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A key open issue for galaxy evolution and formation models is theunderstanding of the different mechanisms of galaxy assembly at various cosmicepochs. The aim of this study is to derive the global and spatially-resolvedmetal content in high-redshift galaxies. Using VLT/SINFONI IFU spectroscopy ofa first sample of 50 galaxies at z~1.2 in the MASSIV survey, we are able tomeasure the Ha and [NII]6584 emission lines. Using the N2 ratio as a proxy foroxygen abundance in the interstellar medium, we measure the metallicity of thesample galaxies. We develop a tool to extract spectra in annular regions ofthese galaxies, leading to a spatially-resolved estimate of the oxygenabundance in each galaxy. We derive a metallicity gradient for 26 galaxies inour sample and discover a significant fraction of galaxies with a "positive"gradient. Using a simple chemical evolution model, we derive infall rates ofpristine gas onto the disks. Seven galaxies display a positive gradient at ahigh confidence level. Four out of these are interacting and one is a chaingalaxy. We suggest that interactions might be responsible for shallowing andeven inverting the abundance gradient. We also identify two interestingcorrelations in our sample: a) galaxies with higher gas velocity dispersionhave shallower/positive gradients; and b) metal-poor galaxies tend to show apositive gradient whereas metal-rich ones tend to show a negative one. Thislast observation can be explained by the infall of metal-poor gas into thecenter of the disks. We address the question of the origin of this infall underthe influence of gas flows triggered by interactions and/or cold gas accretion.

MASSIV: Mass Assembly Survey with SINFONI in VVDS. III. Evidence for positive metallicity gradients in z~1.2 star-forming galaxies