The stellar halos of ETGs in the IllustrisTNG simulations: The photometric and kinematic diversity of galaxies at large radii

Abstract

Context. Early-type galaxies (ETGs) are found to follow a wide variety of merger and accretion histories in cosmological simulations. Aims. We characterize the photometric and kinematic properties of simulated ETG stellar halos, and compare them to the observations. Methods. We selected a sample of 1114 ETGs in the TNG100 simulation and 80 in the higher-resolution TNG50. These ETGs span a stellar mass range of 1010.3 − 1012 M⊙ and they were selected within the range of g − r colour and λ-ellipticity diagram populated by observed ETGs. We determined photometric parameters, intrinsic shapes, and kinematic observables in their extended stellar halos. We compared the results with central IFU kinematics and ePN.S planetary nebula velocity fields at large radii, studying the variation in kinematics from center to halo, and connecting it to a change in the intrinsic shape of the galaxies. Results. We find that the simulated galaxy sample reproduces the diversity of kinematic properties observed in ETG halos. Simulated fast rotators (FRs) divide almost evenly in one third having flat λ profiles and high halo rotational support, a third with gently decreasing profiles, and another third with low halo rotation. However, the peak of rotation occurs at larger R than in observed ETG samples. Slow rotators (SRs) tend to have increased rotation in the outskirts, with half of them exceeding λ = 0.2. For M* >  1011.5 M⊙ halo rotation is unimportant. A similar variety of properties is found for the stellar halo intrinsic shapes. Rotational support and shape are deeply related: the kinematic transition to lower rotational support is accompanied by a change towards rounder intrinsic shape. Triaxiality in the halos of FRs increases outwards and with stellar mass. Simulated SRs have relatively constant triaxiality profiles. Conclusions. Simulated stellar halos show a large variety of structural properties, with quantitative but no clear qualitative differences between FRs and SRs. At the same stellar mass, stellar halo properties show a more gradual transition and significant overlap between the two families, despite the clear bimodality in the central regions. This is in agreement with observations of extended photometry and kinematics.