Spitzer-IRS high resolution spectroscopic survey of the 12 micron Seyfert galaxies: II. Results for the Complete Dataset

TL;DR

This study uses high-resolution Spitzer IRS spectra of 91 12-micron Seyfert galaxies to analyze their mid-infrared properties, classify AGN types more accurately, and derive luminosity functions, revealing differences between AGN 1 and AGN 2 populations.

Contribution

It introduces an improved AGN classification based on spectropolarimetric data and provides the first local luminosity functions for mid-IR lines and PAH features in Seyfert galaxies.

Findings

AGN 1 spectra are strongly AGN-dominated.

AGN 2 can be split into two groups, one similar to AGN 1 and another like non-Seyferts.

Detection of [NeV] lines is a reliable AGN indicator.

Abstract

We present the Spitzer 10-37um IRS high resolution (R~600) spectroscopic survey of the Seyfert galaxies of the 12 micron Galaxy Sample. The new spectra of 61 galaxies, with those already published, gives us a total of 91 12micron Seyfert galaxies observed, out of 112. We use an improved AGN classification for Seyfert galaxies: instead of the type 1 and 2 classes, we use the spectropolarimetric data to divide them into "AGN 1" and "AGN 2", where AGN 1's include all broad-line objects, including the Seyfert 2's showing hidden broad lines in polarized light, while AGN 2's contains only Seyferts with no detectable broad lines at all. We present various mid-IR observables and we find that these properties characterize the AGN 1's objects as a single family, with strongly AGN-dominated spectra. In contrast, the AGN 2's can be divided in two groups, the first one with properties similar to the AGN 1's and the second similar to the non-Seyfert galaxies, such as LINERs or starburst galaxies. We computed a semianalytical model to estimate the AGN and the starburst contributions to the mid-IR galaxy emission at 19um. We find that AGN 1 have an AGN contribution >73% and AGN 2 >45% of their total emission at 19um. The detection of [NeV] lines is an almost perfect signature of energy production by an AGN. We present mean spectra of the various AGN categories. We derive the first local luminosity functions for the mid-infrared lines and the PAH feature. No statistical difference is found in the space densities for Seyfert 1's and 2's, nor for the new classes of AGN 1's and 2's. The global output of accretion-powered galactic nuclei in the local universe is derived from the correlation between [NeV] line and the nonstellar IR continuum luminosity.