A strongly star-forming group : three massive galaxies associated with a quasi-stellar object
Carrera, F. J.
Page, M. J.
Stevens, J. A.
Ivison, R. J.
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.
Published inMonthly Notices of the Royal Astronomical Society
RelationsSchool of Physics, Astronomy and Mathematics
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