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ABSTRACT Recent observational studies have uncovered a small number of very metal-poor (VMP) stars with cold kinematics in the Galactic disc and bulge. However, their origins remain enigmatic. We select a total of 138 Milky Way (MW) analogues from the TNG50 cosmological simulation based on their z = 0 properties: discy morphology, stellar mass, and local environment. In order to make more predictive statements for the MW, we further limit the spatial volume coverage of stellar populations in galaxies to that targeted by the upcoming 4MOST high-resolution survey of the Galactic disc and bulge. We find that across all galaxies, ∼20 per cent of VMP ([Fe/H] < −2) stars belong to the disc, with some analogues reaching 30 per cent. About 50 ± 10 per cent of the VMP disc stars are, on average, older than 12.5 Gyr and ∼70 ± 10 per cent come from accreted satellites. A large fraction of the VMP stars belong to the halo (∼70) and have a median age of 12 Gyr. Our results with the TNG50 cosmological simulation confirm earlier findings with simulations of fewer individual galaxies, and suggest that the stellar disc of the MW is very likely to host significant amounts of very- and extremely-metal-poor stars that, although mostly of ex situ origin, can also form in situ, reinforcing the idea of the existence of a primordial Galactic disc. 

ABSTRACT We quantify the impact of galaxy formation on dark matter halo shapes using cosmological simulations at redshift z = 0. Using magnetohydrodynamic simulations from the IllustrisTNG project, we focus on haloes of mass $$10^{10\!-\!14} \, \rm M_{\odot }$$ from the 50 Mpc (TNG50) and 100 Mpc (TNG100) boxes and compare them to dark matter-only (DMO) analogues and other simulations, e.g. Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) and Evolution and Assembly of GaLaxies and their Environments (EAGLE). We further quantify the prediction uncertainty by varying the feedback models using smaller 25 $${\rm Mpc}\, h^{-1}$$ boxes. We find that (i) galaxy formation results in rounder haloes compared to DMO simulations, in qualitative agreement with past results. Haloes of mass $${\approx }2\times 10^{12} \, \rm M_{\odot }$$ are most spherical, with an average minor-to-major axial ratio of $$\langle s \rangle$$ ≈ 0.75 in the inner halo, an increase of 40 per cent compared to their DMO counterparts. No significant difference is present for low-mass $$10^{10} \, \rm M_{\odot }$$ haloes; (ii) stronger feedback, e.g. increasing galactic wind speed, reduces the impact of baryons; (iii) the inner halo shape correlates with the stellar mass fraction, explaining the dependence of halo shapes on feedback models; and (iv) the fiducial and weaker feedback models are most consistent with observational estimates of the Milky Way halo shape. At fixed halo mass, very diverse and possibly unrealistic feedback models all predict inner shapes closer to one another than to the DMO results. Because of the large halo-to-halo variation in halo shape, a larger observational sample is required to statistically distinguish different baryonic prescriptions. 

Abstract The driving of turbulence in galaxies is deeply connected with the physics of feedback, star formation, outflows, accretion, and radial transport in disks. The velocity dispersion of gas in galaxies therefore offers a promising observational window into these processes. However, the relative importance of each of these mechanisms remains controversial. In this work we revisit the possibility that turbulence on galactic scales is driven by the direct impact of accreting gaseous material on the disk. We measure this effect in a disk-like star-forming galaxy in IllustrisTNG, using the high-resolution cosmological magnetohydrodynamical simulation TNG50. We employ Lagrangian tracer particles with a high time cadence of only a few million years to identify accretion and other events. The energies of particles are measured by stacking the events in bins of time around the event. The average effect of each event is measured by fitting explicit models for the kinetic and turbulent energies as a function of time. These measurements are corroborated by cross-correlating the turbulent energy with other time series and searching for signals of causality, i.e., asymmetries across zero time lag. We find that accretion contributes to the large-scale turbulent kinetic energy even if it does not dominate in this ∼5 × 10⁹M_⊙stellar mass galaxy. Extrapolating this finding to a range of galaxy masses, we find that there are regimes where energy from direct accretion may dominate the turbulent energy budget, particularly in disk outskirts, galaxies less massive than the Milky Way, and at redshift ∼2. 

ABSTRACT The sensitivity of X-ray facilities and our ability to detect fainter active galactic nuclei (AGNs) will increase with the upcoming Athena mission and the AXIS and Lynx concept missions, thus improving our understanding of supermassive black holes (BHs) in a luminosity regime that can be dominated by X-ray binaries. We analyse the population of faint AGNs ($$L_{\rm x, 2{-}10 \, keV}\leqslant 10^{42}\, \rm erg\,s^{ -1}$$) in the Illustris, TNG100, EAGLE, and SIMBA cosmological simulations, and find that the properties of their host galaxies vary from one simulation to another. In Illustris and EAGLE, faint AGNs are powered by low-mass BHs located in low-mass star-forming galaxies. In TNG100 and SIMBA, they are mostly associated with more massive BHs in quenched massive galaxies. We model the X-ray binary (XRB) populations of the simulated galaxies, and find that AGNs often dominate the galaxy AGN + XRB hard X-ray luminosity at z > 2, while XRBs dominate in some simulations at z < 2. Whether the AGN or XRB emission dominates in star-forming and quenched galaxies depends on the simulations. These differences in simulations can be used to discriminate between galaxy formation models with future high-resolution X-ray observations. We compare the luminosity of simulated faint AGN host galaxies to observations of stacked galaxies from Chandra. Our comparison indicates that the simulations post-processed with our X-ray modelling tend to overestimate the AGN + XRB X-ray luminosity; luminosity that can be strongly affected by AGN obscuration. Some simulations reveal clear AGN trends as a function of stellar mass (e.g. galaxy luminosity drop in massive galaxies), which are not apparent in the observations. 

ABSTRACT The James Webb Space Telescope will have the power to characterize high-redshift quasars at z ≥ 6 with an unprecedented depth and spatial resolution. While the brightest quasars at such redshift (i.e. with bolometric luminosity $$L_{\rm bol}\geqslant 10^{46}\, \rm erg/s$$) provide us with key information on the most extreme objects in the Universe, measuring the black hole (BH) mass and Eddington ratios of fainter quasars with $$L_{\rm bol}= 10^{45}-10^{46}\, \rm erg\,s^{ -1}$$ opens a path to understand the build-up of more normal BHs at z ≥ 6. In this paper, we show that the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA large-scale cosmological simulations do not agree on whether BHs at z ≥ 4 are overmassive or undermassive at fixed galaxy stellar mass with respect to the MBH − M⋆ scaling relation at z = 0 (BH mass offsets). Our conclusions are unchanged when using the local scaling relation produced by each simulation or empirical relations. We find that the BH mass offsets of the simulated faint quasar population at z ≥ 4, unlike those of bright quasars, represent the BH mass offsets of the entire BH population, for all the simulations. Thus, a population of faint quasars with $$L_{\rm bol}= 10^{45}-10^{46}\, \rm erg\,s^{ -1}$$ observed by JWST can provide key constraints on the assembly of BHs at high redshift. Moreover, this will help constraining the high-redshift regime of cosmological simulations, including BH seeding, early growth, and co-evolution with the host galaxies. Our results also motivate the need for simulations of larger cosmological volumes down to z ∼ 6, with the same diversity of subgrid physics, in order to gain statistics on the most extreme objects at high redshift. 

ABSTRACT We present a post-processing catalogue of globular clusters (GCs) for the 39 most massive groups and clusters in the TNG50 simulation of the IlllustrisTNG project (virial masses $$M_{200} =[5\times 10^{12} \rm {\!-\!} 2 \times 10^{14}$$] M⊙). We tag GC particles to all galaxies with stellar mass M* ≥ 5 × 106 M⊙, and we calibrate their masses to reproduce the observed power-law relation between GC mass and halo mass for galaxies with M200 ≥ 1011 M⊙ (corresponding to M* ∼ 109 M⊙). Here, we explore whether an extrapolation of this MGC–M200 relation to lower mass dwarfs is consistent with current observations. We find a good agreement between our predicted number and specific frequency of GCs in dwarfs with $$\rm {\it M}_*=[5 \times 10^6 \rm {\!-\!} 10^9]$$ M⊙ and observations. Moreover, we predict a steep decline in the GC occupation fraction for dwarfs with M* < 109 M⊙ that agrees well with current observational constraints. This declining occupation fraction is due to a combination of tidal stripping in all dwarfs plus a stochastic sampling of the GC mass function for dwarfs with M* < 107.5 M⊙. Our simulations also reproduce available constraints on the abundance of intracluster GCs in Virgo and Centaurus A. These successes provide support to the hypothesis that the MGC–M200 relation holds, albeit with more scatter, all the way down to the regime of classical dwarf spheroidals in these environments. Our GC catalogues are publicly available as part of the IllustrisTNG data release. 

Abstract Physical and chemical properties of the interstellar medium (ISM) at subgalactic (∼kiloparsec) scales play an indispensable role in controlling the ability of gas to form stars. In this paper, we use the TNG50 cosmological simulation to explore the physical parameter space of eight resolved ISM properties in star-forming regions to constrain the areas of this hyperspace where most star-forming environments exist. We deconstruct our simulated galaxies spanning a wide range of mass ( M ⋆ = 10 7 –10 11 M ⊙ ) and redshift (0 ≤ z ≤ 3) into kiloparsec-sized regions and statistically analyze the gas/stellar surface densities, gas metallicity, vertical stellar velocity dispersion, epicyclic frequency, and dark-matter volumetric density representative of each region in the context of their star formation activity and environment (radial galactocentric location). By examining the star formation rate (SFR) weighted distributions of these properties, we show that stars primarily form in two distinct environmental regimes, which are brought about by an underlying bicomponent radial SFR profile in galaxies. We examine how the relative prominence of these regimes depends on galaxy mass and cosmic time. We also compare our findings with those from integral field spectroscopy observations and find similarities as well as departures. Further, using dimensionality reduction, we characterize the aforementioned hyperspace to reveal a high degree of multicollinearity in relationships among ISM properties that drive the distribution of star formation at kiloparsec scales. Based on this, we show that a reduced 3D representation underpinned by a multivariate radius relationship is sufficient to capture most of the variance in the original 8D space. 

ABSTRACT We present the radial gas-phase, mass-weighted metallicity profiles and gradients of the TNG50 star-forming galaxy population measured at redshifts z = 0–3. We investigate the redshift evolution of gradients and examine relations between gradient (negative) steepness and galaxy properties. We find that TNG50 gradients are predominantly negative at all redshifts, although we observe significant diversity among these negative gradients. We determine that the gradients of all galaxies grow more negative with redshift at a roughly constant rate of approximately $$-0.02\ \mathrm{dex\, kpc^{-1}}/\Delta z$$. This rate does not vary significantly with galaxy mass. We observe a weak negative correlation between gradient (negative) steepness and galaxy stellar mass at z < 2. However, when we normalize gradients by a characteristic radius defined by the galactic star formation distribution, we find that these normalized gradients do not vary significantly with either stellar mass or redshift. We place our results in the context of previous simulations and show that TNG50 high-redshift gradients are more negative than those of models featuring burstier feedback, which may further highlight high-redshift gradients as important discriminators of galaxy formation models. We also find that z = 0 and z = 0.5 TNG50 gradients are consistent with the gradients observed in galaxies at these redshifts, although the preference for flat gradients observed in redshift z ≳ 1 galaxies is not present in TNG50. If future JWST (James Webb Space Telescope) and ELT (Extremely Large Telescope) observations validate these flat gradients, it may indicate a need for simulation models to implement more powerful radial gas mixing within the ISM (interstellar medium), possibly via turbulence and/or stronger winds. 

ABSTRACT Supermassive black holes (SMBHs) that reside at the centres of galaxies can inject vast amounts of energy into the surrounding gas and are thought to be a viable mechanism to quench star formation in massive galaxies. Here, we study the $$10^{9-12.5}\, \mathrm{M_\odot }$$ stellar mass central galaxy population of the IllustrisTNG simulation, specifically the TNG100 and TNG300 volumes at z = 0, and show how the three components – SMBH, galaxy, and circumgalactic medium (CGM) – are interconnected in their evolution. We find that gas entropy is a sensitive diagnostic of feedback injection. In particular, we demonstrate how the onset of the low-accretion black hole (BH) feedback mode, realized in the IllustrisTNG model as a kinetic, BH-driven wind, leads not only to star formation quenching at stellar masses $$\gtrsim 10^{10.5}\, \mathrm{M_\odot }$$ but also to a change in thermodynamic properties of the (non-star-forming) gas, both within the galaxy and beyond. The IllustrisTNG kinetic feedback from SMBHs increases the average gas entropy, within the galaxy and in the CGM, lengthening typical gas cooling times from $$10\!-\!100\, \mathrm{Myr}$$ to $$1\!-\!10\, \mathrm{Gyr}$$, effectively ceasing ongoing star formation and inhibiting radiative cooling and future gas accretion. In practice, the same active galactic nucleus (AGN) feedback channel is simultaneously ‘ejective’ and ‘preventative’ and leaves an imprint on the temperature, density, entropy, and cooling times also in the outer reaches of the gas halo, up to distances of several hundred kiloparsecs. In the IllustrisTNG model, a long-lasting quenching state can occur for a heterogeneous CGM, whereby the hot and dilute CGM gas of quiescent galaxies contains regions of low-entropy gas with short cooling times.