Paper detail

A strongly starforming group: three massive galaxies associated with a QSO

We present here photometric redshift confirmation of the presence of large scale structure around the z=1.82 QSO RXJ0941, which shows an overdensity of submm sources. Radio imaging confirms the presence of the submm sources and pinpoints their likely optical-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*/Msun)~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 SCUBA sources. The emission of RXJ0941 is dominated by reprocessed AGN emission in the observed 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 ~24um, with star formation rates SFR~2000Msun/yr, central dust masses log(Mdust/Msun)~9+-0.5 and hence central gas masses log(Mgas/Msun)~10.7. There is very little room for an AGN contribution. From X-ray upper limits and the observed 24um flux, we derive a maximum 2-10keV X-ray luminosity of 1e44 erg/s for any putative AGN, even if they are heavily obscured. This in turn points to relatively small black holes with log(MBH/Msun)<~8 and hence stellar-to-black hole mass ratios about one 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 masses ratios can be reached if ~1.3% of the available nuclear gas mass is accreted.