HerMES: Far-infrared properties of known AGN in the HerMES fields

TL;DR

This study analyzes Herschel FIR data of confirmed AGN in the HerMES fields, revealing their FIR color distinctions, starburst contributions, and a correlation between starburst and AGN luminosities, advancing understanding of AGN-host galaxy co-evolution.

Contribution

It provides the first detailed FIR color analysis of spectroscopically confirmed AGN in HerMES, demonstrating the necessity of starburst components and identifying a luminosity correlation.

Findings

One third of AGN have 5-sigma detections at 250um.

AGN colors separate from star-forming galaxies in certain FIR color ratios.

Starburst luminosity correlates with AGN accretion luminosity, L(SB) ∝ L(acc)^0.35.

Abstract

Nuclear and starburst activity are known to often occur concomitantly. Herschel-SPIRE provides sampling of the FIR SEDs of type 1 and type 2 AGN, allowing for the separation between the hot dust (torus) and cold dust (starburst) emission. We study large samples of spectroscopically confirmed type 1 and type 2 AGN lying within the Herschel Multi-tiered Extragalactic Survey (HerMES) fields observed during the science demonstration phase, aiming to understand their FIR colour distributions and constrain their starburst contributions. We find that one third of the spectroscopically confirmed AGN in the HerMES fields have 5-sigma detections at 250um, in agreement with previous (sub)mm AGN studies. Their combined Spitzer-MIPS and Herschel-SPIRE colours - specifically S(250)/S(70) vs. S(70)/S(24) - quite clearly separate them from the non-AGN, star-forming galaxy population, as their 24-um flux is dominated by the hot torus emission. However, their SPIRE colours alone do not differ from those of non-AGN galaxies. SED fitting shows that all those AGN need a starburst component to fully account for their FIR emission. For objects at z > 2, we find a correlation between the infrared luminosity attributed to the starburst component, L(SB), and the AGN accretion luminosity, L(acc), with L(SB) propto L(acc)^0.35. Type 2 AGN detected at 250um show on average higher L(SB) than type 1 objects but their number is still too low to establish whether this trend indicates stronger star-formation activity.