SMALL-SCALE STRUCTURES IN THE FAR-INFRARED BACKGROUND

Four fields with areas ranging from 80 to 2000 arcmin(2) have been map ped with the photometer on board of the Infrared Space Observatory (IS O) at 90 and around 180 mu m in regions of bright and faint cirrus. We examined the spatial characteristics of the infrared background emiss ion with high spatial resolution and found that the fluctuations in th e background emission limit the detection sensitivity of ISOPHOT for m ost of our observations. At 90 mu m the power law relation between the power in the fluctuations and the spatial frequencies established fro m IRAS data could be extended to twice as high spatial frequencies. At 180 mu m the small-scale fluctuations were studied for the first time by a cold space telescope with arcminute-resolution. A similar power law and spectral index down to spatial scales of 3' as for the 90 mu m component is found. For cirrus clouds the spatial frequency spectrum in the far-infrared has a similar shape as that derived from 21 cm lin e observations of the interstellar neutral hydrogen. In faint regions the fluctuations are caused presumably by randomly distributed extraga lactic sources. Future 3 m class space telescopes surveying the sky ar ound 200 mu m will not be hampered by cirrus over most of the sphere.