Sizes, colour gradients and resolved stellar mass distributions for the massive cluster galaxies in XMMUJ2235-2557 at z = 1.39

Abstract

We analyse the sizes, colour gradients and resolved stellar mass distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557 at z = 1.39 using optical and near-infrared Hubble Space Telescope (HST) imaging. We derive light-weighted Sérsic fits in five HST bands (I₇₇₅, z₈₅₀, Y₁₀₅, J₁₂₅, H₁₆₀), and find that the size decreases by ∼20 per cent going from I₇₇₅ to H₁₆₀ band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between M_{{ast }}/L_{H_{160}} and (z₈₅₀ - H₁₆₀) and use these to derive mass-weighted Sérsic fits: the mass-weighted sizes are ∼41 per cent smaller than their rest-frame r-band counterparts compared with an average of ∼12 per cent at z ∼ 0. We attribute this evolution to the evolution in the M_{{ast }}/L_{H_{160}} and colour gradient. Indeed, as expected, the ratio of mass-weighted to light-weighted size is correlated with the M_*/L gradient, but is also mildly correlated with the mass surface density and mass-weighted size. The colour gradients (∇_{z - H}) are mostly negative, with a median value of ∼0.45 mag dex^-1, twice the local value. The evolution is caused by an evolution in age gradients along the semimajor axis (a), with ∇_age = dlog (age)/dlog (a) ∼- 0.33, while the survival of weaker colour gradients in old, local galaxies implies that metallicity gradients are also required, with ∇_Z = dlog (Z)/dlog (a) ∼- 0.2. This is consistent with recent observational evidence for the inside-out growth of passive galaxies at high redshift, and favours a gradual mass growth mechanism, such as minor mergers.