Evidence for Merger Remnants in Early-Type Host Galaxies of Low-Redshift QSOs

TL;DR

This study uses deep HST imaging to find that most low-redshift QSO host galaxies classified as ellipticals show signs of recent mergers, suggesting mergers may trigger QSO activity with some delay.

Contribution

It provides evidence that early-type QSO hosts have undergone recent mergers, challenging the idea they are passively evolved since high redshift.

Findings

Most hosts show fine structures like shells and tidal tails.

Merger signatures are consistent with starburst ages and QSO activity timing.

Majority of studied hosts experienced mergers within the last Gyr.

Abstract

We present results from a pilot HST ACS deep imaging study in broad-band V of five low-redshift QSO host galaxies classified in the literature as ellipticals. The aim of our study is to determine whether these early-type hosts formed at high redshift and have since evolved passively, or whether they have undergone relatively recent mergers that may be related to the triggering of the nuclear activity. We perform two-dimensional modeling of the light distributions to analyze the host galaxies' morphology. We find that, while each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the majority of them (4/5) reveal significant fine structure such as shells and tidal tails. These structures contribute between ~5% and 10% to the total V-band luminosity of each host galaxy within a region of r ~ 3 r_eff and are indicative of merger events that occurred between a few hundred Myr and a Gyr ago. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy. Our results thus support a consistent scenario in which most of the QSO host galaxies suffered mergers with accompanying starbursts that likely also triggered the QSO activity in some way, but we are also left with considerable uncertainty on physical mechanisms that might have delayed this triggering for several hundred Myr after the merger.