Testing the evolutionary link between submillimetre galaxies and quasars: CO observations of QSOs at z ∼ 2

We have used the IRAM Plateau de Bure millimetre interferometer and the UKIRT 1-5 mu m 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 (M-B similar to -28) submm-detected QSOs with BH masses MBH similar or equal to 10(9)-10(10) M-circle dot, confirming the presence of large gas reservoirs of M-gas similar or equal to 3.4 x 10(10) M-circle dot. 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, M-BH/M-sph, although this result is dependent on the size and inclination of the CO-emitting region. However, we find that their BH masses are similar to 30 times too large and their surface density is similar to 300 times too small to be related to typical SMGs in an evolutionary sequence. Conversely, we measure weaker CO emission in four fainter (M-B similar to -25) submm-detected QSOs with properties, BH masses (M-BH similar or equal to 5 x 10(8) M-circle dot), and surface densities similar to SMGs. These QSOs appear to lie near the local M-BH/M-sph 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 similar to 10 Myr, 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.