Spitzer Observations of High Redshift Radio Galaxies

Abstract

We present the results of a comprehensive Spitzer survey of 70 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 1.6 μm (H-band). The resultant stellar luminosities imply stellar masses of 10^(11−12)M_⊙, independent of redshift, indicating that radio galaxies form early and are amongst the most massive galaxies observed over this redshift range. These powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the inferred stellar contribution to their shorter-wavelength, mid-IR SEDs. The observed rest-frame mid-IR luminosities are typically above 10^(11)L_⊙, similar to lower redshift AGN. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. A weak, but significant, correlation of stellar mass with radio luminosity is found, consistent with earlier results.

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