MILLIMETER OBSERVATIONS OF A SAMPLE OF HIGH-REDSHIFT OBSCURED QUASARS
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
We present observations at 1.2 mm with Max-Planck Millimetre Bolometer Array (MAMBO-II) of a sample of z approx> 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{sup -1} and five out of 21 sources (24%) are detected at a significance of >=3sigma. Stacking all sources leads to a statistical detection of (S{sub 1.2mm}) = 0.96 +- 0.11 mJy and stacking only the non-detections also yields a statistical detection, with (S{sub 1.2mm}) = 0.51 +- 0.13 mJy. At the typical redshift of the sample, z = 2, 1 mJy corresponds to a far-infrared luminosity L{sub FIR}approx4 x 10{sup 12} L{sub 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 approx700 M{sub sun} yr{sup -1}. This far-infrared luminosity implies a dust mass of M{sub d}approx3 x 10{sup 8} M{sub sun}, which is expected to be distributed on approxkpc 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 mum 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 approx2-3 kpc. Comparison of our sample to other samples of z approx 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{sub CO}DELTAnu = 630 +- 50 mJy km s{sup -1}, corresponding to L{sub CO(3-2)} = 3.2 x10{sup 7} L{sub sun}, or a brightness-temperature luminosity of L'{sub CO(3-2)} = 2.4 x 10{sup 10} K km s{sup -1} pc{sup 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'{sub CO(4-3)} < 1 x 10{sup 10} K km s{sup -1} pc{sup 2}. Under the assumption that in these objects the high-J transitions are thermalized, we can estimate the molecular gas contents to be M{sub H{sub 2}}=1.9x10{sup 10} M {sub sun} and <8 x 10{sup 9} M{sub sun}, respectively. The estimated gas depletion timescales are tau{sub g} = 4 Myr and <16 Myr, and low gas-to-dust mass ratios of M{sub g}/M {sub d} = 19 and <20 are inferred. These values are at the low end but consistent with those of other high-redshift galaxies.
- OSTI ID:
- 21378224
- Journal Information:
- Astrophysical Journal, Vol. 706, Issue 1; Other Information: DOI: 10.1088/0004-637X/706/1/184; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
BOLOMETERS
BRIGHTNESS
CARBON MONOXIDE
DUSTS
EMISSION
ENERGY SPECTRA
GALAXIES
GALAXY NUCLEI
INTERFEROMETERS
LUMINOSITY
MASS
QUASARS
RED SHIFT
STARS
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
COSMIC RADIO SOURCES
MEASURING INSTRUMENTS
OPTICAL PROPERTIES
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SPECTRA