We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects) cosmological simulations to investigate the effects of baryonic physics on the time evolution of Dark Matter central density profiles. The sample is made of ≈ 70 independent high resolution hydrodynamical simulations of galaxy formation and covers a wide mass range:, from dwarfs to L ⋆ . We confirm previous results on the dependence of the inner dark matter density slope, α, on the ratio between stellar-to-halo mass, M star /M halo . We show that this relation holds approximately at all redshifts (with an intrinsic scatter of ∼ 0.18 in α measured between 1 − 2% of the virial radius). This implies that in practically all haloes the shape of their inner density profile changes quite substantially over cosmic time, as they grow in stellar and total mass. Thus, depending on their final M star /M halo ratio, haloes can either form and keep a substantial density core (R core ∼ 1 kpc), or form and then destroy the core and re-contract the halo, going back to a cuspy profile, which is even steeper than CDM predictions for massive galaxies (10 12 M ⊙ ). We show that results from the NIHAO suite are in good agreement with recent observational measurements of α in dwarf galaxies. Overall our results suggest that the notion of a universal density profile for dark matter haloes is no longer valid in the presence of galaxy formation.

The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2% of stars are within < 2 kpc), observed with the HERMES spectrograph at the Anglo-Australian Telescope. This release (hereafter GALAH+ DR3) includes all observations from GALAH Phase 1 (bright, main, and faint survey, 70%), K2-HERMES (17%), TESS-HERMES (5%), and a subset of ancillary observations (8%) including the bulge and > 75 stellar clusters. We derive stellar parameters Teff, log g, [Fe/H], vmic, vbroad, and vradusing our modified version of the spectrum synthesis code Spectroscopy Made Easy (sme) and 1D marcs model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from Gaia DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65% dwarfs, 34% giants, and 1% other/unclassified stars. Based on unflagged chemical composition and age, we find 62% young low-α, 9% young high-α, 27% old high-α, and 2% stars with [Fe/H] ≤ −1. Based on kinematics, 4% are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after GaiaeDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.

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The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs. [α/Fe] plane, showing a distinct high-α and low-α sequence whose origin is still under debate. We examine the [Fe/H]-[α/Fe] abundance plane in cosmological hydrodynamical simulations of Milky Way like galaxies from the NIHAO-UHD project and show that the bimodal α-sequence is a generic consequence of a gas-rich merger at some time in the Galaxy's evolution. The high-α sequence evolves first in the early galaxies, extending to high metallicities, while it is the low-α sequence that is formed after the gas-rich merger. The merger brings in fresh metal-poor gas diluting the interstellar medium's metallicity while keeping the [α/Fe] abundance almost unchanged. The kinematic, structural and spatial properties of the bimodal α-sequence in our simulations reproduces that of observations. In all simulations, the high-α disk is old, radially concentrated towards the galaxy's center and shows large scale heights. In contrast, the low-α disk is younger, more radially extended and concentrated to the disk mid-plane. Our results show that the abundance plane is well described by these two populations that have been distributed radially across the disk by migration: at present-day in the solar neighbourhood, low-α stars originate from both the inner and outer disk while high-α stars have all migrated from the inner disk. We show that age dating the stars in the [Fe/H]-[α/Fe] plane can constrain the time of the low-α sequence forming merger and conclude that α-bimodality is likely a not uncommon feature of disk galaxies.

We use ∼ 100 cosmological galaxy formation 'zoom-in' simulations using the smoothed particle hydrodynamics code gasoline to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf (M 200 ∼ 10 10 M ⊙ ) to Milky Way (M 200 ∼ 10 12 M ⊙ ) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ∼ 10 changes in the enclosed dark matter mass at 1 per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation:In addition, we report a new correlation with the compactness of the stellar system: ǫ R ≡ r 1/2 /R 200 . We provide an analytic formula depending on ǫ SF and ǫ R for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f , the overall effect is expansion, with more expansion with larger f . For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes.

Abstract We investigate the impact of cosmic rays (CR) and different modes of CR transport on the properties of Milky Way-mass galaxies in cosmological magneto-hydrodynamical simulations in the context of the AURIGA project. We systematically study how advection, anisotropic diffusion and additional Alfvén-wave cooling affect the galactic disc and the circum-galactic medium (CGM). Global properties such as stellar mass and star formation rate vary little between simulations with and without various CR transport physics, whereas structural properties such as disc sizes, CGM densities or temperatures can be strongly affected. In our simulations, CRs affect the accretion of gas onto galaxies by modifying the CGM flow structure. This alters the angular momentum distribution which manifests itself as a difference in stellar and gaseous disc size. The strength of this effect depends on the CR transport model: CR advection results in the most compact discs while the Alfvén-wave model resembles more the AURIGA model. The advection and diffusion models exhibit large (r ∼ 50 kpc) CR pressure-dominated gas haloes causing a smoother and partly cooler CGM. The additional CR pressure smoothes small-scale density peaks and compensates for the missing thermal pressure support at lower CGM temperatures. In contrast, the Alfvén-wave model is only CR pressure dominated at the disc-halo interface and only in this model the gamma-ray emission from hadronic interactions agrees with observations. In contrast to previous findings, we conclude that details of CR transport are critical for accurately predicting the impact of CR feedback on galaxy formation.

Simulating thin and extended galactic disks has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way mass galaxies and study stellar disk properties such as stellar mass, size and rotation velocity which agree well with observations of the Milky Way and local galaxies. In particular, the simulations reproduce the age-velocity dispersion relation and a multi-component stellar disk as observed for the Milky Way. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disk merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar disks is already set at time of birth as a result of the inside-out and upside-down formation. Subsequent evolution modifies this structure by increasing both the scale length and height of all mono-age populations. Thus, our results put tight constraints on how much dynamical memory stellar disks can retain over cosmological timescales. Our simulations demonstrate that it is possible to form thin galactic disks in cosmological simulations provided there are no significant stellar mergers at low redshifts. Most of the stellar mass is formed in-situ with only a few percent ( ∼ < 5%) brought in by merging satellites at early times. Redshift zero snapshots and halo catalogues are publicly available.

Many massive star forming disc galaxies in the redshift range 3 to 0.5 are observed to have a clumpy morphology showing giant clumps of size ∼1 kpc and masses of about 10 7 M to 10 10 M . The nature and fate of these giant clumps is still under debate. In this work we use 19 high-resolution simulations of disc galaxies from the NIHAO sample to study the formation and the evolution of clumps in the discs of high redshift galaxies. We use mock HST -CANDELS observations created with the radiative transfer code GRASIL-3D to carry out, for the first time, a quantitative comparison of the observed fraction of clumpy galaxies and its evolution with redshift with simulations. We find a good agreement between the observed clumpy fraction and the one of the NIHAO galaxies. We find that dust attenuation can suppress intrinsically bright clumps and enhance less luminous ones. In our galaxy sample we only find clumps in light (u-band) from young stars but not in stellar mass surface density maps. This means that the NIHAO sample does not show clumpy stellar discs but rather a clumpy light distribution originating from clumpy star formation events. The clumps found in the NIHAO sample match observed age/color gradients as a function of distance from the galaxy center but they show no sign of inward migration. Clumps in our simulations disperse on timescales of a about a hundred Myr and their contribution to bulge growth is negligible.