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Register a new userThe far-infrared--submm spectral energy distribution of high-redshift quasars
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We combine submm photometric data of z>4 quasars, to obtain a mean far-infrared (FIR) (rest-frame) spectral energy distribution (SED) of thermal emission from dust, parameterised by a single temperature (T) and power-law emissivity index (beta). Best-fit values are T=41+/-5K, beta=1.95+/-0.3. The redshift spread of this set of quasars is exploited to allow us to sample the SED at a greater number of rest wavelengths than is possible for a single object. This parameterisation is of use to any study that extrapolates from a flux at a single submm wavelength, for example to infer dust masses and FIR luminosities. We then interpret the submm component as arising from dust heated by star-formation in the quasars host galaxy, and investigate a simple scheme of AGN--host coevolution, in which the timescale for formation of the host galaxy is c.0.5-1.0Gyr, with star formation proceeding at a constant rate c.1000Msol/yr. The luminous quasar phase occurs towards the end of the star-forming period, just before the galaxys reservoir of cold gas is depleted. Given the youth of the Universe at z=4 (1.6Gyr), the coexistence of a massive black hole and a luminous starburst can be a powerful constraint on models of quasar host-galaxy formation.
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ISO provides a key new far-infrared window through which to observe the multi-wavelength spectral energy distributions (SEDs) of quasars and active galactic nuclei (AGN). It allows us, for the first time, to observe a substantial fraction of the quasar population in the far-IR, and to obtain simultaneous, multi-wavelength observations from 5--200 microns. With these data we can study the behavior of the IR continuum in comparison with expectations from competing thermal and non-thermal models. A key to determining which mechanism dominates, is the measurement of the peak wavelength of the emission and the shape of the far-IR--mm turnover. Turnovers which are steeper than frequency^2.5 indicate thermal dust emission in the far-IR. Preliminary results from our ISO data show broad, fairly smooth, IR continuum emission with far-IR turnovers generally too steep to be explained by non-thermal synchrotron emission. Assuming thermal emission throughout leads to a wide inferred temperature range of 50-1000 K. The hotter material, often called the AGN component, probably originates in dust close to and heated by the central source, e.g. the ubiquitous molecular torus. The cooler emission is too strong to be due purely to cool, host galaxy dust, and so indicates either the presence of a starburst in addition to the AGN or AGN-heated dust covering a wider range of temperatures than present in the standard, optically thick torus models.
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We investigate the intrinsic spectral energy distribution (SED) of active galactic nuclei (AGNs) at infrared (IR) bands with 42 $z < 0.5$ optically luminous Palomar Green survey quasars through SED decomposition. We decompose the SEDs of the 42 quasars by combining an AGN IR template library Siebenmorgen2015 that covers a wide range of the AGN parameter space with three commonly used galaxy template libraries. We determine the median AGN SED from the best-fitting results. The far-IR (FIR) contribution of our median AGN SED is significantly smaller than that of Symeonidis et al. 2016, but roughly consistent with that of Lyu et al. 2017. The AGN IR SED becomes cooler with increasing bolometric luminosity, which might be due to that more luminous AGNs might have stronger radiative feedback to change torus structures and/or their tori might have higher metallicities. Our conclusions do not depend on the choice of galaxy template libraries. However, since the predicted polycyclic aromatic hydrocarbon (PAH) emission line flux is galaxy template-dependent, cautions should be taken on deriving galaxy FIR contribution from PAH fluxes.
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