Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars

TL;DR

This study uses Hubble and Spitzer data to reveal significant optically-obscured star formation in the host galaxies of some type-2 quasars, highlighting complex extinction processes involving dusty disks and the torus.

Contribution

It provides new multi-wavelength imaging and spectral analysis of type-2 quasar hosts, demonstrating the presence of obscured star formation and linking host galaxy properties to AGN obscuration.

Findings

Star formation rates range from 3 to 90 solar masses per year.

Host galaxies show signs of disturbance and dusty, star-forming disks.

Obscuration arises from both the dusty torus and extended star-forming disks.

Abstract

We present Hubble Space Telescope images, and spectral energy distributions from optical to infrared wavelengths for a sample of six 0.3<z<0.8 type-2 quasars selected in the mid-infrared using data from the Spitzer Space Telescope. All the host galaxies show some signs of disturbance. Most seem to possess dusty, star-forming disks. The disk inclination, estimated from the axial ratio of the hosts, correlates with the depth of the silicate feature in the mid-infrared spectra, implying that at least some of the reddening towards the AGN arises in the host galaxy. The star formation rates in these objects, as inferred from the strengths of the PAH features and far-infrared continuum, range from 3-90 Msun/yr, but are mostly much larger than those inferred from the [OII]3727 emission line luminosity, due to obscuration. Taken together with studies of type-2 quasar hosts from samples selected in the optical and X-ray, this is consistent with previous suggestions that two types of extinction processes operate within the type-2 quasar population, namely a component due to the dusty torus in the immediate environment of the AGN, and a more extended component due to a dusty, star forming disk.