Infrared to millimetre observations of active galaxies

Abstract

This thesis presents measurements of the continuum emission at infrared (IR) to millimetre wavelengths from the nuclear regions of a number of active galaxies. In particular many observations have been made for the first time in the submillimetre regime. The observing methods at these wavelengths and the unique astrophysical information on the nature of active galaxies that results from submillimetre data are described.
The central 1.5 kpc of the starburst galaxy M82 was mapped at 800 and 1100pm. The far-infrared (FIR) to millimetre continuum is due to thermal reradiation from dust grains at a temperature of 47 K, and the spatial distribution of the thermal emission shows a halo of grains extending to 500 pc from the plane of the galaxy centred on the 2.2 pm nucleus. The similar morphologies of the inillimetre, radio and X—ray emissions suggests that inverse Compton scattering of FIR photons dominates the observed X—ray luminosity of M82.
An improved method of background subtraction from the IRAS database has been developed and applied to a 2° square field of the Leo Triplet, an interacting system of galaxies. The success of the technique is demonstrated by the detection
of a low surface brightness FIR counterpart to the optical and HI tidal plume associated with NGC3628.
The most sensitive submillimetre observations to date of radio—quiet quasars are described. Although no detections were obtained the upper limits were sufficient to show that, for one quasar PG0710+457, non—thermal emission is unlikely to be the source of the FIR luminosity. A submillimetre detection of a less luminous active emission-line galaxy, Mkn1183, was achieved at a level of 69 miy at 800pm. In this instance the FIR luminosity was attributable to thermal emission from dust.
The interpretation of the continuum emission of an active galactic nucleus is complicated by the contribution of emission from the underlying galaxy to the nuclear fluxes. Endeavours to solve this problem have been made at IR, FIR and millimetre wavelengths and it is only by mapping the interstellar medium of an active galaxy in all of these wavebands that progress can be made. The beginning of such a programme of observations is presented in this thesis and is a major component of current research.