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Description
A deeper detection of the spiral structure of the Sbc galaxy M 51 is the main goal of this paper. New UV data in the 2000A range were obtained with a high altitude balloon of the Laboratoire d'Astronomie 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 defined by the string of HII regions. Since these stars have a relatively well-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 relation between spiral structure and star formation, which is at least partly due to the density wave shock mechanism, can be studied from the differential positions of the low-mass stars observed in the UV and the massive stars coexisting with the ionized gas. To facilitate such detailed UV-H{alpha} comparison, we have also carried out new deep integrations, photometry, and accurate mapping of the H{alpha} emission. A catalogue of 478 HII regions in M 51 has been established from the analysis of narrow band H{alpha} images taken with a combination of the Special Astronomical Observatory (SAO) Russian image tube, and the Observatoire de Marseille (OM) focal reducer f/1.5 (9m equivalent focal length) attached at the prime focus of the 6-m telescope of the SAO. 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 displacements observed in these observations are compatible with a scenario in which the star formation is triggered by the density wave.
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