Mid-infrared spectroscopy of high-redshift obscured quasars
Abstract
We present mid-infrared observations of 18 sources from a sample of 21 z ~ 2 radio-intermediate obscured (type 2) quasars. The mid-infrared spectra of the sources are continuum dominated, and 12 sources show deep silicate absorption with τ_9.7 ~ 1–2. Combining mid-infrared and optical spectra, we achieve 86% spectroscopic completeness which allows us to confirm that most (63^(+14)_(−22%)) z ~ 2 radio-intermediate quasars are obscured. The new spectra also prove that many high-redshift type 2 quasars do not show any rest-frame ultraviolet emission lines. From the 18 individual mid-infrared spectra, we classify most of the sources into three subsamples: those with hints of the 7.7 and 6.2 μm polyaromatic hydrocarbons (3/18 sources show PAHs; subsample A), those with an excess of emission around 8 μm but no hint of the 6.2 μm PAH (7/18 cases; subsample B), and pure-continuum sources with no visible excess (4/18 sources; subsample C). The remaining 4/18 sources have spectra that are featureless or too noisy for any features to be visible. In subsample A, averaging the spectra leads to a statistical detection of both 6.2 and 7.7 μm PAHs over the continuum, with the strength of the 7.7 μm PAH comparable to that of submillimeter-selected galaxies (SMGs) at similar redshifts. These sources are in a phase of coeval growth of a supermassive black hole and a host galaxy.
Additional Information
© 2008 The American Astronomical Society. Print publication: Issue 2 (2008 February 20) Received 2007 July 3, accepted for publication 2007 October 31. We thank Dave Bonfield for help with the William Herschel Telescope (WHT) observations, Henrik Spoon for access to the IRS guaranteed time observer spectra and for useful comments, Kate Brand, Hans-Rainer Klockner, and Dan Smith for communicating results prior to publication, and Roberto Maiolino for discussions. We also thank the anonymous referee for useful comments. This 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. The WHT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofısica de Canarias.Attached Files
Published - MARapj08a.pdf
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Additional details
- Eprint ID
- 14125
- Resolver ID
- CaltechAUTHORS:20090430-131332393
- GO-20705
- NASA
- GO-30634
- NASA
- Created
-
2009-08-11Created from EPrint's datestamp field
- Updated
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2021-11-08Created from EPrint's last_modified field
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)