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Millimeter Observations of a Sample of High-Redshift Obscured Quasars

Martínez-Sansigre, Alejo and Karim, Alexander and Schinnerer, Eva and Omont, Alain and Smith , Daniel J. B. and Wu, Jingwen and Hill, Gary J. and Klöckner, Hans-Rainer and Lacy, Mark and Rawlings, Steve and Willott, Chris J. (2009) Millimeter Observations of a Sample of High-Redshift Obscured Quasars. Astrophysical Journal, 706 (1). pp. 184-202. ISSN 0004-637X.

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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_(H_2) = 1.9 × 10^(10) 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.

Item Type:Article
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Karim, Alexander0000-0002-8414-9579
Schinnerer, Eva0000-0002-3933-7677
Lacy, Mark0000-0002-3032-1783
Additional Information:© 2009 American Astronomical Society. Print publication: Issue 1 (2009 November 20); received 2009 July 3; accepted for publication 2009 October 7; published 2009 October 28. We thank the IRAM staff for help with the observations and data reduction for this program. We are particularly grateful to St´ephane Leon for extensive help with the observations at the 30 m, Robert Zylka for making the MOPSIC package publicly available, Jan Martin Winters for help with the PdBI observations, Philippe Salom´e for help reducing the PdBI data on AMS16 and Roberto Neri for use of his software. We also thank Javier Rod´on, Veronica Roccatagliata, and Aurora Sicilia-Aguilar for help with software. We thank the support and assistance provided by the CSO staff and SHARC-II team at Caltech during the observations and data reduction. We are also grateful to the CLUMPY group16 for making their models publicly available. This manuscriptwas improved by the suggestions of an anonymous referee. The work was partially supported by grants associated with Spitzer programs GO- 20705 and GO-30634, and is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies: active; galaxies: high-redshift; galaxies: nuclei; galaxies: starburst; infrared: galaxies; quasars: general
Issue or Number:1
Record Number:CaltechAUTHORS:20091125-103340666
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Official Citation:Millimeter Observations of a Sample of High-Redshift Obscured Quasars Alejo Martínez-Sansigre, Alexander Karim, Eva Schinnerer, Alain Omont, Daniel J. B. Smith, Jingwen Wu, Gary J. Hill, Hans-Rainer Klöckner, Mark Lacy, Steve Rawlings, and Chris J. Willott 2009 ApJ 706 184-202 doi: 10.1088/0004-637X/706/1/184
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16808
Deposited By: Jason Perez
Deposited On:25 Nov 2009 22:42
Last Modified:01 Dec 2017 19:00

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