Mid-infrared Spectral Properties of Post-Starburst Quasars

TL;DR

This study analyzes mid-infrared spectra of 16 post-starburst quasars, revealing their hybrid nature with properties between ULIRGs and QSOs, and linking host galaxy type to star formation and AGN activity.

Contribution

First detailed mid-infrared spectral analysis of post-starburst quasars showing their hybrid AGN-starburst characteristics and host galaxy correlations.

Findings

Spectra show strong PAH features indicating ongoing star formation.

Mid-infrared properties are intermediate between ULIRGs and QSOs.

Host galaxy type correlates with star formation and AGN activity.

Abstract

We present Spitzer InfraRed Spectrograph (IRS) low-resolution spectra of 16 spectroscopically selected post-starburst quasars (PSQs) at z ~ 0.3. The optical spectra of these broad-lined active galactic nuclei (AGNs) simultaneously show spectral signatures of massive intermediate-aged stellar populations making them good candidates for studying the connections between AGNs and their hosts. The resulting spectra show relatively strong polycyclic aromatic hydrocarbon (PAH) emission features at 6.2 and 11.3\micron and a very weak silicate feature, indicative of ongoing star formation and low dust obscuration levels for the AGNs. We find that the mid-infrared composite spectrum of PSQs has spectral properties between ULIRGs and QSOs, suggesting that PSQs are hybrid AGN and starburst systems as also seen in their optical spectra. We also find that PSQs in early-type host galaxies tend to have relatively strong AGN activities, while those in spiral hosts have stronger PAH emission, indicating more star formation.