Galaxy Pairs in the Sloan Digital Sky Survey XV: Properties of Ionised Outflows

TL;DR

This study systematically examines ionised outflows in galaxy pairs and post-mergers versus isolated galaxies, finding outflow properties depend mainly on AGN activity and luminosity, not on merger stage or environment.

Contribution

It provides the first large-scale analysis of ionised outflows across different galaxy interaction stages, highlighting the role of AGN luminosity over merger environment.

Findings

Ionised outflows are present in about 15% of the sample.

Outflow incidence and velocity increase with [OIII] luminosity.

No significant difference in outflows between merging and isolated galaxies.

Abstract

Powerful outflows are thought to play a critical role in galaxy evolution and black hole growth. We present the first large-scale systematic study of ionised outflows in paired galaxies and post-mergers compared to a robust control sample of isolated galaxies. We isolate the impact of the merger environment to determine if outflow properties depend on merger stage. Our sample contains $\sim$4,000 paired galaxies and $\sim$250 post-mergers in the local universe ($0.02 \leq z \leq 0.2$) from the SDSS DR 7 matched in stellar mass, redshift, local density of galaxies, and [OIII] $\lambda$5007 luminosity to a control sample of isolated galaxies. By fitting the [OIII] $\lambda$5007 line, we find ionised outflows in $\sim$15 per cent of our entire sample. Outflows are much rarer in star-forming galaxies compared to AGN, and outflow incidence and velocity increase with [OIII] $\lambda$5007 luminosity. Outflow incidence is significantly elevated in the optical+mid-infrared selected AGN compared to purely optical AGN; over 60 per cent show outflows at the highest luminosities ($L_{\mathrm{[OIII] \lambda5007}}$ $\gtrsim$ 10$^{42}$ erg s$^{-1}$), suggesting mid-infrared AGN selection favours galaxies with powerful outflows, at least for higher [OIII] $\lambda$5007 luminosities. However, we find no statistically significant difference in outflow incidence, velocity, and luminosity in mergers compared to isolated galaxies, and there is no dependence on merger stage. Therefore, while interactions are predicted to drive gas inflows and subsequently trigger nuclear star formation and accretion activity, when the power source of the outflow is controlled for, the merging environment has no further impact on the large-scale ionised outflows as traced by [OIII] $\lambda5007$.