2023A&A...671L...9T

Context. The wavelength dependence of the projection of the fundamental plane along the velocity dispersion axis, namely the Kormendy relation, is well characterised at low redshift but poorly studied at intermediate redshifts. The Kormendy relation provides information on the evolution of the population of early-type galaxies (ETGs). Therefore, by studying it, we may shed light on the assembly processes of these objects and their size evolution. As studies at different redshifts are generally conducted in different rest-frame wavebands, it is important to investigate whether the Kormendy relation is dependent on wavelength. Knowledge of such a dependence is fundamental to correctly interpreting the conclusions we might draw from these studies.
Aims. We analyse the Kormendy relations of the three Hubble Frontier Fields clusters, Abell S1063 at z = 0.348, MACS J0416.1-2403 at z = 0.396, and MACS J1149.5+2223 at z = 0.542, as a function of wavelength. This is the first time the Kormendy relation of ETGs has been explored consistently over such a large range of wavelengths at intermediate redshifts.
Methods. We exploit very deep Hubble Space Telescope photometry, ranging from the observed B-band to the H-band, and VLT/MUSE integral field spectroscopy. We improve the structural parameter estimation we performed in a previous work by means of a newly developed PYTHON package called MORPHOFIT.
Results. With its use on cluster ETGs, we find that the Kormendy relation slopes increase smoothly with wavelength from the optical to the near-infrared (NIR) bands in all three clusters, with the intercepts becoming fainter at lower redshifts due to the passive ageing of the ETG stellar populations. The slope trend is consistent with previous findings at lower redshifts.
Conclusions. The slope increase with wavelength implies that smaller ETGs are more centrally concentrated than larger ETGs in the NIR with respect to the optical regime. As different bands probe different stellar populations in galaxies, the slope increase also implies that smaller ETGs have stronger internal gradients with respect to larger ETGs.