Color gradients reflect an inside-out growth in early-type galaxies of the cluster MACS J1206.2-0847

Aims: Color gradients of galaxies are a powerful tool for resolving the variations of stellar populations within galaxies. We use this approach to explore the evolution of early-type galaxies in the core of the massive galaxy cluster MACS J1206.2-0847 at z = 0.44.
Methods: We used imaging data in 12 filters (covering a wavelength range from 400 to 1600 nm) from the Hubble Space Telescope provided by the CLASH survey, as well as additional spectral information from its follow-up program, CLASH-VLT. We performed multiwavelength optimized model fitting using Galapagos-2 from the MegaMorph project to measure their photometric parameters (total integrated magnitudes, effective radii r_e, and Sérsic indices n). We used them to derive color gradients for the colors g₄₇₅ - I₈₁₄, r₆₂₅ - Y₁₀₅, I₈₁₄ - H₁₆₀, and Y₁₀₅ - H₁₆₀ at radii ranging between 0.1 and 2r_e for 79 early-type cluster galaxies. From synthetic spectral models that use simple star formation recipes, we inferred ages and metallicities of the stellar population at different locations within each galaxy and characterized their influence on the radial color trends.
Results: Early-type galaxies show a substantial decrease in effective radii r_e with wavelength. We measure that galaxy sizes are 25% smaller in the red H₁₆₀ filter than in the blue r₆₂₅ filter but maintain a constant (within 3σ) Sérsic index n with wavelength. We find negative color gradients in all colors with slopes ranging between -0.07 and -0.17 mag dex^-1 and with no obvious dependence on total magnitude, stellar mass, or location inside the cluster core. We explain the observed radial trends of color gradients as a result of the ages and metallicities of the respective stellar populations. Red galaxy cores are typically 3 Gyr older and more enriched in metals than the galaxy outskirts, which are of solar metallicity.
Conclusions: Our results support the predictions from hydrodynamical cosmological simulations, which describe a passive evolution combined with an inside-out-growth of early-type galaxies. In this scenario, galaxies assemble their stellar mass primarily in the outskirts through the accretion of mass-poor satellites and thus manifest the observed trends of color-, metallicity- and age gradients.