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.

Aims. Results of different polarimetric campaigns at Complejo Astronómico El Leoncito (Casleo), San Juan, Argentina are presented. The aim of these campaigns was to search for objects exhibiting anomalous polarimetric properties, similar to those shown by the Ld-class asteroid (234) Barbara, among members of the same or similar taxonomic classes. Methods. The data have been obtained with Torino and CASPROF polarimeters at the 2.15 m telescope. The Torino polarimeter is an instrument that allows simultaneous measurement of polarization in five different bands, and CASPROF polarimeter is a two-hole aperture polarimeter with rapid modulation. Results. The campaigns began in 2005, and we found four new asteroids with Barbara-like polarimetric properties: the L-class objects (172) Baucis, (236) Honoria and (980) Anacostia, and the K-class asteroid (679) Pax. The polarimetric properties of the phase-polarization curves of these objects may be produced by a mixture of high-and low-albedo particles in their regolith as a result of the fragmentation of a substrate that is spectrally analog to the CO3/CV3 chondrites.

Aims. To assess the role of dense environments in galaxy interactions, we present an analysis of close galaxy pairs in groups and clusters, obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). Methods. We identified pairs that reside in groups by cross-correlating the total galaxy pair catalog with the SDSS-DR7 group catalog from Zapata et al. (2009, MNRAS, 394, 2229. We classify pair galaxies according to the intensity of interaction inferred from the morphological appearance of the optical images. We analyzed the effect of high-density environments on different classes of galaxygalaxy interactions and we studied the impact of the group global environment on pair galaxies. Results. We find that galaxy pairs are more concentrated towards the group centers than the other group galaxy members, and disturbed pairs are more likely to contain the brightest galaxy in the groups. The color-magnitude relation of pair galaxies differs significantly from that of the control sample (constructed with galaxy group members without close companions), consisting in color tails with a clear excess of extremely blue and red galaxies for merging systems. In addition, pair galaxies show a significant excess of young stellar populations with respect to galaxies in the control sample; this finding suggests that, in dense environments, strong interactions have an important effect in modifying galaxy properties. We find that the fraction of star-forming galaxies decreases toward the group center; however, galaxy pairs have a more efficient star formation activity than galaxies without a close companion. We also find that pair galaxies tend to reside in groups in low density global environments with respect to galaxies of the corresponding control sample. Blue, young stellar population galaxies are also more likely to reside in groups within low density global environments. We find that this behavior is mainly driven by galaxy-galaxy interactions, which are the fundamental physical mechanisms driving this process.