Mid-infrared spectra of optically selected type 2 quasars

TL;DR

This study presents Spitzer mid-infrared spectra of twelve optically selected type 2 quasars, revealing diverse spectral features, high star formation rates, and correlations between spectral features and optical classifications, suggesting evolutionary links and obscuration geometries.

Contribution

It provides the first detailed mid-infrared spectral analysis of a diverse sample of optically selected type 2 quasars, highlighting spectral diversity and correlations with optical properties.

Findings

Diverse spectral features observed, from featureless to strong PAH and silicate absorption.

High median star-formation luminosity (~5x10^11 L_sun), indicating active host galaxy star formation.

Strong correlation between 10 micron feature appearance and optical classification, especially in luminous quasars.

Abstract

Type 2 quasars are luminous Active Galactic Nuclei whose central engines are seen through large amounts of gas and dust. We present Spitzer spectra of twelve type 2 quasars selected on the basis of their optical emission line properties. Within this sample, we find a surprising diversity of spectra, from those that are featureless to those showing strong PAH emission, deep silicate absorption at 10 micron, hydrocarbon absorption, high-ionization emission lines and H_2 rotational emission lines. About half of the objects in the sample are likely Compton-thick, including the two with the deepest Si absorption. The median star-formation luminosity of the objects in our sample measured from the strength of the PAH features is 5x10^11 L_sun, much higher than for field galaxies or for any other AGN sample, but similar to other samples of type 2 quasars. This suggests an evolutionary link between obscured quasars and peak star formation activity in the host galaxy. Despite the high level of star formation, the bolometric output is dominated by the quasar in all cases. For a given strength of 10 micron Si absorption, ULIRGs are significantly colder than are type 2 quasars (their F_nu[14.5 micron]/F_nu[27.5 micron] ratio is 0.5 dex lower), perhaps reflecting different obscuration geometries in these sources. We find that the appearance of the 10 micron feature (i.e., whether it shows in emission or in absorption) is well-correlated with the optical classification in type 1 and type 2 quasars, contrary to some models of clumpy obscuration. Furthermore, this correlation is significantly stronger in quasars (L_bol>10^45 erg/s) than it is in Seyfert galaxies (L_bol<<10^45 erg/s).