The dust emission of high-redshift quasars

TL;DR

This study investigates dust emission in high-redshift quasars, finding no significant NIR spectral slope trends with luminosity or redshift, but detecting strong FIR emission indicating intense star formation and unusually high dust temperatures.

Contribution

It provides the first comprehensive infrared SED analysis of z>5 quasars, revealing high FIR luminosities and temperatures, and assesses NIR spectral slope dependence on luminosity and redshift.

Findings

No significant NIR spectral slope trends with luminosity or redshift.

Detection of strong FIR emission in seven quasars, indicating high star-formation rates.

FIR dust temperatures are higher (~65 K) than typical lower-redshift values.

Abstract

The detection of powerful near-infrared emission in high redshift (z>5) quasars demonstrates that very hot dust is present close to the active nucleus also in the very early universe. A number of high-redshift objects even show significant excess emission in the rest frame NIR over more local AGN spectral energy distribution (SED) templates. In order to test if this is a result of the very high luminosities and redshifts, we construct mean SEDs from the latest SDSS quasar catalogue in combination with MIR data from the WISE preliminary data release for several redshift and luminosity bins. Comparing these mean SEDs with a large sample of z>5 quasars we could not identify any significant trends of the NIR spectral slope with luminosity or redshift in the regime 2.5 < z < 6 and 10^45 < nuL_nu(1350AA) < 10^47 erg/s. In addition to the NIR regime, our combined Herschel and Spitzer photometry provides full infrared SED coverage of the same sample of z>5 quasars. These observations reveal strong FIR emission (L_FIR > 10^13 L_sun) in seven objects, possibly indicating star-formation rates of several thousand solar masses per year. The FIR excess emission has unusally high temperatures (T ~ 65 K) which is in contrast to the temperature typically expected from studies at lower redshift (T ~ 45 K). These objects are currently being investigated in more detail.