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

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 γ of the total density profile for their main progenitors. We find that their γ steepen on average during z ∼ 4 − 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 ≈ 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 γ from z = 4 to z = 2, whereas subsequent AGN feedback (especially in the kinetic mode) makes γ shallower again from z = 2 to z = 1. Afterwards, passive evolution from z = 1 to z = 0, mainly through gas-poor mergers, mildly decreases γ and maintains the overall mass distribution close to isothermal.

SEAGLE – II. Constraints on feedback models in galaxy formation from massive early-type strong-lens galaxies

“…So we assume the effects are currently not significant in our case. A similar result is also reported recently by Wang et al (2019) where they find the density slope to remain nearly invariant after z=1 with a mild increase towards z = 0. However, in our case, a full-scale redshift evolution study is beyond the scope of this work.…”

Total mass density slopes of early-type galaxies using Jeans dynamical modelling at redshifts 0.29 < z < 0.55

“…The evolution with redshift implied by gravitational lensing results is at odds with the predictions of simulations, which indicate steeper slopes with higher redshifts (Remus et al 2013(Remus et al , 2017Wang et al 2019), and is also inconsistent to the observed mass-size growth of early-type galaxies with redshift as inferred from stellar light (Franx et al 2008;van Dokkum et al 2010;van der Wel et al 2014;Mowla et al 2019). Furthermore, gravitational lensing is necessarily biased towards the most massive, dense objects that act as effective lenses, and this can impact knowledge of the distribution of total mass density slopes (Mandelbaum, van de Ven & Keeton 2009).…”

“…However, strong evidence for evolution in the small redshift range of the Frontier Fields sample is not expected, considering also the intrinsic scatter of the sample and associated density slope uncertainties. The IllustrisTNG simulations indicate little evolution in density slope below z ∼ 1, due to evolution via gas-poor mergers (Wang et al 2019). The Magneticum simulations predict a mild trend with redshift given by the relation γ = −0.21z − 2.03 (Remus et al 2017).…”

“…This two-phase evolution is supported by simulation studies that have traced the total mass density slopes through cosmic time. Total mass density slopes γ are found to be steeper at greater lookback times, and approximately constant below redshifts of z ∼ 1, calculated over ∼4 effective radii (R e ) (Remus et al 2013(Remus et al , 2017Xu et al 2017;Springel et al 2018;Wang et al 2019; but see also Peirani et al 2019). At redshifts of zero, the simulations indicate a convergence of density slopes around γ ≈ −2, possibly as this 'isothermal' solution represents a low-energy, high-entropy state that galaxies evolve towards due to multiple stellar accretion events.…”

“…A KS test for the two samples gives a test statistic of 0.35, a critical value of 0.1769, and a p-value of 7 × 10 −10 , indicating the IllustrisTNG and Frontier Fields sample do not share a common distribution. The shallower IllustrisTNG slopes may arise due to the level of included active galactic nucleus (AGN) feedback in the models, which quenches in situ star formation, removes baryonic matter from central regions, and causes density slopes to approach isothermal values (Wang et al 2019(Wang et al , 2020. In particular, Wang et al (2019) find AGN feedback energy through the kinetic mode (the ejection of kinetic energy and momentum into surrounding gas cells without thermal energy) as implemented in the IllustrisTNG simulations dominates the change of density slopes towards shallower values in the redshift range 1 < z < 2, after which the density slope is near invariant.…”