Dust covering factor, silicate emission, and star formation in luminous QSOs

We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high luminosity QSOs at 2 < z < 3.5. When combined with archival IRS observations of local, low luminosity type-I active galactic nuclei ( AGNs), the sample spans five orders of magnitude in luminosity. We find that the continuum dust thermal emission lambda rest = 6.7 mu m is correlated with the optical luminosity, following the non-linear relation lambda L-lambda( 6.7 mu m) proportional to lambda L-lambda(5100 angstrom)(0.82). We also find an anti correlation between lambda L-lambda(6.7 mu m)/lambda L-lambda(5100 angstrom) and the [OIII]lambda 5007 line luminosity. These effects are interpreted as a decreasing covering factor of the circumnuclear dust as a function of luminosity. Such a result is in agreement with the decreasing fraction of absorbed AGNs as a function of luminosity recently found in various surveys. In particular, while X-ray surveys find a decreasing covering factor of the absorbing gas as a function of luminosity, our data provides an independent and complementary confirmation by finding a decreasing covering factor of dust. We clearly detect the silicate emission feature in the average spectrum, but also in four individual objects. These are the silicate emission in the most luminous objects obtained so far. When combined with the silicate emission observed in local, low luminosity type-I AGNs, we find that the silicate emission strength is correlated with luminosity. The silicate strength of all type-I AGNs also follows a positive correlation with the black hole mass and with the accretion rate. The Polycyclic Aromatic Hydrocarbon (PAH) emission features, expected from starburst activity, are not detected in the average spectrum of luminous, high-z QSOs. The upper limit inferred from the average spectrum points to a ratio between PAH luminosity and QSO optical luminosity significantly lower than observed in lower luminosity AGNs, implying that the correlation between star formation rate and AGN power saturates at high luminosities.