Far - infrared mapping of the starburst galaxy NGC 253 with ISOPHOT . ⋆

A 180 μm map and strip maps at 120 and 180 μm were obtained for the edge-on starburst galaxy NGC 253 with ISOPHOT, the photometer on board the Infrared Space Observatory. We compare these observations with those obtained by IRAS at 60 μm and 100 μm and derive the far–infrared spectral energy distribution at different locations in the galaxy. There is evidence for the presence of cold dust (T ≤ 20 K) both in the nucleus and in the disk. Extended emission dominated by cold dust is detected up to ∼ 15′ (∼ 10 kpc) along the major and minor axis; its spatial distribution is similar to that seen in the IRAS and ROSAT PSPC images. The emission along the minor axis is probably related to large-scale outflows of gas (superwinds) which originate in the nuclear starburst and maybe to star formation in the halo. The radial dependence of the dust temperature along the major axis is found using a radiative transfer code: we show that the dust scale length in the disk is ∼ 40% larger