The K-band Hubble diagram of submillimetre galaxies and hyperluminous galaxies

We present the K-band Hubble diagrams (K-z relations) of submillimetre-selected galaxies and hyperluminous galaxies (HLIRGs). We report the discovery of a remarkably tight K-z relation of HLIRGs, indistinguishable from that of the most luminous radio galaxies. Like radio galaxies, the HLIRG K-z relation at z less than or similar to 3 is consistent with a passively evolving similar to3L(*) instantaneous starburst starting from a redshift of z similar to 10 . In contrast, many submillimetre-selected galaxies are greater than or similar to2 mag fainter, and the population has a much larger dispersion. We argue that dust obscuration and/or a larger mass range may be responsible for this scatter. The galaxies so far proved to be hyperluminous may have been biased towards higher AGN bolometric contributions than submillimetre-selected galaxies due to the 60-mum selection of some, so the location on the K-z relation may be related to the presence of the most massive active galactic nucleus. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.