On the composition of H II regions in southern galaxies - I. NGC 300 and 1365

AAT/IPCS spectra have been obtained from six H II regions at differing radial distances from the centre of the Sed galaxy NGC 300 (Sculptor Group) and from three H II regions in the giant barred spiral NGC 1365 (Fornax Group) which has an active nucleus. The spectra have been analysed to determine abundances of the elements represented by observed emission lines for a combination of electron temperatures based on various weak temperature-sensitive lines and interpolation between photoionization models, using the sum of [O II] and [O III] intensities relative to Hβ to select plausible model parameters. NGC 300 turns out to have a radial abundance gradient in oxygen quite similar to those in M33 and M101, but no gradient in N/O, which has a rather low value throughout, whereas in NGC 1365 the abundances are fairly close to solar in all three cases studied, including one region that is not far from the 25 mag arcsec –2 isophotal radius. One object in NGC 300 is unusual in having stronger [O II] than is normally found in giant H II regions in Sed galaxies, and quite weak [O III], but an electron temperature and composition not untypical of its considerable radial distance. Neon, sulphur and argon abundances (relative to oxygen) are in all cases close to those in Orion and the Magellanic Clouds, while the He/H ratio seems to be slightly lower than in Orion.