Mid-IR properties of Seyferts: Spitzer/IRS spectroscopy of the IRAS 12micron Seyfert sample

TL;DR

This study analyzes the mid-infrared spectral properties of a complete sample of local Seyfert galaxies using Spitzer/IRS data, revealing differences in continuum slopes, hot dust features, and PAH emissions, and proposing a new method to estimate AGN contributions.

Contribution

It provides detailed mid-IR spectral analysis of the IRAS 12-micron Seyfert sample and introduces a novel approach to separate AGN and star formation components in galaxy emission.

Findings

Seyfert 1s have a flatter 15-30 micron continuum slope than Seyfert 2s.

Approximately 32% of Seyfert 1s show a 20-micron peak indicating hot dust.

PAH equivalent width decreases with dust temperature but shows no difference between Seyfert types.

Abstract

We performed an analysis of the mid-infared properties of the 12micron Seyfert sample, a complete unbiased 12micron flux limited sample of local Seyfert galaxies selected from the IRAS Faint Source Catalog, based on low resolution spectra obtained with the Infrared Spectrograph (IRS) on-board Spitzer Space Telescope. A detailed presentation of this analysis is dicussed in Wu et al. (2009). We find that on average, the 15-30micron slope of the continuum is -0.85+/-0.61 for Seyfert 1s and -1.53+/-0.84 for Seyfert 2s, and there is substantial scatter in each type. Moreover, nearly 32% of Seyfert 1s, and 9% of Seyfert 2s, display a peak in the mid-infrared spectrum at 20micron, which is attributed to an additional hot dust component. The Polycyclic Aromatic Hydrocarbon (PAH) equivalent width decreases with increasing dust temperature, as indicated by the global infrared color of the host galaxies. However, no statistical difference in PAH equivalent width is detected between the two Seyfert types, 1 and 2, of the same bolometric luminosity. Finally, we propose a new method to estimate the AGN contribution to the integrated 12micron galaxy emission, by subtracting the "star formation" component in the Seyfert galaxies, making use of the tight correlation between PAH 11.2micron luminosity and 12micron luminosity for star forming galaxies.