Dust Emission from Unobscured Active Galactic Nuclei

TL;DR

This study uses mid-infrared spectroscopy of unobscured AGNs to analyze their dusty environments, revealing the nature of the dust distribution and the presence of a compact, clumpy torus, with star formation linked to AGN luminosity.

Contribution

It provides the first detailed comparison of silicate features in unobscured AGNs with radiative transfer models distinguishing dust geometries, and confirms the presence of a clumpy torus.

Findings

Silicate features are weakly in emission, indicating specific dust properties.

The dust distribution is best modeled as compact and clumpy.

Star formation correlates with AGN luminosity but its relative contribution decreases.

Abstract

We use mid-infrared spectroscopy of unobscured active galactic nuclei (AGNs) to reveal their native dusty environments. We concentrate on Seyfert 1 galaxies, observing a sample of 31 with the Infrared Spectrograph aboard the Spitzer Space Telescope, and compare them with 21 higher-luminosity quasar counterparts. Silicate dust reprocessing dominates the mid-infrared spectra, and we generally measure the 10 and 18 micron spectral features weakly in emission in these galaxies. The strengths of the two silicate features together are sensitive to the dust distribution. We present numerical radiative transfer calculations that distinguish between clumpy and smooth geometries, which are applicable to any central heating source, including stars as well as AGNs. In the observations, we detect the obscuring ``torus'' of unified AGN schemes, modeling it as compact and clumpy. We also determine that star formation increases with AGN luminosity, although the proportion of the galaxies' bolometric luminosity attributable to stars decreases with AGN luminosity.