Polycyclic aromatic hydrocarbon emission in powerful high-redshift radio galaxies
We present the mid-infrared (IR) spectra of seven of the most powerful radio-galaxies known to exist at 1.5 < z < 2.6. The radio emission of these sources is dominated by the AGN with 500 MHz luminosities in the range 10^(27.8)–10^(29.1) W Hz^(−1). The AGN signature is clearly evident in the mid-IR spectra; however, we also detect polycyclic aromatic hydrocarbon emission, indicative of prodigious star formation at a rate of up to ∼1000 M_⊙ yr^(−1). Interestingly, we observe no significant correlation between AGN power and star formation in the host galaxy. We also find most of these radio galaxies to have weak 9.7 μm silicate absorption features (τ_(9.7 μm) < 0.8) which implies that their mid-IR obscuration is predominantly due to the dusty torus that surrounds the central engine, rather than the host galaxy. The tori are likely to have an inhomogeneous distribution with the obscuring structure consisting of individual clouds. We estimate that these radio galaxies have already formed the bulk of their stellar mass and appear to lie at a stage in their evolution where the obscured AGN dominates the energy output of the system but star formation is also prevalent.
Additional Information© 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. First published online: December 5, 2012. Accepted 2012 November 6. Received 2012 November 6; in original form 2012 July 18. We thank the anonymous referee and R.C. Hickox for their useful comments which improved the manuscript. JIR acknowledges the support of a Science and Technologies Facilities Council studentship. NS is the recipient of an Australian Research Council Future Fellowship. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech) under contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through JPL and Ames Research Center. This work benefitted from the NASA/IPAC Extragalactic Database (NED), which is operated by the JPL, Caltech, under contract with NASA.
Published - MNRAS-2013-Rawlings-744-56.pdf