributions of luminosity and colours for galaxies up to redshift zphot= 0.6 comparable to the results in the literature. Thus, our k‐correction estimate procedure is a powerful, low computational time algorithm capable of reproducing suitable results that can be used for testing galaxy properties at intermediate redshifts using the large SDSS data base.

We analyse a sample of 71 triplets of luminous galaxies derived from the work of O'Mill et al. (2012). We compare the properties of triplets and their members with those of control samples of compact groups, the ten brightest members of rich clusters, and galaxies in pairs.The triplets are restricted to have members with spectroscopic redshifts in the range 0.01 z 0.14 and absolute r-band luminosities brighter than M r = −20.5. For these member galaxies, we analyse the stellar mass content, the star formation rates, the D n (4000) parameter and (M g − M r ) colour index. Since galaxies in triplets may finally merge in a single system, we analyse different global properties of these systems. We calculate the probability that the properties of galaxies in triplets are strongly correlated. We also study total star formation activity and global colours, and define the triplet compactness as a measure of the percentage of the system total area that is filled by the light of member galaxies. We concentrate in the comparison of our results with those of compact groups to assess how the triplets are a natural extension of these compact systems.Our analysis suggest that triplet galaxy members behave similarly to compact group members and galaxies in rich clusters. We also find that systems comprising three blue, starforming, young stellar population galaxies (blue triplets) are most probably real systems and not a chance configuration of interloping galaxies. The same holds for triplets composed by three red, non star-forming galaxies, showing the correlation of galaxy properties in these systems. From the analysis of the triplet as a whole, we conclude that, at a given total stellar mass content, triplets show a total star formation activity and global colours similar to compact groups. However, blue triplets show a high total star formation activity with a lower stellar mass content. From an analysis of the compactness parameter of the systems we find that light is even more concentrated in triplets than in compact groups.We propose that triplets composed by three luminous galaxies, should not be considered as an analogous of galaxy pairs with a third extra member, but rather they are a natural extension of compact groups.

We analyse optical and radio properties of radio galaxies detected in the Sloan Digital Sky Survey. The sample of radio sources is selected from the catalogue of Kimball & Ivezić with flux densities at 325, 1400 and 4850 MHz, using Westerbork Northern Sky Survey, NRAO VLA Sky Survey and Green Bank 6 cm radio surveys and from flux measurements at 74 MHz taken from Very Large Array Low‐Frequency Sky Survey (Cohen et al. 2006). We study radio galaxy spectral properties using radio colour–colour diagrams and find that our sample follows a single power law from 74 to 4850 MHz. The spectral index versus spectroscopic redshift relation is not significant for our sample of radio sources. We analyse a subsample of radio sources associated with clusters of galaxies identified from the maxBCG catalogue and find that about 40 per cent of radio sources with ultrasteep spectra (USS; , where ) are associated with galaxy clusters or groups of galaxies. We construct a Hubble diagram of USS radio sources in the optical r band up to and compare our results with those for normal galaxies selected from different optical surveys and find that USS radio sources are around as luminous as the central galaxies in the maxBCG cluster sample and typically more than 4 mag brighter than normal galaxies at . We study correlations between spectral index, richness and luminosity of clusters associated with radio sources. We find that USS at low redshift are rare, most of them reside in regions of unusually high ambient density, such of those found in rich cluster of galaxies. Our results also suggest that clusters of galaxies associated with steeper than the average spectra have higher richness counts and are populated by luminous galaxies in comparison with those environments associated to radio sources with flatter than the average spectra. A plausible explanation for our results is that radio emission is more pressure confined in higher gas density environments such as those found in rich clusters of galaxies and as a consequence radio lobes in rich galaxy clusters will expand adiabatically and lose energy via synchrotron and inverse Compton losses, resulting in a steeper radio spectra.