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

TL;DR

This study investigates the evolution of the total density profiles of early-type galaxies using IllustrisTNG simulations, revealing a complex, mass-dependent trend influenced by mergers and AGN feedback from redshift 4 to 0.

Contribution

It provides a detailed analysis of the redshift evolution of galaxy density profiles and links these changes to physical processes like mergers and AGN feedback.

Findings

Density profile slopes steepen from z=4 to 2, then become shallower until z=1.

Post z=1, the slopes remain nearly constant with mild evolution towards z=0.

Mass dependence observed: low-mass galaxies have steeper slopes than high-mass ones.

Abstract

We study the evolutionary trend of the total density profile of early-type galaxies (ETGs) in IllustrisTNG. To this end, we trace ETGs from $z=0$ to $z=4$ and measure the power-law slope $\gamma^{\prime}$ of the total density profile for their main progenitors. We find that their $\gamma^{\prime}$ steepen on average during $z\sim4-2$, then becoming shallower until $z=1$, after which they remain almost constant, aside from a residual trend of becoming shallower towards $z=0$. We also compare to a statistical sample of ETGs at different redshifts, selected based on their luminosity profiles and stellar masses. Due to different selection effects, the average slopes of the statistical samples follow a modified evolutionary trend. They monotonically decrease since $z=3$, and after $z\approx 1$, they remain nearly invariant with a mild increase towards $z=0$. These evolutionary trends are mass-dependent for both samples, with low-mass galaxies having in general steeper slopes than their more massive counterparts. Galaxies that transitioned to ETGs more recently have steeper mean slopes as they tend to be smaller and more compact at any given redshift. By analyzing the impact of mergers and AGN feedback on the progenitors' evolution, we conjecture a multi-phase path leading to isothermality in ETGs: dissipation associated with rapid wet mergers tends to steepen $\gamma^{\prime}$ from $z=4$ to $z=2$, whereas subsequent AGN feedback (especially in the kinetic mode) makes $\gamma^{\prime}$ shallower again from $z=2$ to $z=1$. Afterwards, passive evolution from $z=1$ to $z=0$, mainly through gas-poor mergers, mildly decreases $\gamma^{\prime}$ and maintains the overall mass distribution close to isothermal.