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SCIENTIFIC ABSTRACT That the central regions of many galaxies are powerful infrared energy sources has been, without a doubt, one of the major scientific discoveries of the past two decades. However, we still understand very little of the phenomenology of infrared activity in galaxies: the frequency of its occurrence, its luminosity function, its relation to other galaxy properties such as Hubble type, galaxy environment, the presence of bars, or an active galactic nucleus. Even less therefore, do we understand the underlying astrophysics of infrared activity in galaxies: the physical mechanisms by which such activity is triggered--both interactions and bar-instabilities seem implicated. Indeed, it is hotly debated whether the underlying energy source is accretion onto a compact object or thermonuclear reactions in massive young stars produced in a burst of star formation. We propose to use ISOPHOT and ISOCAM to study the phenomenology of infrared activity in a sample of 120 spiral galaxies. The selection was made by taking those galaxies in the RSA catalogue which are classified as spiral or lenticular with B_T<=12. Then, for each launch season, the 120 with the largest ISO visibility were selected. Galaxies designated as being in the Virgo cluster in Binggeli etal (Astron. J., 1985, 90,1681) were excluded. About 50 % of these are barred or mixed morphological types. About 10 % are interacting or disturbed. This sample will provide statistical information on a variety of scientific questions related to starburst activity in galaxies. Moreover, it will be the fundamental control sample against which to compare the properties of more pathological examples of infrared activity in galaxies. It will be a unique database on the infrared properties of galaxies for the astronomical community for many years. To obtain information about the spatial and spectral distribution of cold dust in the central regions of the target galaxies, they will be observed using the ISOPHOT far-infrared camera at 60, 100, and 180 microns. These observations will complement the IRAS data in terms of spatial resolution by providing diffraction-limited angular resolution. The spectral energy distributions of virtually all galaxies detected by IRAS are still rising at the longest IRAS wavelength, 100 microns, making it difficult to determine such a basic parameter as the dust temperture from the IRAS data. The long wavelength data obtained in this ISO program should resolve this major uncertainty concerning the temperature of the cool interstellar dust in spiral galaxies. To obtain information about the distribution of warm dust in the inner regions of the target galaxies, they will be observed with the 8 to 15 micron CAM "IRAS" filter, in microscanning mode. This will provide a unique database of the highest angular resolution achievable with ISO for the thermal emission from spiral galaxies. OBSERVATION SUMMARY The far infrared (>60 microns) continuum energy distributions of the galaxies will be obtained using C100 with the 60 and 100 micron filters and using C200 with the 180 micron filter - PHT37 and PHT39 for on source and background measurements. An integration time of 16 seconds per filter per sky position will be used which gives, for a flux level of 0.1 Jy at each wavelength, signal to noise ratios of 10 (60 microns), 24 (100 microns), and 10 (180 microns). 75% of the galaxies in the sample were detected by IRAS with fluxes >1Jy at 60 microns and 85% with fluxes >1Jy at 100 microns. The 60/100 micron colour temperatures of the galaxies range from 23-62K so that the 180 micron filter will be measuring the spectrum beyond the peak in the emission, in most cases. We therefore expect to detect all the galaxies in the 3 filters with the photometric accuracy being limited by detector effects. By taking advantage of the improved flat-fielding achievable through micro-scanning, and by noting that the relatively high background fluxes encountered along with the large PSF (relative to the pixel size and raster step size chosen) will reduce the effects of responsive transients for each step of the microscan, it is possible to achieve signal-to-noise ratios of about 50 per CAM pixel for the galaxies of the sample in a 3x3 step microscan (1.3 pixel step size) of total duration 100 seconds using the 3" per pixel CAM f.o.v. (i.e. 90" diameter f.o.v.) and a fundamental on-chip integration time of 2 seconds. A frame for background subtraction is obtained for each source on a nearby reference sky field using identical observational parameters. The CAM observations are concatenated to the PHT observations of the same targets. The justification for this concatenation is that without it these CAM observations, which require about 10 hours when concatenated to the PHT observations, would require 17.5 hours.