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Register a new userThe stellar halos of ETGs in the IllustrisTNG simulations: the photometric and kinematic diversity of galaxies at large radii
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We characterize the photometric and kinematic properties of simulated early-type galaxy (ETG) stellar halos, and compare them to observations. We select a sample of ~1200 ETGs in the TNG100 and TNG50 simulations, spanning a stellar mass range of $10^{10.3}-10^{12}M_{odot}$ and within the range of (g-r) colour and lambda-ellipticity diagram populated by observed ETGs. We determine photometric parameters, intrinsic shapes, and kinematic observables in their extended stellar halos. We study the variation in kinematics from center to halo and connect it to a change in the intrinsic shape of the galaxies. We find that the simulated galaxy sample reproduces the diversity of kinematic properties observed in ETG halos. Simulated fast rotators (FRs) divide almost evenly in one third having flat lambda profiles and high halo rotational support, a third with gently decreasing profiles, and another third with low halo rotation. Slow rotators (SRs) tend to have increased rotation in the outskirts, with half of them exceeding lambda=0.2. For $M_{*}>10^{11.5}M_{odot}$ halo rotation is unimportant. A similar variety of properties is found for the stellar halo intrinsic shapes. Rotational support and shape are deeply related: the kinematic transition to lower rotational support is accompanied by a change towards rounder intrinsic shape. Triaxiality in the halos of FRs increases outwards and with stellar mass. Simulated SRs have relatively constant triaxiality profiles. Simulated stellar halos show a large variety of structural properties, with quantitative but no clear qualitative differences between FRs and SRs. At the same stellar mass, stellar halo properties show a gradual transition and significant overlap between the two families, despite the clear bimodality in the central regions. This is in agreement with observations of extended photometry and kinematics. [abridged]
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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 with stellar masses $10^{10.3}leq M_{*}/M_odotleq 10^{12}$, selected at z=0 within the range of g-r colour and lambda-ellipticity diagram populated by observed ETGs. We study how the rotational support and intrinsic shapes of the stellar halos depend on the fraction of accreted stars, overall and separately by major, minor, and mini mergers. Accretion histories in TNG100 ETGs as well as the radial distributions of ex-situ stars $f_{ex}(R)$ strongly correlate with stellar mass. Low-mass ETGs have characteristic peaked rotation profiles and near-oblate shapes with rounder halos that are completely driven by the in-situ stars. At high $f_{ex}$ major mergers decrease the in-situ peak in rotation velocity, flatten the $V_{*}/sigma_{*}(R)$ 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 $M_{*}>10^{10.6}M_odot$ the local rotational support decreases with the local ex-situ fraction $f_{ex}(R)$ and triaxiality increases with $f_{ex}$. These correlations are followed by fast and slow rotators alike with a continuous and overlapping sequence of properties. Merger events dynamically couple stars and dark matter: in high mass ETGs and at large radii where $f_{ex}gtrsim0.5$, 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 ($M_{*}>10^{10.6}M_odot$) 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. [abridged]
We investigate the evolution of stellar population gradients from $z=2$ to $z=0$ in massive galaxies at large radii ($r > 2R_{mathrm{eff}}$) using ten cosmological zoom simulations of halos with $6 times 10^{12} M_{odot} < M_{mathrm{halo}} < 2 times 10^{13}M_{odot}$. The simulations follow metal cooling and enrichment from SNII, SNIa and AGB winds. We explore the differential impact of an empirical model for galactic winds that reproduces the mass-metallicity relation and its evolution with redshift. At larger radii the galaxies, for both models, become more dominated by stars accreted from satellite galaxies in major and minor mergers. In the wind model, fewer stars are accreted, but they are significantly more metal poor resulting in steep global metallicity ($langle abla Z_{mathrm{stars}} rangle= -0.35$ dex/dex) and color (e.g. $langle abla g-r rangle = -0.13$ dex/dex) gradients in agreement with observations. In contrast, colour and metallicity gradients of the models without winds are inconsistent with observations. Age gradients are in general mildly positive at $z=0$ ($langle abla Age_{mathrm{stars}} rangle= 0.04$ dex/dex) with significant differences between the models at higher redshift. We demonstrate that for the wind model, stellar accretion is steepening existing in-situ metallicity gradients by about 0.2 dex by the present day and helps to match observed gradients of massive early-type galaxies at large radii. Colour and metallicity gradients are significantly steeper for systems which have accreted stars in minor mergers, while galaxies with major mergers have relatively flat gradients, confirming previous results. This study highlights the importance of stellar accretion for stellar population properties of massive galaxies at large radii, which can provide important constraints for formation models.
In this contribution we report on a kinematic study for 33 early type galaxies (ETGs) into their outer halos (average 6 effective radii, Re). We use planetary nebulae (PNe) as tracers of the main stellar population at large radii, where absorption line spectroscopy is no longer feasible. The ePN.S survey is the largest survey to-date of ETG kinematics with PNe, based on data from the Planetary Nebula Spectrograph (PN.S), counter-dispersed imaging, and high-resolution PN spectroscopy. We find that ETGs typically show a kinematic transition between inner regions and halos. Slow rotators have increased rotational support at large radii. Most of the ePN.S fast rotators show a decrease in rotation, due to the fading of the stellar disk in the outer, more slowly rotating spheroid. 30% of these fast rotators are dominated by rotation also at large radii, 40% show kinematic twists or misalignments, indicating a transition from oblate to triaxial in the halo. Despite this variety of kinematic behaviors, the ePN.S ETG halos have similar angular momentum content, independently of fast/slow rotation of the central regions. Estimated kinematic transition radii in units of Re are ~1-3 Re and anti-correlate with stellar mass. These results are consistent with cosmological simulations and support a two-phase formation scenario for ETGs.
The hierarchical theory of galaxy formation rests on the idea that smaller galactic structures merge to form the galaxies that we see today. The past decade has provided remarkable observational support for this scenario, driven in part by advances in spectroscopic instrumentation. Multi-object spectroscopy enabled the discovery of kinematically cold substructures around the Milky Way and M31 that are likely the debris of disrupting satellites. Improvements in high-resolution spectroscopy have produced key evidence that the abundance patterns of the Milky Way halo and its dwarf satellites can be explained by Galactic chemical evolution models based on hierarchical assembly. These breakthroughs have depended almost entirely on observations of nearby stars in the Milky Way and luminous red giant stars in M31 and Local Group dwarf satellites. In the next decade, extremely large telescopes will allow observations far down the luminosity function in the known dwarf galaxies, and they will enable observations of individual stars far out in the Galactic halo. The chemical abundance census now available for the Milky Way will become possible for our nearest neighbor, M31. Velocity dispersion measurements now available in M31 will become possible for systems beyond the Local Group such as Sculptor and M81 Group galaxies. Detailed studies of a greater number of individual stars in a greater number of spiral galaxies and their satellites will test hierarchical assembly in new ways because dynamical and chemical evolution models predict different outcomes for halos of different masses in different environments.
The IllustrisTNG project is a new suite of cosmological magneto-hydrodynamical simulations of galaxy formation performed with the Arepo code and updated models for feedback physics. Here we introduce the first two simulations of the series, TNG100 and TNG300, and quantify the stellar mass content of about 4000 massive galaxy groups and clusters ($10^{13} leq M_{rm 200c}/M_{rm sun} leq 10^{15}$) at recent times ($z leq 1$). The richest clusters have half of their total stellar mass bound to satellite galaxies, with the other half being associated with the central galaxy and the diffuse intra-cluster light. The exact ICL fraction depends sensitively on the definition of a central galaxys mass and varies in our most massive clusters between 20 to 40% of the total stellar mass. Haloes of $5times 10^{14}M_{rm sun}$ and above have more diffuse stellar mass outside 100 kpc than within 100 kpc, with power-law slopes of the radial mass density distribution as shallow as the dark matters ( $-3.5 < alpha_{rm 3D} < -3$). Total halo mass is a very good predictor of stellar mass, and vice versa: at $z=0$, the 3D stellar mass measured within 30 kpc scales as $propto (M_{rm 500c})^{0.49}$ with a $sim 0.12$ dex scatter. This is possibly too steep in comparison to the available observational constraints, even though the abundance of TNG less massive galaxies ($< 10^{11}M_{rm sun}$ in stars) is in good agreement with the measured galaxy stellar mass functions at recent epochs. The 3D sizes of massive galaxies fall too on a tight ($sim$0.16 dex scatter) power-law relation with halo mass, with $r^{rm stars}_{rm 0.5} propto (M_{rm 500c})^{0.53}$. Even more fundamentally, halo mass alone is a good predictor for the whole stellar mass profiles beyond the inner few kpc, and we show how on average these can be precisely recovered given a single mass measurement of the galaxy or its halo.
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