Star forming regions and the IMF along the Hubble sequence

Abstract

This thesis examines the properties of star forming regions in galaxies across the Hubble sequence. It focuses on the stellar populations of giant extragalactic H scII regions and OB associations, addressing in particular the question of Initial Mass Function (IMF) variations with metallicity and/or galaxy morphology. This work is composed of three main sections: (a) 3650-10,000 Å spectroscopy of nearly 100 H scII regions in 20 spiral galaxies (Sa through Sm). Two indicators of the ionizing cluster effective temperature (T ͙) are analyzed: the intensity of the He I λ5876 line, and the 'radiation softness' parameter η = (O⁺O⁺⁺)/(S⁺S⁺⁺). The interpretation of the data relies on CLOUDY photoionization models. A positive T ͙ gradient of 7000-8000 K is found between 2 Z(⊙) and 0.2 Z(⊙). The diagnostic diagrams and the T ͙ - metallicity relation are consistent with an upper mass limit of the IMF of ∼100 M(⊙) and an age of ∼1 Myr, irrespective of chemical abundance or Hubble type. (b) An investigation of extragalactic OB associations, based on Hubble Space Telescope images. The size distribution of the associations (found with an automated search algorithm) is similar in all galaxies examined, with a mean size around 80 pc. An indication is found that the average number of bright blue stars depends on the parent galaxy Hubble type. The upper stellar V luminosity function is comparable among galaxies, with slope d log N/dMᵥ=0.61±0.03. A few star cluster candidates are identified. (c) UBVR and Hα photometry of 266 H II regions in 10 spiral galaxies (Sa through Sd). The Hα equivalent width is independent of Hubble type. The continuum and Hα luminosity functions show similar trends, namely a steeper slope and a smaller characteristic luminosity for early-type galaxies. These results lead to the conclusion that changes in the properties of H II regions and associations along the Hubble sequence are most likely due to variations in the number of stars per star forming region and in the number of regions per unit area, rather than the mass function.