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.
