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 Overall our results suggest that the notion of a universal density profile for dark matter haloes is no longer valid in the presence of galaxy formation.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.