Statistical properties of HII regions in the disc of M100

Abstract

(Abridged) From a new H alpha mosaic image 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. I present the results of a statistical study of properties of the HII regions. The luminosity function of the complete ensemble of HII regions is well fitted by a power-law slope in the higher luminosity range, confirming results for galaxies like M100. Luminosity function slopes for arm and interarm HII regions separately are 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 star-forming clouds, nor the statistical properties of the HII region population. Diameter and radial number density distributions are discussed. 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.