Galactic Winds across the Gas-Rich Merger Sequence II. Lyman Alpha Emission and Highly Ionized O VI and N V Outflows in Ultraluminous Infrared Galaxies

TL;DR

This study investigates the properties of highly ionized gas outflows and Lyα emission in ULIRGs across the merger sequence, revealing correlations with AGN activity and outflow velocities using HST/COS observations.

Contribution

It provides new insights into the connection between ionized outflows, Lyα emission, and AGN properties in ULIRGs, focusing on highly ionized gas tracers O VI and N V.

Findings

Lyα emission detected in 15/19 ULIRGs with blueshifted features.

Lyα emission strength correlates with AGN luminosity and bolometric fraction.

Outflows are more frequent and stronger in X-ray weak or absorbed sources.

Abstract

This paper is the second in a series aimed at examining the gaseous environments of z$\le$0.3 quasars and ultraluminous infrared galaxies (ULIRGs) as a function of AGN/host galaxy properties across the merger sequence. This second paper focuses on the Ly$\alpha$ emission and O VI and N V absorption features, tracers of highly ionized gas outflows, in ULIRGs observed with HST/COS. Ly$\alpha$ emission is detected in 15 out of 19 ULIRGs, and 12 of the 14 clear Ly$\alpha$ detections show emission with blueshifted velocity centroids and/or wings. The equivalent widths of the Ly$\alpha$ emission increase with increasing AGN luminosities and AGN bolometric fractions. The blueshifts of the Ly$\alpha$ emission correlate positively with those of the [O III] emission, where the latter traces the ionized gas outflows. The Ly$\alpha$ escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities, but they do not correlate with nebular line reddening. Among the 12 ULIRGs with good continuum signal-to-noise ratios, O VI and/or N V absorption features are robustly detected in 6 of them, all of which are blueshifted, indicative of outflows. In the combined ULIRG $+$ quasar sample, the outflows are more frequently detected in the X-ray weak or absorbed sources. The absorption equivalent widths, velocities and velocity dispersions of the outflows are also higher in the X-ray weak sources. No other strong correlations are visible between the properties of the outflows and those of the AGN or host galaxies.