Spitzer Observations of Young Red Quasars

TL;DR

This study uses Spitzer infrared data to analyze dust-reddened quasars, revealing their evolutionary stage, accretion rates, and evidence of outflows, supporting models of galaxy and black hole co-evolution.

Contribution

First mid-infrared spectral analysis of a sample of dust-reddened quasars, linking their properties to galaxy evolution models and black hole growth.

Findings

Many quasars have high accretion rates near the Eddington limit.

Objects tend to fall below the local black hole mass–bulge luminosity relation.

Evidence of outflows in some quasars indicates quasar-driven winds.

Abstract

We present mid-infrared spectra and photometry of thirteen redshift 0.4<z<1 dust-reddened quasars obtained with Spitzer IRS and MIPS. We compare properties derived from their infrared spectral energy distributions (intrinsic AGN luminosity and far-infrared luminosity from star formation) to the host luminosities and morphologies from HST imaging, and black hole masses estimated from optical and/or near-infrared spectroscopy. Our results are broadly consistent with models in which most dust reddened quasars are an intermediate phase between a merger-driven starburst triggering a completely obscured AGN, and a normal, unreddened quasar. We find that many of our objects have high accretion rates, close to the Eddington limit. These objects tend to fall below the black hole mass -- bulge luminosity relation as defined by local galaxies, whereas most of our low accretion rate objects are slightly above the local relation, as typical for normal quasars at these redshifts. Our observations are therefore most readily interpreted in a scenario in which galaxy stellar mass growth occurs first by about a factor of three in each merger/starburst event, followed sometime later by black hole growth by a similar amount. We do not, however, see any direct evidence for quasar feedback affecting star formation in our objects, for example in the form of a relationship between accretion rate and star formation. Five of our objects, however, do show evidence for outflows in the OIII 5007 Angstrom emission line profile, suggesting that the quasar activity is driving thermal winds in at least some members of our sample.