Evidence for Quasar Activity Triggered by Galaxy Mergers in HST Observations of Dust-reddened Quasars

TL;DR

This study uses HST images to show that dust-reddened quasars often reside in interacting host galaxies, supporting the idea that galaxy mergers trigger quasar activity and that red quasars are in an early, obscured evolutionary stage.

Contribution

It provides observational evidence linking galaxy mergers to dust-reddened quasars, highlighting a higher interaction rate compared to normal quasars and suggesting an evolutionary connection.

Findings

High fraction of interaction in dust-reddened quasars (~85%)

Weak correlation between dust reddening and host galaxy interaction

No strong dependence of merger activity on host luminosity

Abstract

We present Hubble ACS images of thirteen dust reddened Type-1 quasars selected from the FIRST/2MASS Red Quasar Survey. These quasars have high intrinsic luminosities after correction for dust obscuration (-23.5 > M_B > -26.2 from K-magnitude). The images show strong evidence of recent or ongoing interaction in eleven of the thirteen cases, even before the quasar nucleus is subtracted. None of the host galaxies are well fit by a simple elliptical profile. The fraction of quasars showing interaction is significantly higher than the 30% seen in samples of host galaxies of normal, unobscured quasars. There is a weak correlation between the amount of dust reddening and the magnitude of interaction in the host galaxy, measured using the Gini coefficient and the Concentration index. Although few host galaxy studies of normal quasars are matched to ours in intrinsic quasar luminosity, no evidence has been found for a strong dependence of merger activity on host luminosity in samples of the host galaxies of normal quasars. We thus believe that the high merger fraction in our sample is related to their obscured nature, with a significant amount of reddening occurring in the host galaxy. The red quasar phenomenon seems to have an evolutionary explanation, with the young quasar spending the early part of its lifetime enshrouded in an interacting galaxy. This might be further indication of a link between AGN and starburst galaxies.