The far-infrared continuum of quasars

THE FAR-INFRARED CONTINUUM OF QUASARS. B.J. Wilkes 1 , E.J. Hooper 1 , K.K. McLeod 2 , M.S. Elvis 1 , C.D. Impey 3 , C.J. Lonsdale 4 , M.A. Malkan 5 , J.C. McDowell 1 arXiv:astro-ph/9902084v1 4 Feb 1999 Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138, USA Astronomy Department, Wellesley College, Wellesley, MA 02481, USA. Steward Observatory, University of Arizona, Tucson, AZ 85721, USA IPAC, Caltech, Pasadena, CA 91125, USA Department of Astronomy, University of California, Los Angeles, CA 90095, USA ABSTRACT ISO provides a key new far-infrared window through which to observe the multi-wavelength spectral en- ergy distributions (SEDs) of quasars and active galac- tic nuclei (AGN). It allows us, for the first time, to observe a substantial fraction of the quasar popula- tion in the far-IR, and to obtain simultaneous, multi- wavelength observations from 5–200 µm. With these data we can study the behavior of the IR contin- uum in comparison with expectations from compet- ing thermal and non-thermal models. A key to deter- mining which mechanism dominates, is the measure- ment of the peak wavelength of the emission and the shape of the far-IR–mm turnover. Turnovers which are steeper than ν 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 tem- perature range ∼ 50 − 1000K. 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 star- burst in addition to the AGN or AGN-heated dust covering a wider range of temperatures than present in the standard, optically thick torus models. Key words: ISO; infrared astronomy; quasars. 1. INTRODUCTION AND ANALYSIS The energy output of quasars and AGN is compara- ble thoughout the far-IR to X-ray spectral regions, so studies of their emission mechanisms require observa- tions throughout this range. Prior to ISO, the far-IR continuum was observed by IRAS, providing our first look at quasars in this wavelength range and show- ing them to be strong emitters from 12–100 µm. The poor spatial resolution and limited wavelength range of IRAS restricted observations to the IR-brightest subsets of quasars and AGN and left open many ques- tions as to the range of behavior present and the emis- sion mechanisms responsible in the various types of AGN. With ISO we are now able to address some of these limitations by studying a large number of different kinds of AGN and quasars. We observed 72 quasars and AGN covering a full range of redshift and luminosity and many different types of SEDs. The observations were made with ISOPHOT (Lemke et al. (1996)) and cover 5–200 µm in 8 bands. The status of this project, including a description of the sample and global comparisons of our results with those of IRAS, is summarised else- where (Hooper et al., this volume). Here we discuss our preliminary results in the context of the scientific questions we wish to address, with particular empha- sis on the high-redshift objects in the sample. The data were processed with PIA (Gabriel et al. (1997)) 1 through the AAP level, from which custom scripts (originating with M. Haas & S. Mueller at MPIA) were used to determine source fluxes and un- certainties. Default vignetting values from PIA were applied to the chopped data, and the flux scale was set by FCS observations. Sky noise estimates based on Herbstmeier et al. (1998) were included in the er- ror budget. For further details of the data reduction please refer to Hooper et al. (this volume). 2. SPECTRAL ENERGY DISTRIBUTIONS (SEDs). Figure 1 shows typical examples of the SEDs of a radio-loud (upper, RLQ) and radio-quiet (lower, RQQ) quasar (Elvis et al. (1994)). Throughout the far-IR–ultraviolet (UV) spectral region, the SEDs are remarkably similar, showing the big blue bump in the 1 PIA is a joint development by the ESA Astrophysics Di- vision and the ISOPHOT Consortium