Cover picture: The central regions of the Lagoon Nebula in the light ofHa and forbidden [0 III]'\ 5007 \lA. The false colours are chosen to emphasise the differences in ionization conditions within the nebula as the ratio of the emission lines change. Young hot stars are photoionizing the nebula in a Huorescent process which converts UV radiation into visible light, mostly as emission lines of hydrogen and other light elements. Dark dust lanes and globules show where dense cores of interstellar clouds are still condensing, and where new star formation may still be taking place.

No abstract

Understanding the evolution of the N/O ratio in the interstellar medium (ISM) of galaxies is essential if we are to complete our picture of the chemical evolution of galaxies at high redshift, since most observational calibrations of O/H implicitly depend upon the intrinsic N/O ratio. The observed N/O ratio, however, shows large scatter at low O/H, and is strongly dependent on galactic environment. We show that several heretofore unexplained features of the N/O distribution at low O/H can be explained by the N seen in metal-poor galaxies being mostly primary nitrogen that is returned to the ISM via pre-supernova winds from rapidly rotating massive stars (M ≳ 10 M⊙, v/vcrit ≳ 0.4). This mechanism naturally produces the observed N/O plateau at low O/H. We show that the large scatter in N/O at low O/H also arises naturally from variations in star-formation efficiency. By contrast, models in which the N and O come primarily from supernovae provide a very poor fit to the observed abundance distribution. We propose that the peculiar abundance patterns we observe at low O/H are a signature that dwarf galaxies retain little of their SN ejecta, leaving them with abundance patterns typical of winds.