Global environmental effects versus galaxy interactions

TL;DR

This study investigates how galaxy interactions influence galaxy properties across different environments, revealing that close encounters accelerate galaxy evolution particularly in intermediate density regions, with effects varying by halo mass.

Contribution

It provides a comparative analysis of galaxy properties in different environments, highlighting the role of interactions in galaxy evolution and the influence of local density and halo mass.

Findings

Close galaxy pairs show similar properties to isolated galaxies at low and high densities.

At intermediate densities, close pairs are more likely to be red and morphologically transformed.

The impact of interactions is more pronounced in massive haloes at intermediate densities.

Abstract

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)