Gas-phase oxygen abundances and radial metallicity gradients in the two nearby spiral galaxies NGC7793 and NGC4945

Gas-phase abundances in HII regions of two spiral galaxies, NGC7793 and NGC4945, have been studied to determine their radial metallicity gradients. We used the strong-line method to derive oxygen abundances from spectra acquired with GMOS-S, the multi-object spectrograph on the 8m- Gemini South telescope. We found that NGC7793 has a well-defined gas-phase radial oxygen gradient of -0.321 $\pm$ 0.112 dex R$_{\rm 25}^{-1}$ (or -0.054 $\pm$ 0.019 dex kpc$^{-1}$) in the galactocentric range 0.17$<$R$_{\rm G}$/R$_{\rm 25}$ $<$ 0.82, not dissimilar from gradients calculated with direct abundance methods in galaxies of similar mass and morphology. We also determined a shallow radial oxygen gradient in NGC4945, -0.253 $\pm$ 0.149 dex R$_{\rm 25}^{-1}$ (or -0.019 $\pm$ 0.011 dex kpc$^{-1}$) for 0.04$<$R$_{\rm G}$/R$_{\rm 25}$ $<$ 0.51, where the larger relative uncertainty derives mostly from the larger inclination of this galaxy. NGC7793 and NGC4945 have been selected for this study because they are similar, in mass and morphology, to M33 and the Milky Way, respectively. Since at zeroth order we expect the radial metallicity gradients to depend on mass and galaxy type, we compared our galaxies in the framework of radial metallicity models best suited for M33 and the Galaxy. We found a good agreement between M33 and NGC7793, pointing toward similar evolution for the two galaxies. We notice instead differences between NGC4945 and the radial metallicity gradient model that best fits the Milky Way. We found that these differences are likely related to the presence of an AGN combined with a bar in the central regions of NGC4945, and to its interacting environment.Comment: ApJ, in pres