FAR-UV OBSERVATIONS OF THE WHIRLPOOL GALAXY M-51

Far-UV (2000 angstrom) images of M 51 have been obtained with a balloo n-borne telescope, at a resolution of 15''. A comparison with an Ha im age shows that the UV is systematically located downstream from Ha, th e distance between the Ha maximum and the UV maximum is of 7''-11''. T he mean age of the stars emitting at these wavelengths is not the only reason for this displacement, the extinction also plays a role. Using the total gas content, an extinction correction has been applied to r egions 33'' in diameter. From UV fluxes, we computed local star format ion rates (SFR) and the local star formation efficiency (SFE). The SFR is strongly correlated to the total gas surface density. There is a s light evidence that for R > 5 kpc, the SFE has been influenced by the interaction with the companion galaxy NGC 5195. The mean azimuthal pro files of UV fluxes show that in the UV the arm/interarm contrast exhib its a similar, but stronger, contrast than at longer wavelengths. A sk ewness of the IMF towards massive stars is not necessary to explain th e outward increase of the contrast.