A strongly star-forming group : three massive galaxies associated with a quasi-stellar object

Abstract

We present here photometric redshift confirmation of the presence of large-scale structure around the z = 1.82 quasi-stellar object (QSO) RX J0941, which shows an overdensity of submillimetre (submm) sources. Radio imaging confirms the presence of the submm sources and pinpoints their likely optical near-infrared (NIR) counterparts. Four of the five submm sources present in this field (including the QSO) have counterparts with redshifts compatible with z = 1.82. We show that our photometric redshifts are robust against the use of different spectral templates. We have measured the galaxy stellar mass of the submm galaxies from their rest-frame K-band luminosity obtaining log(M-*/M-circle dot) similar to 11.5 +/- 0.2, slightly larger than the Schechter mass of present-day galaxies, and hence indicating that most of the stellar mass is already formed. We present optical-to-radio spectral energy distributions (SEDs) of the five Submillimetre Common-User Bolometer Array (SCUBA) sources. The emission of RX J0941 is dominated by reprocessed active galactic nucleus (AGN) emission in the observed mid-IR (MIR) range, while the starburst contribution completely dominates in the submm range. The SEDs of the other three counterparts are compatible with a dominant starburst contribution above similar to 24 mu m, with star formation rates similar to 2000 M-circle dot yr-1, central dust masses log(M-dust/M-circle dot) similar to 9 +/- 0.5 and hence central gas masses log(M-gas/M-circle dot) similar to 10.7. There is very little room for an AGN contribution. From X-ray upper limits and the observed 24 mu m flux, we derive a maximum 2-10 keV X-ray luminosity of 1044 erg s-1 for any putative AGN, even if they are heavily obscured. This in turn points to relatively small black holes with log(M-center dot/M-circle dot) less than or similar to 8 and hence stellar-to-black hole mass ratios about 1 order of magnitude higher than those observed in the present Universe: most of their central black hole masses are still to be accreted. Local stellar-to-black hole mass ratios can be reached if similar to 1.3 per cent of the available nuclear gas mass is accreted.