3.2 Millimetre and infrared emission

Figure 5 gives the average spectrum between 10¹¹ Hz and 10¹⁴ Hz extracted from Fig. 1. This continuum spectral energy distribution is characterised by a power law of index of 0.7 ± 0.1 (between 10 µm and 100 µm) and a "bump" around a few microns (see below).

Figure 5. The average millimetre-infrared spectrum of 3C 273. Data from Fig. 1. Panels as in Fig. 4.

Early infrared data are presented in (Neugebauer et al. 1979) and in (Rieke & Lebofsky 1979). (Neugebauer et al. 1979) note that the 3 µm flux of 3C 273 and other quasars is in excess of a power law that would link the 1 µm and the far infrared fluxes. They suggest that this bump (easier to see in the . f representation) around 3 µm may be due to the presence of heated dust, the emission of which is superimposed on the non thermal emission that extends smoothly from the radio domain. A similar conclusion is reached by (Allen 1980). The presence of dust within the nucleus might also explain the ratio of Ly to H fluxes which is an order of magnitude less than the theoretical predictions (Hyland et al. 1978). Dust located within the line emitting region could, according to these authors, lower this line ratio through reddening effects. Indeed, even small amounts of reddening will considerably decrease the Ly flux while the H flux will remain nearly unperturbed. Continuum observations suggest, however, that this dust does not redden the continuum in the same way. There is in fact no indication of substantial reddening in either the UV domain or the X-ray domain (see below) in excess to that caused within our Galaxy.