Belinda Wilkes 1Eric Hooper 1, Kim McLeod 2, Martin Elvis 1, Chris Impey 4, Carol Lonsdale 5, Matt Malkan 6, & Jonathon McDowell 7
1 SAO
2 Wellesley
3 SAO
4 UofA
5 IPAC
6 UCLA
7 SAO
Over the past decade studies of the spectral energy distributions (SEDs) of quasars have become widespread largely due to technological advances which facilitate deeper observations over wider wavelength ranges. Our understanding of the energy generation mechanisms at work has correspondingly increased. The ISO satellite has provided important new observational capability in the far-infrared (IR), with nearly simultaneous coverage from 5-200 microns. Observations with ISO extend the IRAS database, which covers 12-100 microns in 4 bands, in wavelength range, spatial resolution and to somewhat deeper flux levels. This allowed observations to be made of a larger proportion of the quasar population and extension to high redshift for the first time.
Our program has obtained photometry with ISOPHOT in 8 bands from 5 to 200 microns for 72 quasars and active galaxies having a wide variety of properties. The sample includes subsets which are X-ray, optically and IR selected and covers a wide range of redshift. This paper will present preliminary analysis of the nine high redshift quasars (1<z<4.7) in our sample. The IR SEDs are broad, generally peaking at 25-60 microns in the rest frame but with no sharp cut-off visible in the observed range. These results are consistent with currently accepted scenarios in which the IR emission originates in dust at a range of temperatures both from the host galaxy and an inner, heated region such as a molecular torus. A comparison of the SEDs with those of low-redshift quasars reveals a wide range of behavior extending from very similar to two orders-of-magnitude stronger IR emission than is typical at low-redshift. The latter case is difficult to reconcile with current models for the IR emission from quasars. Comparison with typical low-redshift galaxy SEDs indicates that the amount of cool dust consistent with a pure dust model in the far-IR requires host galaxies to be several orders-of-magnitude more luminous than host galaxies at low redshift and/or similar to ultra-luminous IR galaxies.