Dust in PG quasars as seen by ISO

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Infrared to millimeter spectral energy distributions (SEDs) have been obtained for a random sample of 17 Palomar Green quasars. The SEDs provide evidence for considerable dust emission and do not support the existence of dust-free, naked quasars. The SEDs resemble those of warm ultra-luminous infrared galaxies (ULIRGs). The variety of SED shapes suggests different heating mechanisms. While AGNs might dominate the power-law shaped and mid-infrared (MIR) peaking SEDs, the far-infrared (FIR) peaking SEDs look as though they definitely require a starburst. The mid- and far-infrared luminosities are correlated, which can be understood as follows: a turbulent environment which favours a strong starburst (showing up in the far-infrared) also likely disturbs the angular momentum of gas clouds which are then channeled inwards and feed the AGN (which heats the mid-infrared dust emission). PG quasars have blue optical colors and low extinction, but are also dust rich. Thus the dust might be distributed in a non-spherical disk/torus geometry which is seen face-on or at intermediate angles. Then from the dust mass a high edge-on optical depth is derived even in the mid-infrared. Hence, any candidate for an edge-on PG quasar will appear to us like a cool ULIRG. But not every ULIRG needs to be a dust-enshrouded quasar. Finally, when looking for a possible connection between PG quasars and other related objects, unified schemes are considered. Also, an attractive picture could be the evolutionary chain: merger of two spirals — ULIRG — radio quiet (PG) quasar — radio loud quasar — radio galaxy — elliptical.