Physical Properties of Luminous Dust Poor Quasars

We identify and characterize a population of luminous dust poor quasars at 0<z<5, similar in photometric properties to the objects found at z>6 previously. This class of active galactic nuclei has been known to show little IR emission from a dusty structure, but is yet poorly understood in terms of number evolution or of dependence on physical quantities. In order to better understand the luminous dust poor quasar properties, we compiled a rest-frame UV to IR library of 41,000 optically selected type-1 quasars with $L_{bol}>10^{45.7} erg s^{-1}$. After fitting the broad-band spectral energy distributions (SEDs) with accretion disk and dust components, we find 0.6% of our sample to be hot dust poor with a rest-frame 2.3 micron to 0.51 micron flux density ratio of -0.5 dex or less. The dust poor SEDs are blue in the UV-optical and weak in the MIR, such that their accretion disks are less obscured, and that hot dust emission traces that of warm dust down to the dust poor regime. At a given bolometric luminosity, dust poor quasars are lower in black hole mass and higher in Eddington ratio than general luminous quasars, suggesting that they are in a rapidly growing evolutionary state in which the dust poor phase appears as a short or rare phenomenon. The dust poor fraction increases with redshift, and possible implications for the evolution of the dust poor fraction are discussed.

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Date added: Mon, 14 Oct 13