The star-formation rates of QSOs

TL;DR

This study analyzes the far-infrared properties of over 5,000 quasars across redshifts 0.5 to 2.65, decomposing their IR emission into star formation and AGN components, revealing how AGN activity influences IR emission and star formation rates.

Contribution

It provides a detailed decomposition of IR emission in QSOs, showing the relationship between AGN power and IR components, and highlights the limitations of IR as a star formation proxy in luminous AGNs.

Findings

AGN IR contribution increases with AGN power.

Star formation rates are primarily redshift-dependent, not correlated with AGN luminosity.

IR emission trends can be modeled as a sum of independent star formation and AGN components.

Abstract

We examine the far-IR properties of a sample of 5391 optically selected QSOs in the 0.5<z<2.65 redshift range down to log[nuLnu,2500 (erg/s)]>44.7, using SPIRE data from Herschel-ATLAS. We split the sample in a grid of 74 luminosity-redshift bins and compute the average optical-infrared spectral energy distribution (SED) in each bin. By normalising an intrinsic AGN template to the AGN optical power (at 5100A) we decompose the total infrared emission (L_IR; 8-1000um) into an AGN (L_IR,AGN) and star-forming component (L_IR,SF). We find that the AGN contribution to L_IR increases as a function of AGN power which manifests as a reduction of the `far-IR bump' in the average QSO SEDs. We note that L_IR,SF does not correlate with AGN power; the mean star formation rates (SFRs) of AGN host galaxies are a function of redshift only and they range from ~6 Msun/yr at z~0 to a plateau of <200 Msun/yr at z~2.6. Our results indicate that the accuracy of far-IR emission as a proxy for SFR decreases with increasing AGN luminosity. We show that, at any given redshift, observed trends between infrared luminosity (whether monochromatic or total) and AGN power (in the optical or X-rays) can be explained by a simple model which is the sum of two components: (A) the infrared emission from star-formation, uncorrelated with AGN power and (B) the infrared emission from AGN, directly proportional to AGN power in the optical or X-rays.