Herschel Far-Infrared Photometry of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. II. SPIRE Observations

TL;DR

This study uses Herschel SPIRE data to analyze the far-infrared properties of local AGN selected from the Swift BAT catalog, revealing that most FIR emission is not directly related to the AGN activity, but some show excess emission likely linked to accretion disk coronae.

Contribution

First comprehensive FIR spectral energy distributions of local, moderate luminosity AGN from Herschel SPIRE observations, highlighting the relationship between FIR excess emission and AGN properties.

Findings

46% of the sample detected at all three SPIRE bands

No correlation between FIR luminosities and X-ray luminosity for most AGN

Identification of a subset with excess 500 micron emission related to AGN activity

Abstract

We present far-infrared (FIR) and submillimeter photometry from the Herschel Space Observatory's Spectral and Photometric Imaging Receiver (SPIRE) for 313 nearby z<0.05 active galactic nuclei (AGN). We selected AGN from the 58 month Swift Burst Alert Telescope (BAT) catalog, the result of an all-sky survey in the 14-195 keV energy band, allowing for a reduction in AGN selection effects due to obscuration and host galaxy contamination. We find 46% (143/313) of our sample is detected at all three wavebands and combined with our PACS observations represents the most complete FIR spectral energy distributions of local, moderate luminosity AGN. We find no correlation between the 250, 350, and 500 micron luminosities with 14-195 keV luminosity, indicating the bulk of the FIR emission is not related to the AGN. However, Seyfert 1s do show a very weak correlation with X-ray luminosity compared to Seyfert 2s and we discuss possible explanations. We compare the SPIRE colors (F250/F350 and F350/F500) to a sample of normal star-forming galaxies, finding the two samples are statistically similar, especially after matching in stellar mass. But a color-color plot reveals a fraction of the Herschel-BAT AGN are displaced from the normal star-forming galaxies due to excess 500 micron emission E500). Our analysis shows E500 is strongly correlated with the 14-195 keV luminosity and 3.4/4.6 micron flux ratio, evidence the excess is related to the AGN. We speculate these sources are experiencing millimeter excess emission originating in the corona of the accretion disk.