Mid-infrared spectra of high-redshift (z > 2) radio galaxies

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 mm. 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, similar to 10(9) M(circle dot). Powerful radio galaxies at lower redshifts tend to have weak or M, 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 tau(9.7 mu 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.