Journal Article

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

F. J. Carrera, M. J. Page, J. A. Stevens, R. J. Ivison, T. Dwelly, J. Ebrero and S. Falocco

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 4, pages 2791-2807
Published in print June 2011 | ISSN: 0035-8711
Published online May 2011 | e-ISSN: 1365-2966 | DOI:
A strongly star-forming group: three massive galaxies associated with a quasi-stellar object

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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) ∼ 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 ∼24 μm, with star formation rates ∼2000 M yr−1, central dust masses log(Mdust/M) ∼ 9 ± 0.5 and hence central gas masses log(Mgas/M) ∼ 10.7. There is very little room for an AGN contribution. From X-ray upper limits and the observed 24 μm 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/M) ≲ 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 ∼1.3 per cent of the available nuclear gas mass is accreted.

Keywords: galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: starburst

Journal Article.  11718 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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