The radial metallicity gradient and the history of elemental enrichment in M 81 through emission-line probes

Aims. We present a new set of weak-line abundances of HII regions in M 81, which are based on Gemini Multi-Object Spectrograph (GMOS) observations. The aim is to derive plasma and abundance analysis for a sizable set of emission-line targets to study the galactic chemical contents in the framework of galactic metallicity gradients. Methods. We used the weak-line abundance approach by deriving electron density and temperatures for several HII regions in M 81. Our gradient analysis is based on oxygen abundances. Results. Together with a set of HII region abundances determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new data yield a radial oxygen gradient of −0.088 ± 0.013 dex kpc-1, which is steeper than the metallicity gradient obtained for planetary nebulae (−0.044 ± 0.007 dex kpc-1). This result could be interpreted as gradient evolution with time: Models of galactic evolution with inside-out disk formation associated to pre-enriched gas infall would produce such a difference in gradients, although stellar migration effects would also induce a difference in the metallicity gradients between the old and young populations. Conclusions. By comparing the M 81 metallicity gradients with those of other spiral galaxies, which were all consistently derived from weak-line analysis, we can infer that a similar gradient difference is common among spirals. The metallicity gradient slopes for HII regions and PNe seem to be steeper in M 81 than in other galactic disks, which is probably because M 81 belongs to a galaxy group. We also found that M 81 has experienced an average oxygen enrichment of 0.14 ± 0.08 dex in the spatial domain defined by the observations. Our data are compatible with a break in the radial oxygen gradient slope around R25 as inferred by other authors both in M 81 and in other galaxies