Mid-infrared spectroscopy of high-redshift obscured quasars

Abstract

We present mid-infrared observations of 18 sources from a sample of 21 z ~ 2 radio-intermediate obscured (type 2) quasars. The mid-infrared spectra of the sources are continuum dominated, and 12 sources show deep silicate absorption with τ_9.7 ~ 1–2. Combining mid-infrared and optical spectra, we achieve 86% spectroscopic completeness which allows us to confirm that most (63^(+14)_(−22%)) z ~ 2 radio-intermediate quasars are obscured. The new spectra also prove that many high-redshift type 2 quasars do not show any rest-frame ultraviolet emission lines. From the 18 individual mid-infrared spectra, we classify most of the sources into three subsamples: those with hints of the 7.7 and 6.2 μm polyaromatic hydrocarbons (3/18 sources show PAHs; subsample A), those with an excess of emission around 8 μm but no hint of the 6.2 μm PAH (7/18 cases; subsample B), and pure-continuum sources with no visible excess (4/18 sources; subsample C). The remaining 4/18 sources have spectra that are featureless or too noisy for any features to be visible. In subsample A, averaging the spectra leads to a statistical detection of both 6.2 and 7.7 μm PAHs over the continuum, with the strength of the 7.7 μm PAH comparable to that of submillimeter-selected galaxies (SMGs) at similar redshifts. These sources are in a phase of coeval growth of a supermassive black hole and a host galaxy.