Letter to the Editor

Herschel-PACS far-infrared photometry of two z > 4 quasars^*

¹ Max-Planck Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany e-mail: leipski@mpia-hd.mpg.de
² Service d'Astrophysique (SAp)/IRFU/DSM/CEA Saclay - Bât. 709, 91191 Gif-sur-Yvette Cedex, France
³ Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁵ Astronomisches Institut Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
⁶ Institut d'Astrophysique et de Géophysique, University of Liège, Allé du 6 Août 17, 4000 Liège, Belgium
⁷ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium

Received: January 1900
Accepted: 5 May 2010

Abstract

We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z = 6.42) and BR 1202–0725 (z = 4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Herschel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For λ_rest ≲ 20 μm, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160 μm with a distance of ~10´´ from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z = 6.42 quasar.