Stellar populations and the star formation histories of low surface brightness galaxies. II. H II regions

摘要

The luminosities, colors, and Ha emission for 429 H II regions in 54 low surface brightness (LSB) galaxies are presented. While the number of H II regions per galaxy is lower in LSB galaxies compared to star-forming irregulars and spirals, there is no indication that the size or luminosity function of H II regions differs from other galaxy types. The lower number of Hii regions per galaxy is consistent with their lower total star formation rates. The fraction of the total L_(H α) contributed by H II regions varies from 10 to 90 in LSB galaxies (the rest of the Ha emission being associated with a diffuse component) with no correlation with galaxy stellar or gas mass. Bright H II regions have bluer colors, similar to the trend in spirals; their number and luminosities are consistent with the hypothesis that they are produced by the same Hii luminosity function as spirals. Comparison with stellar population models indicates that the brightest Hii regions in LSB galaxies range in cluster mass from a few 10~3 M_⊙ (e.g., ρ Oph) to globular-cluster-sized systems (e.g., #130 Dor) and that their ages are consistent with clusters from 2 to 15 Myr old. The faintest H II regions are comparable to those in the LMC powered by a single O or B star. Thus, star formation in LSB galaxies covers the full range of stellar cluster mass.