The ALHAMBRA survey: 2-D analysis of the stellar populations in massive early-type galaxies at z < 0.3

We present a technique that permits the analysis of stellar population gradients in a relatively low cost way compared to IFU surveys analyzing a vastly larger samples as well as out to larger radii. We developed a technique to analyze unresolved stellar populations of spatially resolved galaxies based on photometric multi-filter surveys. We derived spatially resolved stellar population properties and radial gradients by applying a Centroidal Voronoi Tesselation and performing a multi-color photometry SED fitting. This technique has been applied to a sample of 29 massive (M$_{\star}$ > 10$^{10.5}$ M$_{\odot}$), early-type galaxies at $z$ < 0.3 from the ALHAMBRA survey. We produced detailed 2D maps of stellar population properties (age, metallicity and extinction). Radial structures have been studied and luminosity-weighted and mass-weighted gradients have been derived out to 2 - 3.5 R$_\mathrm{eff}$. We find the gradients of early-type galaxies to be on average flat in age ($\nabla$log Age$_\mathrm{L}$ = 0.02 $\pm$ 0.06 dex/R$_\mathrm{eff})$ and negative in metallicity ($\nabla$[Fe/H]$_\mathrm{L}$ = - 0.09 $\pm$ 0.06 dex/R$_\mathrm{eff}$). Overall, the extinction gradients are flat ($\nabla$A$_\mathrm{v}$ = - 0.03 $\pm$ 0.09 mag/R$_\mathrm{eff}$ ) with a wide spread. These results are in agreement with previous studies that used standard long-slit spectroscopy as well as with the most recent integral field unit (IFU) studies. According to recent simulations, these results are consistent with a scenario where early-type galaxies were formed through major mergers and where their final gradients are driven by the older ages and higher metallicity of the accreted systems. We demonstrate the scientific potential of multi-filter photometry to explore the spatially resolved stellar populations of local galaxies and confirm previous spectroscopic trends from a complementary technique.