Complete infrared spectral energy distributions of mm detected quasars at z>5

TL;DR

This study presents detailed infrared spectral energy distributions of high-redshift quasars, revealing an additional FIR component likely due to intense star formation, and clarifies the contributions of AGN and star formation to their emission.

Contribution

It provides the first comprehensive SED fits from 0.1 to 400 microns for z>5 quasars, identifying a significant FIR component beyond the dusty torus emission.

Findings

FIR component has T_FIR ~ 40-60K and L_FIR ~ 10^13 L_sun.

Star formation rates exceed 1000 solar masses per year.

AGN torus emission is negligible at wavelengths >50 microns.

Abstract

We present Herschel far-infrared (FIR) photometry of eleven quasars at redshift z>5 that have previously been detected at 1.2mm. We perform full spectral energy distribution (SED) fits over the wavelength range lambda_rest ~0.1-400mu for those objects with good Herschel detections. These fits reveal the need for an additional far-infrared (FIR) component besides the emission from a dusty AGN-powered torus. This additional FIR component has temperatures of T_FIR ~ 40-60K with luminosities of L_(8-1000mu) ~ 10^13 L_sun (accounting for 25-60% of the bolometric FIR luminosity). If the FIR dust emission is due to star formation it would suggest star formation rates in excess of 1000 solar masses per year. We show that at long wavelengths (lambda_rest > 50mu) the contribution of the AGN-powered torus emission is negligible. This explains how previous FIR studies of high-redshift quasars that relied on single component fits to (ground-based) observations at lambda_obs > 350mu reached T_FIR and L_FIR values similar to our complete SED fits. Stacking the Herschel data of four individually undetected sources reveals a significant average signal in the PACS bands but not in SPIRE. The average SED of sources with individual Herschel detections shows a striking surplus in near- and mid-infrared emission when compared to common AGN templates. The comparison between two average SEDs (sources with and without individual Herschel detections) matched in the UV/optical indicates that for these objects the strength of the MIR emission may correlate with the strength of the FIR emission.