Star formation and nuclear activity in close pairs of early-type galaxies

TL;DR

This study analyzes close pairs of early-type galaxies from SDSS to understand how interactions influence star formation and nuclear activity, revealing increased AGN activity and residual star formation during early merger stages.

Contribution

It provides new insights into the impact of galaxy interactions on star formation and AGN activity in early-type galaxy pairs, using spectral analysis and a large SDSS sample.

Findings

Increased AGN activity correlates with decreasing pair separation.

Residual star formation is elevated in close pairs regardless of interaction signs.

Interaction triggers initial star formation, followed by AGN activation and quiescence.

Abstract

We extract from the Sloan Digital Sky Survey a sample of 347 systems involving early type galaxies separated by less than 30 kpc, in projection, and 500 km/s in radial velocity. These close pairs are likely progenitors of dry mergers. The (optical) spectra is used to determine how the interaction affects the star formation history and nuclear activity of the galaxies. The emission lines (or lack thereof) are used to classify the sample into AGN, star forming or quiescent. Increased AGN activity and reduced star formation in early-type pairs that already appear to be interacting indicate that the merging process changes the nature of nebular activity, a finding that is also supported by an increase in AGN luminosity with decreasing pair separation. Recent star formation is studied on the absorption line spectra, both through principal component analysis as well as via a comparison of the spectra with composite stellar population models. We find that the level of recent star formation in close pairs is raised relative to a control sample of early-type galaxies. This excess of residual star formation is found throughout the sample of close pairs and does not correlate with pair separation or with visual signs of interaction. Our findings are consistent with a scenario whereby the first stage of the encounter (involving the outer parts of the halos) trigger residual star formation, followed by a more efficient inflow towards the centre -- switching to an AGN phase -- after which the systems are quiescent.