We explore properties of close galaxy pairs and merging systems selected from the SDSS-DR4 in different environments with the aim to assess the relative importance of the role of interactions over global environmental processes. For this purpose, we
perform a comparative study of galaxies with and without close companions as a function of local density and host-halo mass, carefully removing sources of possible biases. We find that at low and high local density environments, colours and morphologies of close galaxy pairs are very similar to those of isolated galaxies. At intermediate densities, we detect significant differences, indicating that close pairs could have experienced a more rapid transition onto the red sequence than isolated galaxies. The presence of a correlation between colours and morphologies indicates that the physical mechanism responsible for the colour transformation also operates changing galaxy morphologies. Regardless of dark matter halo mass, we show that the percentage of red galaxies in close pairs and in the control sample are comparable at low and high local density environments. However, at intermediate local densities, the gap in the red fraction between close pairs and the control galaxies increases from ~10% in low mass haloes up to ~50% in the most massive ones. Our findings suggest that in intermediate density environments galaxies are efficiently pre-processed by close encounters and mergers before entering higher local density regions. (Abridge)
Several observational works have attempted to isolate the effects of galaxy interactions by comparing galaxies in pairs with isolated galaxies. However, different authors have proposed different ways to build these so-called control samples (CS). B
y using mock galaxy catalogues of the SDSS-DR4 built up from the Millennium Simulation, we explore how the way of building a CS might introduce biases which could affect the interpretation of results. We make use of the fact that the physics of interactions is not included in the semianalytic model, to infer that any difference between the mock control and pair samples can be ascribed to selection biases. Thus, we find that galaxies in pairs artificially tend to be older and more bulge-dominated, and to have less cold gas and different metallicities than their isolated counterparts. Also because of a biased selection, galaxies in pairs tend to live in higher density environments, and in haloes of larger masses. We find that imposing constraints on redshift, stellar masses and local densities diminishes the selection biases by ~70%. Based on these findings, we suggest observers how to build an unique and unbiased CS in order to reveal the effect of galaxy interactions.
