Herschel-ATLAS: The link between accretion luminosity and star formation in quasar host galaxies

TL;DR

This study investigates the relationship between star formation, quasar accretion luminosity, and redshift using Herschel far-infrared data, revealing a correlation likely driven by shared cold gas reservoirs in host galaxies.

Contribution

It introduces a maximum likelihood estimator to analyze correlations between far-infrared and optical luminosities in quasars, establishing a link between star formation and black hole activity.

Findings

Star formation correlates with quasar luminosity and redshift.

The relationship follows L_IR ∝ L_QSO^{0.22} (1+z)^{1.6}.

No intrinsic dispersion found in the L_QSO-L_IR correlation.

Abstract

We use the science demonstration field data of the Herschel-ATLAS to study how star formation, traced by the far-infrared Herschel data, is related to both the accretion luminosity and redshift of quasars selected from the Sloan Digital Sky Survey and the 2SLAQ survey. By developing a maximum likelihood estimator to investigate the presence of correlations between the far-infrared and optical luminosities we find evidence that the star-formation in quasar hosts is correlated with both redshift and quasar accretion luminosity. Assuming a relationship of the form L_IR \propto L_QSO^{\theta} (1 + z)^{\zeta}, we find {\theta} = 0.22 +/- 0.08 and {\zeta} = 1.6 +/- 0.4, although there is substantial additional uncertainty in {\zeta} of order +/- 1, due to uncertainties in the host galaxy dust temperature. We find evidence for a large intrinsic dispersion in the redshift dependence, but no evidence for intrinsic dispersion in the correlation between L_QSO and L_IR, suggesting that the latter may be due to a direct physical connection between star formation and black hole accretion. This is consistent with the idea that both the quasar activity and star formation are dependent on the same reservoir of cold gas, so that they are both affected by the influx of cold gas during mergers or heating of gas via feedback processes.