The near-to-mid infrared spectrum of quasars

We analyse a sample of 85 luminous (log(nuLnu(3\um)/erg s-1)>45.5) quasars with restframe ~2-11 \um spectroscopy from AKARI and Spitzer. Their high luminosity allows a direct determination of the near-infrared quasar spectrum free from host galaxy emission. A semi-empirical model consisting of a single template for the accretion disk and two blackbodies for the dust emission successfully reproduces the 0.1-10 \um spectral energy distributions (SEDs). Excess emission at 1-2 \um over the best-fitting model suggests that hotter dust is necessary in addition to the ~1200 K blackbody and the disk to reproduce the entire near-infrared spectrum. Variation in the extinction affecting the disk and in the relative strength of the disk and dust components accounts for the diversity of individual SEDs. Quasars with higher dust-to-disk luminosity ratios show slightly redder infrared continua and less prominent silicate emission. We find no luminosity dependence in the shape of the average infrared quasar spectrum. We generate a new quasar template that covers the restframe range 0.1-11 \um, and separate templates for the disk and dust components. Comparison with other infrared quasar composites suggests that previous ones are less reliable in the 2-4 \um range. Our template is the first one to provide a detailed view of the infrared emission on both sides of the 4 \um bump.
Comment: 26 pages, 14 figures, 3 tables. Accepted for publication in MNRAS