Spectral energy distributions of type 2 quasi-stellar objects: obscured star formation at high redshifts

Abstract

We present new mid-infrared and submillimetre observations for a sample of eight high-redshift type 2 quasi-stellar objects (QSOs) located in the Chandra Deep Field-South. The sources are X-ray absorbed with luminosities in excess of 10⁴⁴ ergs^-1. Two of the targets have robust detections, S/N > 4, while further three targets are marginally detected with S/N >= 2.5. All sources are detected in multiple mid-infrared bands with the Spitzer Space Telescope. The multiwavelength spectral energy distributions (SEDs) of the type 2 QSOs are compared to those of two local ultraluminous galaxies (Arp220 and IR22491) in order to assess contributions from a star-forming component in various parts of the SED. We suggest that their submm emission is possibly due to a starburst while a large fraction of the mid-infrared energy is likely to originate in the obscured central quasar. Using the mid-infrared and submm observations, we derive infrared luminosities which are found to be in excess of L > 10¹²L_solar. The submm (850 μm) to X-ray (2 keV) spectral indices (α_SX) span a wide range. About half of the type 2 QSOs have values typical for a Compton-thick active galactic nuclei with only 1 per cent of the nuclear emission seen through scattering, and the remaining with values typical of submm-bright galaxies. Combining the available observational evidence, we outline a possible scenario for the early stages of evolution of these sources.