A characteristic oxygen abundance gradient in galaxy disks unveiled with CALIFA

We present the largest catalog of Hii regions and associations derived so far. The catalog comprises more than 7000 ionized regions, extracted from 306 galaxies observed by the CALIFA survey. We describe the procedures to detect, select, and analyse the spectroscopic properties of these ionized regions.

In the current study we focus on the characterization of the radial gradient of the oxygen abundance of the ionized gas, based on the study of the deprojected distribution of Hii regions. We found that all galaxies without clear evidence of an interaction present a common gradient in the oxygen abundance, with a characteristic slope of O/H = -0.1 dex/re between 0.3-2 disk effective radii, and a scatter compatible with random fluctuations around this value, when the gradient is normalized to the disk effective radius. The slope does not present significant differences depending on the morphology, incidence of bars, absolute magnitude or masses. Only those galaxies with evidence of interactions and/or clear merging systems present a significant shallower gradient, consistent with previous results. The majority of the 94 galaxies with Hii regions detected beyond 2 disk effective radius present a flattening in the oxygen abundance. The flattening is statistically significant. We cannot provide with a conclusive answer regarding the origin of this flattening. However, our results indicate that its origin is most probably related to the secular evolution of galaxies.

Finally, we find a drop/truncation of the oxygen abundance in the inner regions for 26 of the galaxies. All of them are non interacting, mostly unbarred, Sb/Sbc galaxies. This feature is associated with a central star-forming ring, which suggest that both features are produced by radial gas flows induced by resonance processes. Our result suggest that galaxy disks grow inside-out, with metal enrichment being driven by the local star-formation history, and with a small variation galaxy-by-galaxy. At a certain galactocentric distance, the oxygen abundance seems to be well correlated with the stellar mass density and total stellar mass of the galaxies, indepedently of other properties of the galaxies. Other processes, like radial mixing and inflows/outflows, although they are not ruled out, seem to have a limited effect on shaping of the radial distribution of oxygen abundances.