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 RX J0941, 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∗/M) ∼ 11.5±0.2, slightly larger than the Schechter mass of present day galaxies, and hence indicating that most of the stel-lar mass is already formed. We present optical-to-radio spectral energy distributions (SEDs) of the five SCUBA sources. The emission of RX J0941 is dominated by re-processed 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 ∼ 24 µm, with sta