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

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 lowerredshift 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.

Early-type galaxy density profiles from IllustrisTNG – II. Evolutionary trend of the total density profile

“…The total power-law slope also correlates with the in-situ formed stellar mass ratio and redshift, in line with previous simulation studies (Xu et al 2017;Remus et al 2017;Bellstedt et al 2018). Although a slightly negative total density slope/central velocity dispersion correlation and a shallower slope with time are in tension with observational trends, the uncertainties and systematics in observations may have obscured the true trends (Xu et al 2016(Xu et al , 2017Tagore et al 2018). In this work, we will hence investigate the redshift evolution of the density profiles of z = 0 ETG progenitors, and statistical ETG samples at higher redshifts selected based on their morphology.…”

“…These galaxies show correlations between the total power-law slope with galaxy size, stellar mass, surface density and central dark matter fraction that are broadly consistent with both local and distant observations. The total power-law slope also correlates with the in-situ formed stellar mass ratio and redshift, in line with previous simulation studies (Xu et al 2017;Remus et al 2017;Bellstedt et al 2018). Although a slightly negative total density slope/central velocity dispersion correlation and a shallower slope with time are in tension with observational trends, the uncertainties and systematics in observations may have obscured the true trends (Xu et al 2016(Xu et al , 2017Tagore et al 2018).…”

“…Galaxies are identified as gravitationally-bound systems by subfind (Springel et al 2001;Dolag et al 2009) in Illus-trisTNG. In Xu et al (2017) and , the selection and classification of the simulated galaxies have been described in detail, and we apply the same methods in this work. We derive the optical luminosity of the galaxy with the SPS model galaxev (Bruzual & Charlot 2003) based on the age and metallicity of every stellar particle in the galaxy, and apply a basic dust attenuation treatment that varies with the viewing angle.…”

“…However regarding the redshift evolution of ETGs' total power-law density slopes, no consensus has been reached neither among different cosmological hydrodynamic simulations nor between the simulations and observations, despite the many advances in cosmological simulations (Vogelsberger et al 2019a). While the Magneticum pathfinder simulation (Remus et al 2017) and the Illustris Simulations (Xu et al 2017) produce shallower total density profile with time, the Horizon-AGN simulations (Peirani et al 2019) produce steeper total density profile with time, in better agreement with the redshift evolution trend found in observations. However, the latter simulation has smaller slope values compared to the former two, which are closer to the observed slope values due to different implementation of feedback models etc.…”

The last 6 Gyr of dark matter assembly in massive galaxies from the Kilo Degree Survey

“…Tortora et al 2014a;Poci et al 2017) and at high redshifts, where gas and in situ star formation dominate the galaxy evolution, the ETGs from cosmological simulations have a total density slope very steep (α ∼ −3), with merging events driving the galaxy to a nearly isothermal profile (Remus et al 2013(Remus et al , 2017. Xu et al (2017) from the Illustris simulation find a that α is constant with redshift, which would leave the trend unaffected. While, using strong lenses, Bolton et al (2012) and Sonnenfeld et al (2013) have shown that the total mass density slope is steeper at lower redshift.…”