MILLIMETER OBSERVATIONS OF A SAMPLE OF HIGH-REDSHIFT OBSCURED QUASARS

We present observations at 1.2 mm with Max-Planck Millimetre Bolometer Array (MAMBO-II) of a sample of z ≳ 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 〈 S 1.2 mm 〉 = 0.96 ± 0.11 mJy and stacking only the non-detections also yields a statistical detection, with 〈 S 1.2 mm 〉 = 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 × 10 12 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 × 10 8 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 CO Δν = 630 ± 50 mJy km s −1 , corresponding to L CO(3-2) = 3.2 × 10 7 L ☉ , or a brightness-temperature luminosity of L ' CO(3-2) = 2.4 × 10 10 K km s −1 pc 2 . 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 × 10 10 K km s −1 pc 2 . Under the assumption that in these objects the high- J transitions are thermalized, we can estimate the molecular gas contents to be M ☉ and <8 × 10 9 M ☉ , respectively. The estimated gas depletion timescales are τ g = 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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