Galaxy Pairs in the Sloan Digital Sky Survey - II: The Effect of Environment on Interactions

TL;DR

This study examines how galaxy interactions and merger-induced star formation vary across different environments using SDSS data, revealing that low-density regions favor star formation triggered by interactions, while high-density regions mainly show tidal distortions.

Contribution

It provides a detailed analysis of how local galaxy density influences merger activity and star formation, highlighting environmental effects on galaxy evolution.

Findings

Lower density environments have more close pairs with conducive interaction parameters.

Star formation activity triggered by interactions is prominent only in low-to-intermediate density environments.

Closest pairs in cluster centers show the highest asymmetry, indicating environmental impact on interaction outcomes.

Abstract

We use a sample of close galaxy pairs selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4) to investigate in what environments galaxy mergers occur and how the results of these mergers depend on differences in local galaxy density. The galaxies are quantified morphologically using two-dimensional bulge-plus-disk decompositions and compared to a control sample matched in stellar mass, redshift and local projected density. Lower density environments have fractionally more galaxy pairs with small projected separations (r_p) and relative velocities (Delta v), but even high density environments contain significant populations of pairs with parameters that should be conducive to interactions. Metrics of asymmetry and colour are used to identify merger activity and triggered star formation. The location of star formation is inferred by distinguishing bulge and disk colours and calculating bulge fractions from the SDSS images. Galaxies in the lowest density environments show the largest changes in star formation rate, asymmetry and bulge-total fractions at small separations, accompanied by bluer bulge colours. At the highest local densities, the only galaxy property to show an enhancement in the closest pairs is asymmetry. We interpret these results as evidence that whilst interactions (leading to tidal distortions) occur at all densities, triggered star formation is seen only in low-to-intermediate density environments. We suggest that this is likely due to the typically higher gas fractions of galaxies in low density environments. Finally, by cross-correlating our sample of galaxy pairs with a cluster catalogue, we investigate the dependence of interactions on clustercentric distance. It is found that for close pairs the fraction of asymmetric galaxies is highest in the cluster centres.