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

Abstract

We present a new set of weak-line abundances of HII regions in M81, 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. We used the weak-line abundance approach by deriving electron density and temperatures for several HII regions in M81. Gradient analysis is based on oxygen abundances.Together with a set of HII region abundances determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new data yield to a radial oxygen gradient of -0.088$\pm$0.013 dex kpc$^{-1}$, which is steeper than the metallicity gradient obtained for planetary nebulae (-0.044$\pm$0.007 dex kpc$^{-1}$). This result could be interpreted as gradient evolution with time: Models of galactic evolution with inside-out disk formation associated to pre-enriched gas infall would produce such difference of gradients, although stellar migration effects would also induce a difference in the metallicity gradients between the old and young populations. By comparing the M81 metallicity gradients with those of other spiral galaxies, all consistently derived from weak-line analysis, we can infer that similar gradient difference is common among spirals. The metallicity gradient slopes for HII regions and PNe seem to be steeper in M81 than in other galactic disks, which is likely due to the fact that M81 belongs to a galaxy group. We also found that M81 has experienced an average oxygen enrichment of 0.14$\pm$0.08 dex in the spatial domain defined by the observations. Our data are compatible with a break in the radial oxygen gradient slope around R$_{25}$ as inferred by other authors both in M81 and in other galaxies.