Millimeter Observations of a Sample of High-Redshift Obscured Quasars
Abstract
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 sun. 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 sun yr-1. This far-infrared luminosity implies a dust mass of M d~3 × 108 M sun, 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 sun, 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_H_{2}=1.9× 10^{10} M sun and <8 × 109 M sun, 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.
Based on observations carried out with the IRAM 30 m Telescope and the Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2009
- DOI:
- 10.1088/0004-637X/706/1/184
- arXiv:
- arXiv:0910.1099
- Bibcode:
- 2009ApJ...706..184M
- Keywords:
-
- galaxies: active;
- galaxies: high-redshift;
- galaxies: nuclei;
- galaxies: starburst;
- infrared: galaxies;
- quasars: general;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted by ApJ, 25 pages, 11 figures, 4 tables