Astrophys. J., 706, 184-202 (2009/November-3)
Millimeter observations of a sample of high-redshift obscured quasars.
MARTINEZ-SANSIGRE A., KARIM A., SCHINNERER E., OMONT A., SMITH D.J.B., WU J., HILL G.J., KLOCKNER H.-R., LACY M., RAWLINGS S. and WILLOTT C.J.
Abstract (from CDS):
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 and five out of 21 sources (24%) are detected at a significance of ≥3σ. Stacking all sources leads to a statistical detection of = 0.96±0.11 mJy and stacking only the non-detections also yields a statistical detection, with = 0.51±0.13 mJy. At the typical redshift of the sample, z = 2, 1 mJy corresponds to a far-infrared luminosity LFIR∼4 x1012 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. This far-infrared luminosity implies a dust mass of Md∼3x108 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 SCOΔν = 630±50 mJy km/s, corresponding to LCO(3–2)= 3.2 x107 L☉, or a brightness-temperature luminosity of L'CO(3–2)= 2.4x1010 K km/s 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)< 1x1010 K km/s pc2. Under the assumption that in these objects the high-J transitions are thermalized, we can estimate the molecular gas contents to be M☉ and <8x109 M☉, respectively. The estimated gas depletion timescales are τg = 4 Myr and <16 Myr, and low gas-to-dust mass ratios of Mg/Md= 19 and <20 are inferred. These values are at the low end but consistent with those of other high-redshift galaxies.
galaxies: active - galaxies: high-redshift - galaxies: nuclei - galaxies: starburst - infrared: galaxies - quasars: general
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