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Published August 2007 | Published
Journal Article Open

The Massive Hosts of Radio Galaxies across Cosmic Time


We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1 < z < 5.2. Using IRAC (3.6-8.0 μm), IRS (16 μm), and MIPS (24-160 μm) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H band from stars is >60% for ~75% of the high-redshift radio galaxies. As expected from unified models of AGNs, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5 μm) luminosity. In addition, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anticorrelates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 10^(11)-10^(11.5) M_☉ even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGNs, despite the stellar contribution to their mid-IR SEDs at shorter wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly accreting AGN. We find a weak correlation of stellar mass with radio luminosity.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 2 (2007 August); received 2007 January 2; accepted for publication 2007 March 2. We would like to thank the referee for the constructive comments provided on the clarification of this paper. N. S. and D. S. thank ESO for generous hospitality on several occasions over the last few years. We thank Dave Frayer and Ranga-Ram Chary for advice on reducing the MIPS data and the IRAC instrument team for allowing publication of GTOdata on 4C 41.17 and 6C 0140+326. 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. The work by W. v. B. was performed under the auspices of the US Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48. W. v. B. also acknowledges support for radio galaxy studies at UC Merced, including the work reported here, with the Spitzer Space Telescope via NASA grants SST 1264353, 1265551, 1279182, and 1281587.

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