Mid-infrared spectra of optically-selected type 2 quasars

Type 2 quasars are luminous active galactic nuclei (AGNs) whose central engines are seen through large amounts of gas and dust. We present Spitzer spectra of 12 type 2 quasars selected on the basis of their optical emission line properties. Within this sample, we find a surprising diversity of spectra, from those that are featureless to those showing strong polycyclic aromatic hydrocarbon (PAH) emission, deep silicate absorption at 10 mu m, hydrocarbon absorption, high-ionization emission lines, and H(2) rotational emission lines. About half of the objects in the sample are likely Compton-thick, including the two with the deepest Si absorption. Themedian star-formation luminosity of the objects in our sample measured from the strength of the PAH features is 5 x 10(11) L(circle dot), much higher than for field galaxies or for any other AGN sample, but similar to other samples of type 2 quasars. This suggests an evolutionary link between obscured quasars and peak star-formation activity in the host galaxy. Despite the high level of star formation, the bolometric output is dominated by the quasar in all cases. For a given strength of 10 mu m Si absorption, ultra-luminous infrared galaxies are significantly colder than type 2 quasars (their F(nu)[14.5 mu m]/F(nu)[27.5 mu m] ratio is 0.5 dex lower), perhaps reflecting different obscuration geometries in these sources. We find that the appearance of the 10 mu m feature (i.e., whether it shows in emission or in absorption) is well correlated with the optical classification in type 1 and type 2 quasars, contrary to some models of clumpy obscuration. Furthermore, this correlation is significantly stronger in quasars (L(bol) greater than or similar to 10(45) erg s(-1)) than it is in Seyfert galaxies (Lbol << 10(45) erg s(-1)).