The inner structure of early-type galaxies in the Illustris simulation

TL;DR

This study uses the Illustris simulation to analyze the properties and evolution of early-type galaxies, revealing correlations among their internal structures and identifying biases in observational measurements, with implications for galaxy formation models.

Contribution

It provides new insights into the internal properties and evolution of early-type galaxies in simulations, highlighting systematic biases in observational methods and comparing results with real surveys.

Findings

Lower-mass and higher-redshift galaxies are bluer, gas-rich, and have more tangential orbits.

Projected dark matter fractions show mild mass dependence but correlate with effective radius.

Biases in density slope measurements are due to assumptions of isotropy and power-law profiles.

Abstract

Early-type galaxies provide unique tests for the predictions of the cold dark matter cosmology and the baryonic physics assumptions entering models for galaxy formation. In this work, we use the Illustris simulation to study correlations of three main properties of early-type galaxies, namely, the stellar orbital anisotropies, the central dark matter fractions and the central radial density slopes, as well as their redshift evolution since $z=1.0$. We find that lower-mass galaxies or galaxies at higher redshift tend to be bluer in rest-frame colour, have higher central gas fractions, and feature more tangentially anisotropic orbits and steeper central density slopes than their higher-mass or lower-redshift counterparts, respectively. The projected central dark matter fraction within the effective radius shows a very mild mass dependence but positively correlates with galaxy effective radii due to the aperture effect. The central density slopes obtained by combining strong lensing measurements with single aperture kinematics are found to differ from the true density slopes. We identify systematic biases in this measurement to be due to two common modelling assumptions, isotropic stellar orbital distributions and power-law density profiles. We also compare the properties of early-type galaxies in Illustris to those from existing galaxy and strong lensing surveys, we find in general broad agreement but also some tension, which poses a potential challenge to the stellar formation and feedback models adopted by the simulation.