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

Abstract

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 − 1600nm) 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 g 475 − I 814 , r 625 − Y 105 , I 814 − H 160 , and Y 105 − H 160 at radii ranging between 0.1 -2 r 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 H 160 filter than in the blue r 625 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.

On the Origin of the Scatter in the Red Sequence: An Analysis of Four CLASH Clusters

“…Elliptical galaxies in clusters have been shown to have a color gradient (e.g., Vader et al 1988;Tamura et al 2000), with cores that are redder than their outskirts. This effect has even been seen in another cluster in the CLASH data set (Marian et al 2018). As we are sampling galaxies covering three orders of magnitude in brightness, we can neither use apertures of fixed size (which do not scale) nor can we fit spatial profiles to each galaxy (despite using the same imaging data as we did, the analysis of Marian et al 2018 stopped ∼3 magnitudes brighter than in this work).…”

“…This effect has even been seen in another cluster in the CLASH data set (Marian et al 2018). As we are sampling galaxies covering three orders of magnitude in brightness, we can neither use apertures of fixed size (which do not scale) nor can we fit spatial profiles to each galaxy (despite using the same imaging data as we did, the analysis of Marian et al 2018 stopped ∼3 magnitudes brighter than in this work). Additionally, as noted by Scodeggio (2001), measurements of the red sequence using fixed apertures are inherently biased due to the presence of color gradients.…”

The BUFFALO HST Survey

“…Very effective star formation quenching is observed in clusters at all redshifts, where the fraction of star-forming galaxies is lower than in the field (Hashimoto et al 1998), and the fraction of early-type morphologies (lenticulars, ellipticals) is the highest (Dressler 1980). A major thrust of ongoing research is to understand these transitions, particularly using deep HST imaging of clusters (Martinet et al 2017;Wagner et al 2017;Marian et al 2018;Olave-Rojas et al 2018;Connor et al 2019;Rodríguez-Muñoz et al 2019).…”

The differences between mass- and light-derived structural parameters over time for MaNGA Elliptical galaxies

“…For two clusters, they report that the mean of 𝑅 H /𝑅 V (rest-frame) is 0.62 ± 0.07 (𝑧 = 0.31) and 0.83 ± 0.03 (𝑧 = 0.64); the uncertainties correspond to the error of the mean. (b) Marian et al (2018) studied 79 massive early-type galaxies in the core of a massive cluster at 𝑧 = 0.44. The mean and error of the mean of their measured (rest-frame) 𝐻-to-𝑟 size ratios are 𝑅 e,H /𝑅 e,r = 0.77±0.02.…”

“…Remarkably, the change with 𝑧 of the color or mass-to-light ratio (Υ ★ ) gradients of quiescent galaxies has been also suggested to explain part of their claimed strong size evolution (e.g., La Barbera & de Carvalho 2009;Kennedy et al 2015;Ciocca et al 2017;Marian et al 2018;Suess et al 2019a,b). The latter authors, using data from the CANDELS survey, have shown that the radial variations in Υ ★ cause that the galaxy's light profile is different from its mass profile, in such a way that the half-light radius is a biased tracer of galaxy mass distribution.…”