An Infrared Comparison of Type-1 and Type-2 Quasars

TL;DR

This study compares the infrared properties of type-1 and type-2 quasars to test unification models, revealing differences in dust features and star formation rates that are consistent with orientation-based explanations.

Contribution

It provides detailed infrared spectral energy distributions for matched samples of type-1 and type-2 quasars, analyzing dust features and star formation to support unification schemes.

Findings

Type-1 quasars show significant 3 micron emission and silicate emission.

Type-2 quasars exhibit diverse silicate features and higher star formation rates.

Differences are consistent with orientation-based unification models.

Abstract

We model the optical to far-infrared SEDs of a sample of six type-1 and six type-2 quasars selected in the mid-infrared. The objects in our sample are matched in mid-IR luminosity and selected based on their Spitzer IRAC colors. We obtained new targeted Spitzer IRS and MIPS observations and used archival photometry to examine the optical to far-IR SEDs. We investigate whether the observed differences between samples are consistent with orientation-based unification schemes. The type-1 objects show significant emission at 3 micron. They do not show strong PAH emission and have less far-IR emission on average when compared to the type-2 objects. The SEDs of the type-2 objects show a wide assortment of silicate features, ranging from weak emission to deep silicate absorption. Some also show strong PAH features. In comparison, silicate is only seen in emission in the type-1 objects. This is consistent with some of the type-2s being reddened by a foreground screen of cooler dust, perhaps in the host galaxy itself. We investigate the AGN contribution to the far-IR emission and find it to be significant. We also estimate the star formation rate for each of the objects by integrating the modeled far-IR flux and compare this with the SFR found from PAH emission. We find the type-2 quasars have a higher average SFR than the type-1 quasars based on both methods, though this could be due to differences in bolometric luminosities of the objects. While we find pronounced differences between the two types of objects, none of them are inconsistent with orientation-based unification schemes.