Statistical properties of HII regions in the disc of M100

Abstract

From a new mosaic image in the H line of the complete disc of the spiral galaxy M100, a catalogue is composed listing 1948 individual Hii regions. I give details of the data collection and reduction procedure, and of the production of the Hii region catalogue. For each Hii region, the catalogue gives its position relative to the centre of the galaxy, its deprojected distance to the centre, its radius, and its calibrated luminosity. An indication is included as to whether the Hii region is located in the arms, between them, or in the circumnuclear star-forming region. I present the results of a statistical study of properties of the Hii regions. The luminosity function of the complete ensemble of Hii regions shows a characteristic shape well fitted by a power-law slope in the higher luminosity range, and complying with literature values for galaxies like M100. Luminosity function slopes for arm and interarm Hii region populations separately are found to be equal within the errors of the fits, indicating that whereas the density wave accumulates material into the arm regions, and may trigger star formation there, it does not in fact change the mass distribution of the starforming clouds, nor the statistical properties of the Hii region population. Diameter distributions and the radial number density distribution are discussed. The latter indicates those areas where most star formation occurs: the circumnuclear region and the spiral arms. The huge number of Hii regions allowed the construction of a number of independent luminosity functions at different distances to the nucleus. The slope of the luminosity function shows a marginal decrease with increasing distance from the centre, which could indicate a gradual change toward shallower IMF slopes with increasing galactocentric distance, or an evolutionary effect.