| Résumé | We present observations at 1.2 mm with Max-Planck Millimetre Bolometer Array (MAMBO-II) of a sample of z gsim 2 radio-intermediate obscured quasars, as well as CO observations of two sources with the Plateau de Bure Interferometer. The typical rms noise achieved by the MAMBO observations is 0.55 mJy beam–1 and five out of 21 sources (24%) are detected at a significance of ≥3σ. Stacking all sources leads to a statistical detection of langS 1.2 mmrang = 0.96 ± 0.11 mJy and stacking only the non-detections also yields a statistical detection, with langS 1.2 mmrang = 0.51 ± 0.13 mJy. At the typical redshift of the sample, z = 2, 1 mJy corresponds to a far-infrared luminosity L FIR~4 × 1012 L ☉. If the far-infrared luminosity is powered entirely by star formation, and not by active galactic nucleus heated dust, then the characteristic inferred star formation rate is ~700 M ☉ yr–1. This far-infrared luminosity implies a dust mass of M d~3 × 108 M ☉, which is expected to be distributed on ~kpc scales. We estimate that such large dust masses on kpc scales can plausibly cause the obscuration of the quasars. Combining our observations at 1.2 mm with mid- and far-infrared data, and additional observations for two objects at 350 μm using SHARC-II, we present dust spectral energy distributions (SEDs) for our sample and derive a mean SED for our sample. This mean SED is not well fitted by clumpy torus models, unless additional extinction and far-infrared re-emission due to cool dust are included. This additional extinction can be consistently achieved by the mass of cool dust responsible for the far-infrared emission, provided the bulk of the dust is within a radius ~2-3 kpc. Comparison of our sample to other samples of z ~ 2 quasars suggests that obscured quasars have, on average, higher far-infrared luminosities than unobscured quasars. There is a hint that the host galaxies of obscured quasars must have higher cool-dust masses and are therefore often found at an earlier evolutionary phase than those of unobscured quasars. For one source at z = 2.767, we detect the CO(3-2) transition, with S COΔν = 630 ± 50 mJy km s–1, corresponding to L CO(3-2) = 3.2 ×107 L ☉, or a brightness-temperature luminosity of L'CO(3-2) = 2.4 × 1010 K km s–1 pc2. For another source at z = 4.17, the lack of detection of the CO(4-3) line suggests the line to have a brightness-temperature luminosity L'CO(4-3) < 1 × 1010 K km s–1 pc2. Under the assumption that in these objects the high-J transitions are thermalized, we can estimate the molecular gas contents to be $M_{\rm H_{2}}=1.9\times 10^{10}$ M ☉ and <8 × 109 M ☉, respectively. The estimated gas depletion timescales are τg = 4 Myr and <16 Myr, and low gas-to-dust mass ratios of M g/M d = 19 and <20 are inferred. These values are at the low end but consistent with those of other high-redshift galaxies. |
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