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Mid-Infrared Spectra of High-Redshift (z > 2) Radio Galaxies

Seymour, N. and Ogle, P. and de Breuck, C. and Fazio, G. G. and Galametz, A. and Haas, M. and Lacy, M. and Sajina, A. and Stern, D. and Willner, S. P. and Vernet, J. (2008) Mid-Infrared Spectra of High-Redshift (z > 2) Radio Galaxies. Astrophysical Journal Letters, 681 . L1-L4. ISSN 2041-8205. doi:10.1086/590081.

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We present the first mid-infrared Spitzer/Infrared Spectrograph (IRS) observations of powerful radio galaxies at z > 2. These radio galaxies, 4C +23.56 (z = 2.48) and 6C J1908+7220 (z = 3.53), both show strong mid-infrared continua, but with 6C J1908+7220 also showing strong PAH emission at rest-frame 6.2 and 7.7 μm. In 4C +23.56 we see no obvious PAH features above the continuum. The PAH emission in 6C J1908+7220 is the among the most distant observed to date and implies that there is a large instantaneous star formation rate (SFR). This is consistent with the strong detection of 6C J1908+7220 at far-IR and sub-mm wavelengths, indicative of large amounts of cold dust, ~10^9 M☉. Powerful radio galaxies at lower redshifts tend to have weak or undetectable PAH features and typically have lower far-IR luminosities. In addition, 4C +23.56 shows moderate silicate absorption as seen in less luminous radio galaxies, indicating τ_(9.7) μ m = 0.3 ± 0.05. This feature is shifted out of the observed wavelength range for 6C J1908+7220. The correlation of strong PAH features with large amounts of cold dust, despite the presence of a powerful AGN, is in agreement with other recent results and implies that star formation at high redshift is, in some cases at least, associated with powerful, obscured AGN.

Item Type:Article
Related URLs:
URLURL TypeDescription
Seymour, N.0000-0003-3506-5536
Ogle, P.0000-0002-3471-981X
de Breuck, C.0000-0002-6637-3315
Fazio, G. G.0000-0002-0670-0708
Galametz, A.0000-0002-1504-8117
Lacy, M.0000-0002-3032-1783
Sajina, A.0000-0002-1917-1200
Stern, D.0000-0003-2686-9241
Willner, S. P.0000-0002-9895-5758
Vernet, J.0000-0002-8639-8560
Additional Information:© 2008. The American Astronomical Society. Received 2008 March 13; accepted 2008 May 14; published 2008 June 13. We thank the referee for help improving the presentation of this Letter. This work 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. Support for this work was provided by NASA through an award issued by JPL/Caltech.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies: active, galaxies: evolution, galaxies: individual (4C +23.56, 4C +72.26)
Record Number:CaltechAUTHORS:20090717-132154373
Persistent URL:
Official Citation:Mid-Infrared Spectra of High-Redshift (z > 2) Radio Galaxies N. Seymour, P. Ogle, C. De Breuck, G. G. Fazio, A. Galametz, M. Haas, M. Lacy, A. Sajina, D. Stern, S. P. Willner, and J. Vernet 2008 ApJ 681 L1-L4 doi: 10.1086/590081
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14610
Deposited By: Ruth Sustaita
Deposited On:27 Aug 2009 00:13
Last Modified:08 Nov 2021 23:13

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