Dust emission and star formation toward a redshift 5.5 QSO

We report observations of the low-luminosity z =5 :50 quasar RD J030117+002025 (RD 0301 hereafter) at 250 GHz (1.20 mm) using the Max-Planck Millimeter Bolometer (MAMBO) array at the IRAM 30-meter telescope. The quasar was detected with a 1.2 mm flux density of 0:87 0:20 mJy. The lack of detectable 1.4 GHz radio emission indicates that the millimeter emission is of thermal nature, making RD 0301 the most distant dust- emission source known. When matching a 50 K grey body thermal far-infrared (FIR) spectrum to the observed millimeter flux we imply a FIR luminosity4 10 12 L, which is comparable to the quasar's optical luminosity. If the FIR luminosity arises from massive star formation, the implied star formation rate would be600M yr 1 , comparable to that of the starburst galaxies which dominate the average star formation and FIR emission in the early Universe. The FIR luminosity of RD 0301 is close to the average of that found in optically far more luminous high-redshift quasars. The comparably high millimeter to optical brightness ratio of RD 0301 is further evidence for that there is no strong correlation between the optical and millimeter brightness of high-redshift quasars, supporting the idea that in high-redshift quasars the dust is not heated by the AGN, but by starbursts.