Prior to the launch of IRAS rather little information on the spatial extent of far infrared emission in galaxies was available. Some of the earlier work has been reviewed by Telesco (1988). One of the most significant pre-IRAS studies was by Smith (1982), who produced a 170 µ map of the disk of M51, which showed that the bulk of the far infrared emission in M51 is produced by dust associated with the diffuse gas in the disk of the galaxy.
IRAS extended data is still under active study by several groups. Detailed maps have been produced of M31 (Habing et al 1986, Soifer et al 1987 and Walterbos and Schwering 1987), M33 (Rice et al 1990) and of the Magellanic Clouds (Schwering 1988). Rice et al (1988) have published coadded IRAS maps for all galaxies with optical extent greater than 8'. Higher resolution images may be expected for many of these galaxies from the use of maximum entropy and other deconvolution techniques now under active study at IPAC and elsewhere (e.g. Canterna et al 1990).
Subsequent studies have for the most part concentrated on wavelengths longer than 100 µ. Stark et al (1989) have mapped 4 Virgo spirals at 160 and 350 µ and shown that that there is no evidence for a grain component whose emission peaks beyond 200 µ, a prediction of grain models with emissivity Q at wavelengths > 100 µ. Eales and Wynn-Williams (1989) have measured 350 and 450 µ fluxes at locations centered on several galaxies with a 100 beam. 160 µm maps have been published of NGC4449 (Thronson et al 1987), NGC4214 (Tnronson et al 1988), NGC 4485 and 4490 () and NGC1569 and 3593 (Hunter et al 1989). have published maps of IC10 at 95 and 160 µ and given fluxes for several other galaxies. Eckart et al (1990) have mapped Centaurus A at 50 and 100 µ, and have separated the cirrus and starburst components. have mapped M82 at 450 µm, have mapped M83 at 100 µm and Engargiola and Harper (1990) have mapped NGC6946 at 100, 160 and 250 µ. Hughes et al (1989) have mapped 8 IRAS galaxies at 800 µm with JCMT and given some 450 and 1100 µm data for some of them. The importance of the longer wavelengths is that the most reliable estimates of dust mass can be obtained at these wavelengths. Fig 13 shows far infrared spectra of selected galaxies from this latter study, with theoretical fits derived from the models described in section 5. Fig 14a-d show colour-colour diagrams derived from the work of Thronson et al (1990), Hughes et al (1989) and Eales and Wynn-Williams (1989) compared with the predictions of the models of section 5.
Figure 13. Far infrared and submillimeter spectra of galaxies mapped by at 800 µm, compared with models of section 5. For NGC 2076 a pure cirrus model, with depletion of the smallest grains, is satisfactory. For the other galaxies both cirrus and starburst components are required (the dotted curve indicates the total predicted flux). Parameters for the models are given in Table 2.
Figure 14. Far infrared colour-colur diagrams, compared with predictions of interstellar grain model of section 5. (a) 100/60 versus 100/800 (data from Hughes et al 1989), (b) 100/60 and (c) 450/350 versus 100/350 (data from Eales et al 1990). (d) 160/100 versus 100/60 (data from Thronson et al 1990: solid curves, n B (T) fits; broken curve, cirrus model of section 5).