The near-to-mid infrared spectrum of quasars

Abstract

We analyse a sample of 85 luminous (log(nuLnu(3\\um)/erg s-1)>45.5) quasars\nwith restframe ~2-11 \\um spectroscopy from AKARI and Spitzer. Their high\nluminosity allows a direct determination of the near-infrared quasar spectrum\nfree from host galaxy emission. A semi-empirical model consisting of a single\ntemplate for the accretion disk and two blackbodies for the dust emission\nsuccessfully reproduces the 0.1-10 \\um spectral energy distributions (SEDs).\nExcess emission at 1-2 \\um over the best-fitting model suggests that hotter\ndust is necessary in addition to the ~1200 K blackbody and the disk to\nreproduce the entire near-infrared spectrum. Variation in the extinction\naffecting the disk and in the relative strength of the disk and dust components\naccounts for the diversity of individual SEDs. Quasars with higher dust-to-disk\nluminosity ratios show slightly redder infrared continua and less prominent\nsilicate emission. We find no luminosity dependence in the shape of the average\ninfrared quasar spectrum. We generate a new quasar template that covers the\nrestframe range 0.1-11 \\um, and separate templates for the disk and dust\ncomponents. Comparison with other infrared quasar composites suggests that\nprevious ones are less reliable in the 2-4 \\um range. Our template is the first\none to provide a detailed view of the infrared emission on both sides of the 4\n\\um bump.\n