PAH Emission in Powerful High-Redshift Radio Galaxies
  We present the mid-infrared 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. The AGN signature is clearly evident in the mid-infrared
spectra, however, we also detect polycyclic aromatic hydrocarbons emission,
indicative of prodigious star formation at a rate of up to ~1000 Msun/yr.
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 um silicate absorption features (tau_{9.7 um} < 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.
