Serjeant, Stephen; Farrah, Duncan; Geach, James; Takagi, Toshinobu; Verma, Aprajita; Kaviani, Ali and Fox, Matt
The K-band Hubble diagram of submillimetre galaxies and hyperluminous galaxies.
Monthly Notices of the Royal Astronomical Society, 346(4) L51-L56.
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<~ 3 is consistent with a passively evolving ~3L* instantaneous starburst starting from a redshift of z~ 10. In contrast, many submillimetre-selected galaxies are >~2 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.
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