THE SPIRAL STRUCTURE OF M-51 FROM H-ALPHA AND 2000 ANGSTROM UV IMAGES- A NEW TRACER OF DENSITY WAVES

A deeper detection of the spiral structure of the Sbc galaxy M 51 is t he main goal of this paper. New UV data in the 2000 Angstrom range wer e obtained with a high altitude balloon of the Laboratoire d'Astronomi e Spatiale de Marseille and the Observatoire de Geneve, (LAS-OG). The data reveal the location of hot, evolved low-mass stars predominantly along the outside edge of, and down stream from, the spiral arms defin ed by the string of HII regions. Since these stars have a relatively w ell-defined age compared to the ''mixed'' population of stars seen in the arms when observed in visible light, the UV data is well-suited to derive time-scales of spiral arm components. In particular, the relat ion between spiral structure and star formation, which is at least par tly due to the density wave shock mechanism, can be studied from the d ifferential positions of the low-mass stars observed in the UV and the massive stars coexisting with the ionized gas. To facilitate such det ailed UV-H alpha comparison, we have also carried out new deep integra tions, photometry, and accurate mapping of the Ha emission. A catalogu e of 478 HII, regions in M 51 has been established from the analysis o f narrow band H alpha images taken with a combination of the Special A stronomical Observatory (SAG) Russian image tube, and the Observatoire de Marseille (OM) focal reducer f/1.5 (9 m equivalent focal length) a ttached at the prime focus of the 6-m telescope of the SAG. The radial and tangential shifts of the main and secondary spiral structures in UV and H alpha are also compared with other spiral tracers. The displa cements observed in these observations are compatible with a scenario in which the star formation is triggered by the density wave.