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

Aims. The estimate of radial abundance gradients in high-redshift galaxies allows to constrain their star formation history and their interplay with the surrounding intergalactic medium. Methods. We present VLT/SINFONI integral-field spectroscopy of a first sample of 50 galaxies at z similar to 1.2 in the MASSIV survey. Using the N2 ratio between the [N II]6584 and H alpha rest-frame optical emission lines as a proxy for oxygen abundance in the interstellar medium, we measured the metallicity of the sample galaxies. We developed a tool to extract spectra in annular regions, leading to a spatially resolved estimate of the oxygen abundance in each galaxy. We were able to derive a metallicity gradient for 26 galaxies in our sample and discovered a significant fraction of galaxies with a positive gradient. Using a simple chemical evolution model, we derived infall rates of pristine gas onto the disks. Results. Seven galaxies display a positive gradient at a high confidence level. Four out of these are interacting, and one is a chain galaxy. We suggest that interactions might be responsible for shallowing and even inverting the abundance gradient. We also identify two interesting correlations in our sample: a) galaxies with higher gas velocity dispersion have shallower/positive gradients; and b) metal-poor galaxies tend to show a positive gradient, whereas metal-rich ones tend to show a negative one. This last observation can be explained by the infall of metal-poor gas into the center of the disks. We address the question of the origin of this infall under the influence of gas flows triggered by interactions and/or cold gas accretion. All the data published in this paper are publicly available at the time of publication following this link: http://cosmosdb.lambrate.inaf.it/VVDS-SINFONI.