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Outer-disk reddening and gas-phase metallicities: The CALIFA connection

R. A. Marino, A. Gil de Paz, S. F. Sánchez, P. Sánchez-Blázquez, N. Cardiel, A. Castillo-Morales, S. Pascual, J. M. Vílchez, C. Kehrig, M. Mollá, J. Méndez-Abreu, C. Catalán-Torrecilla, E. Florido, I. Pérez, T. Ruiz-Lara, S. Ellis, Á. R. López-Sánchez, R. M. González Delgado, A. de Lorenzo-Cáceres, R. García-Benito, L. Galbany, S. Zibetti, C. Cortijo, V. Kalinova, D. Mast, J. Iglesias-Páramo, P. Papaderos, C. J. Walcher, J. Bland-Hawthorn, and the CALIFA collaboration

Abstract
We study, for the first time in a statistically significant and well-defined sample, the relation between the outer-disk ionized-gas metallicity gradients and the presence of breaks in the surface brightness profiles of disk galaxies. Sloan Digital Sky Survey (SDSS) g'- and r'-band surface brightness, (g' - r') color, and ionized-gasoxygen abundance profiles for 324 galaxies within the Calar Alto Legacy Integral Field Area (CALIFA) survey are used for this purpose. We perform a detailed light-profile classification, finding that 84% of our disks show down- or up-bending profiles (Type II and Type III, respectively), while the remaining 16% are well fitted by one single exponential (Type I). The analysis of the color gradients at both sides of this break shows a U-shaped profile for most Type II galaxies with an average minimum (g' - r') color of ~0.5 mag and an ionized-gas metallicity flattening associated with it only in the case of low-mass galaxies. Comparatively, more massive systems show a rather uniform negative metallicity gradient. The correlation between metallicity flattening and stellar mass for these systems results in p-values as low as 0.01. Independent of the mechanism having shaped the outer light profiles of these galaxies, stellar migration or a previous episode of star formation in a shrinking star-forming disk, it is clear that the imprint in their ionized-gas metallicity was different for low- and high-mass Type II galaxies. In the case of Type III disks, a positive correlation between the change in color and abundance gradient is found (the null hypothesis is ruled out with a p-value of 0.02), with the outer disks of Type III galaxies with masses ≤1010 M showing a weak color reddening or even a bluing. This is interpreted as primarily due to a mass downsizing effect on the population of Type III galaxies that recently experienced an enhanced inside-out growth.

Keywords
galaxies: abundances, galaxies: evolution, galaxies: photometry, galaxies: ISM, ISM: abundances, Hii regions

Notes
Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, jointly operated by the Max-Planck-Institut für Astronomie Heidelberg and the Instituto de Astrofísica de Andalucía (CSIC ).

Astronomy and Astrophysics
Volume 585, Article Number A47, Number of pages 29
2016 January

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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