Mon. Not. R. Astron. Soc., 389, 45-62 (2008/September-1)
Testing the evolutionary link between submillimetre galaxies and quasars: CO observations of QSOs at z ∼ 2.
COPPIN K.E.K., SWINBANK A.M., NERI R., COX P., ALEXANDER D.M., SMAIL I., PAGE M.J., STEVENS J.A., KNUDSEN K.K., IVISON R.J., BEELEN A., BERTOLDI F. and OMONT A.
Abstract (from CDS):
We have used the IRAM Plateau de Bure millimetre interferometer and the UKIRT 1-5µm Imager Spectrometer (UIST) to test the connection between the major phases of spheroid growth and nuclear accretion by mapping CO emission in nine submillimetre-detected QSOs at z = 1.7-2.6 with black hole (BH) masses derived from near-infrared spectroscopy. When combined with one QSO obtained from the literature, we present sensitive CO(3-2) or CO(2-1) observations of 10 submillimetre-detected QSOs selected at the epoch of peak activity in both QSOs and submillimetre (submm) galaxies (SMGs). CO is detected in 5/6 very optically luminous (MB∼ -28) submm-detected QSOs with BH masses MBH≃ 109-1010M☉, confirming the presence of large gas reservoirs of Mgas≃ 3.4x1010M☉. Our BH masses and dynamical mass constraints on the host spheroids suggest, at face value, that these optically luminous QSOs at z = 2 lie about an order of magnitude above the local BH-spheroid relation, MBH/Msph, although this result is dependent on the size and inclination of the CO-emitting region. However, we find that their BH masses are ∼30 times too large and their surface density is ∼300 times too small to be related to typical SMGs in an evolutionary sequence. Conversely, we measure weaker CO emission in four fainter (MB∼ -25) submm-detected QSOs with properties, BH masses (MBH≃ 5x108M☉), and surface densities similar to SMGs. These QSOs appear to lie near the local MBH/Msph relation, making them plausible `transition objects' in the proposed evolutionary sequence linking QSOs to the formation of massive young galaxies and BHs at high redshift. We show that SMGs have a higher incidence of bimodal CO line profiles than seen in our QSO sample, which we interpret as an effect of their relative inclinations, with the QSOs seen more face-on. Finally, we find that the gas masses of the four fainter submm-detected QSOs imply that their star formation episodes could be sustained for ∼10Myr, and are consistent with representing a phase in the formation of massive galaxies which overlaps a preceding SMG starburst phase, before subsequently evolving into a population of present-day massive ellipticals.
© 2008 The Authors. Journal compilation © 2008 RAS
galaxies: evolution - galaxies: formation - galaxies: high-redshift - galaxies: kinematics and dynamics - quasars: emission lines - submillimetre
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