Induced nuclear activity in galaxy pairs with different morphologies (E+E), (E+S) and (S+S)

TL;DR

This study examines nuclear activity in galaxy pairs using SDSS data, finding that weak interactions are not the main trigger for low-luminosity AGN, and highlighting the importance of bulges and gas reservoirs.

Contribution

It provides new insights into the role of galaxy morphology and interactions in triggering nuclear activity, challenging the Unified Model for AGN.

Findings

Approximately 48% of paired galaxies with emission lines host AGN.

AGN incidence is similar in paired and isolated galaxies, around 40-43%.

Type 1 AGN are nearly absent in galaxy pairs, conflicting with the Unified Model.

Abstract

We analysed 385 galactic spectra from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) that belong to the catalog of isolated pairs of galaxies by Karachentsev. The spectra corresponds to physical pairs of galaxies as defined by V $\leq$ 1200 Km/s and a pair separation $\leq$ 100 kpc. We search for the incidence of nuclear activity, both thermal (star-forming) and non-thermal -Active Galactic Nuclei (AGN). After a careful extraction of the nuclear spectra, we use diagnostic diagrams and find that the incidence of AGN activity is 48 \% in the paired galaxies with emission lines and 40\% for the total sample (as compared to $\sim$ 43 \% and 41\% respectively in a sample of isolated galaxies). These results remain after dissecting the effects of morphological type and galactic stellar mass (with only a small, non significant, enhancement of the AGN fraction in pairs of objects). These results suggest that weak interactions are not necessary or sufficient to trigger low-luminosity AGN. Since the fraction of AGN is predominant in early type spiral galaxies, we conclude that the role of a bulge, and a large gas reservoir are both essential for the triggering of nuclear activity. The most striking result is that type 1 galaxies are almost absent from the AGN sample. This result is in conflict with the Unified Model, and suggests that high accretion rates are essential to form the Broad Line Region in active galaxies.