The stellar halos of ETGs in the IllustrisTNG simulations

Abstract

Stellar halos in early-type galaxies (ETGs) are shaped by their accretion and merger histories. We use a sample of 1114 ETGs in the TNG100 simulation of the IllustrisTNG suite with stellar masses 1010.3 ≤ M*/M⊙ ≤ 1012, selected at z = 0 within the range of the g − r colour and λ-ellipticity diagram populated by observed ETGs. We study how the rotational support and intrinsic shapes of the stellar halos depend on the fraction of stars accreted, both overall and separately, by major, minor, and mini mergers. Accretion histories in TNG100 ETGs as well as the final radial distributions of ex-situ stars fexsitu(R) relative to in-situ (“accretion classes”) strongly correlate with stellar mass. Low-mass galaxies have characteristic peaked rotation profiles and near-oblate shapes with rounder halos that are completely driven by the in-situ stars. At high fexsitu, major mergers decrease the in-situ peak in rotation velocity, flatten the V*/σ*(R) profiles, and increase the triaxiality of the stellar halos. Kinematic transition radii do not trace the transition between in-situ- and ex-situ-dominated regions, but for systems with M* > 1010.6 M⊙ the local rotational support of the stellar halos decreases with the local ex-situ fraction fexsitu(R) at fixed M*, and their triaxiality increases with fexsitu(R). These correlations between rotational support, intrinsic shapes, and local fexsitu are followed by fast and slow rotators alike with a continuous and overlapping sequence of properties, but slow rotators are concentrated at the high-fexsitu end dominated by dry major mergers. We find that in ∼20% of high-mass ETGs, the central regions are dominated by stars from a high-redshift compact progenitor. Merger events dynamically couple stars and dark matter: in high-mass galaxies and at large radii where fexsitu ≳ 0.5, both components tend to have similar intrinsic shapes and rotational support, and nearly aligned principal axes and spin directions. Based on these results we suggest that extended photometry and kinematics of massive ETGs (M* > 1010.6 M⊙) can be used to estimate the local fraction of ex-situ stars, and to approximate the intrinsic shapes and rotational support of the co-spatial dark matter component.

The halo of M105 and its group environment as traced by planetary nebula populations: II. Using kinematics of single stars to unveil the presence of intragroup light around the Leo I galaxies NGC 3384 and M105

“…Instead, the characteristic peaked and outwardly decreasing rotation profile of stars in the inner halo (see top panel of Fig. 13) is similar to that of the in-situ stars in low-mass ETGs in cosmological hydrodynamical simulations (Pulsoni et al 2021). In both panels, the dotted grey line at λ R = 0.1 separates fast from slow rotator regions assuming constant ellipticity.…”

“…In addition to the inferences based on their metal-rich nature, the kinematics of stars in the inner halo point towards an in-situ origin or they may have been brought in through few massive and ancient mergers. The outwardly decreasing rotation and LOS velocity dispersion profiles are similar to those of in-situ stars in massive ETGs in cosmological hydrodynamical simulations such as IllustrisTNG (Pulsoni et al 2021). The formation of the metal-rich inner stellar halo through massive and ancient mergers is also observed in IllustrisTNG (Zhu et al 2021).…”

“…Based on results from the IllustrisTNG cosmological hydrodynamical simulations Pulsoni et al (2021) argued that the observed kinematic transition radii do not trace the transition between in-situ and ex-situ dominated regions. The innermost transition radius at 1 R eff therefore does not necessarily represent a transition to a component dominated by accreted stars.…”

The Outer Stellar Mass of Massive Galaxies: A Simple Tracer of Halo Mass with Scatter Comparable to Richness and Reduced Projection Effects

“…While we cannot separate the ex situ component from the M distribution directly when using observational data alone, recent simulations and observations suggest that the ex situ stars dominate the outskirts of massive galaxies (e.g., Lackner et al 2012;Rodriguez-Gomez et al 2016;Pulsoni et al 2021). It is therefore interesting to test whether the stellar mass in the outer envelope is a useful Mvir proxy using the Top-N tests.…”

Dynamical modelling of the twisted galaxy PGC 046832

“…A second assumption is that the galaxy is a single triaxial system. It has been observed in simulations that stars in the outer parts (beyond ∼ 1 effective radius) trace the shape of the dark matter halo (Pulsoni et al 2020a), of which the axes are not necessarily aligned with those of its central galaxy.…”