IMAGES IN THE ROCKET ULTRAVIOLET - STAR FORMATION AND EXTINCTION IN THE M51 SYSTEM

UV images of M51 with up to 15″ resolution were obtained by a rocket-borne telescope. The bandwidth was 970 Å with maximum response at 2650 Å. The two most prominent features of M51 in the UV are the bright central region and a region of intense star formation about 215 NE of the nucleus toward the companion NGC 5195. This complex is the source of 20% of the total UV flux in M51. The companion is much less prominent in the UV than in optical bands. Spiral arms show much higher contrast in UV and U bands than in the R band. The nuclear region in the UV band shows clumped emission from probable star-forming regions, possibly associated with the inner Lindblad resonance. These regions are probably the UV counterparts of FIR sources discovered in an EW scan across the nucleus. UV/U colors of most bright H II regions with known extinction are consistent with O star spectra reddened by amounts estimated from radio and Balmer line measurements. However, some of the brightest H II regions have redder UV/U colors, probably indicating the presence of cooler stars which contribute in the U band. CO and IR observations indicate that the companion NGC 5195 may be a reddened starburst in spite of its faintness in the UV.