Gas and dust in a z= 2.8 obscured quasar★

We present new detections of the CO(5−4), CO(7−6), [CI] and [CI] molecular and atomic line transitions towards the unlensed, obscured quasar AMS12 (z= 2.7672), observed with the Institut de Radioastronomie Millimetrique (IRAM) Plateau de Bure Interferometer (PdBI). This is the first unlensed, high-redshift source to have both atomic carbon ([CI]) transitions detected. Continuum measurements between 70 m and 3 mm are used to constrain the far-infrared (FIR) spectral energy distribution (SED), and we find a best-fitting FIR luminosity of log10[LFIR/L⊙] = 13.5 ± 0.1, dust temperature TD= 88 ± 8 K and emissivity index β= 0.6 ± 0.1. The highly excited molecular gas probed by CO(3−2), (5−4) and (7−6) is modelled with large velocity gradient models. The gas kinetic temperature TG, density n(H2) and the characteristic size r0 are determined using the dust temperature from the FIR SED as a prior for the gas temperature. The best-fitting parameters are TG= 90 ±8 K, n(H2) = 103.9 ± 0.1 cm−3 and r0= 0.8 ± 0.04 kpc. The ratio of the [CI] lines gives a [CI] excitation temperature of 43 ± 10 K, indicating that the [CI] and the high-excitation CO are not in thermal equilibrium. The [CI] excitation temperature is below that of the dust temperature and the gas kinetic temperature of the high-excitation CO, perhaps because [CI] lies at a larger radius where there may also be a large reservoir of CO at a cooler temperature, perhaps detectable through the CO(1−0) line. Using the [CI] line we can estimate the strength of the CO(1−0) line and hence the gas mass. This suggests that a significant fraction (∼30 per cent) of the molecular gas is missed from the high-excitation line analysis, giving a gas mass higher than that inferred from the assumption that the high-excitation gas is a good tracer of the low-excitation gas. The stellar mass was estimated from the mid-/near-infrared SED to be M★∼ 3 × 1011 M⊙. The Eddington-limited black hole mass is found from the bolometric luminosity to be M•≳ 1.5 × 109 M⊙. These give a black hole–bulge mass ratio of M•/M★≳ 0.005. This is in agreement with studies on the evolution of the M•/M★ relationship at high redshifts, which find a departure from the local value of ∼0.002. We discuss the implications for the evolution of the black hole in AMS12 and its host galaxy.