Shin Oya , Fumihide Iwamuro , Jun'ichi Iwai , Hiroyuki Tsukamoto , & Toshinori Maihara
Department of Physics, Faculty of Science, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, JAPAN
The spectral energy distribution (SED) of QSO, in general, consists of two major emission components: one in the UV to near-infrared region, and the other in the mid- to far-infrared region. The former is supposed to be emanated from the very central region including an accretion disk, while the latter is attributed to the thermal emission of dust grains passively heated by the central primary radiation. This means that we are able to deduce a rough, but quantitative picture of the geometry and optical properties of the torus by comparing observed mid- to far-infrared SED with a model.
However, this method has so far been restricted to low-redshift, and hence to relatively low-luminosity QSOs, because the sensitivity at longer wavelengths is not sufficient for observations of apparently faint, but intrinsically luminous QSOs at redshifts larger than 2. In this report, we present a observation of such a QSO, making the best use of high sensitivity of ISOPHOT together with magnification by a gravitational lensing effect.
Far-infrared photometric observation of a gravitationally lensed QSO,
MG 0414+0534, at redshift of 2.64 were made with ISOPHOT at the wavelength of
60, 100 and 160 
.
The obtained rest-frame mid- to far-infrared SED of
the lensed quasar shows a peak around 10 ,
which is naturally
explained by
thermal emission from a dusty torus around the central engine of the quasar.
Based on a optically-thick dusty torus model we found that the dusty torus of
the quasar has outer radius of almost the same size ( 1 kpc) as that of
low-luminosity objects, whereas it has the larger inner radius ( 10 pc),
and that the ratio of the mid- to far-infrared luminosity relative to
the optical- to UV-luminosity is smaller than that of nearby QSOs,
even if we take a probable difference in magnification factors of
the respective emissions into account, which implies smaller thickness
(or covering factor) of the dusty torus.