We examine the stellar haloes of the Auriga simulations, a suite of thirty cosmological magneto-hydrodynamical high-resolution simulations of Milky Way-mass galaxies performed with the moving-mesh code AREPO. We study halo global properties and radial profiles out to ∼150 kpc for each individual galaxy. The Auriga haloes are diverse in their masses and density profiles; mean metallicity and metallicity gradients; ages; and shapes, reflecting the stochasticity inherent in their accretion and merger histories. A comparison with observations of nearby late-type galaxies shows very good agreement between most observed and simulated halo properties. However, Auriga haloes are typically too massive. We find a connection between population gradients and mass assembly history: galaxies with few significant progenitors have more massive haloes, possess large negative halo metallicity gradients and steeper density profiles. The number of accreted galaxies, either disrupted or under disruption, that contribute 90% of the accreted halo mass ranges from 2 to 15, with a median of 6.5, and their stellar masses span over three orders of magnitude. The observed halo mass--metallicity relation is well reproduced by Auriga and is set by the stellar mass and metallicity of the dominant satellite contributors. This relationship is found not only for the accreted component but also for the total (accreted + in-situ) stellar halo. Our results highlight the potential of observable halo properties to infer the assembly history of galaxies. [go to paper]

Minor axis projected SB profile for spatially selected haloes (without kinematic selection) for all Auriga simulations. Shaded areas correspond to 5 and 95 percentiles. Coloured dashed lines are the profiles of few individual Auriga galaxies. Different symbols represent the observed minor axis data for the stellar haloes of GHOSTS galaxies (Harmsen et al. 2017) and M31 (Gilbert et al. 2012). See Monachesi et al. 2018 for full figure.

We present and analyse mock stellar catalogues that match the selection criteria and observables (including uncertainties) of the Gaia satellite data release 2 (DR2). The source are six cosmological high-resolution magneto-hydrodynamic LambdaCDM zoom simulations of the formation of Milky Way analogues from the AURIGA project. Mock data are provided for stars with V < 16 mag, and V < 20 mag at |b|>20 degrees. The mock catalogues are made using two different methods: the public SNAPDRAGONS code, and a method based on that of Lowing et al. that preserves the phase-space distribution of the model stars. The catalogues contain 5-parameter astrometry, radial velocities, multi-band photometry, stellar parameters, dust extinction values, and uncertainties in all these quantities. In addition, we provide theoretical quantities from the simulations, including the gravitational potential and information on the origin of each star. By way of demonstration we apply the mock catalogues to the young stellar disc and the stellar halo. We show that: i) the young (< 300 Myr) outer stellar disc exhibits a flared distribution that is reflected in the height and vertical velocity distribution of A- and B-dwarf stars up to radii comparable to about 15 kpc, and ii) the spin of the stellar halo out to 100 kpc can be accurately measured with Gaia DR2 RR Lyrae stars. We discuss the limitations of both methods and conclude that these mock stellar catalogues, which we make publicly available, are well suited for comparisons with observations and should help to: i) develop and test analysis methods for the Gaia DR2 data, ii) gauge the limitations and biases of the data and iii) interpret the data in the light of theoretical predictions from realistic ab initio simulations of galaxy formation in the LambdaCDM cosmological model. [go to paper]

We introduce a suite of 30 cosmological magneto-hydrodynamical zoom simulations of the formation of galaxies in isolated Milky Way mass dark haloes. These were carried out with the moving mesh code arepo, together with a comprehensive model for galaxy formation physics, including active galactic nuclei (AGN) feedback and magnetic fields, which produces realistic galaxy populations in large cosmological simulations. We demonstrate that our simulations reproduce a wide range of present-day observables, in particular, two-component disc-dominated galaxies with appropriate stellar masses, sizes, rotation curves, star formation rates and metallicities. We investigate the driving mechanisms that set present-day disc sizes/scalelengths, and find that they are related to the angular momentum of halo material. We show that the largest discs are produced by quiescent mergers that inspiral into the galaxy and deposit high-angular momentum material into the pre-existing disc, simultaneously increasing the spin of dark matter and gas in the halo. More violent mergers and strong AGN feedback play roles in limiting disc size by destroying pre-existing discs and by suppressing gas accretion on to the outer disc, respectively. The most important factor that leads to compact discs, however, is simply a low angular momentum for the halo. In these cases, AGN feedback plays an important role in limiting central star formation and the formation of a massive bulge. [go to paper]

Recent studies have revealed an oscillating asymmetry in the vertical structure of the Milky Way's disc. Here we analyze 16 high-resolution, fully cosmological simulations of the evolution of individual Milky Way-sized galaxies, carried out with the MHD code AREPO. At redshift zero, about 70% of our galactic discs show strong vertical patterns, with amplitudes that can exceed 2 kpc. Half of these are typical 'integral sign' warps (left). The rest are oscillations similar to those observed in the Milky Way. Such structures are thus expected to be common. The associated mean vertical motions can be as large as 30 km/s. Cold disc gas typically follows the vertical patterns seen in the stars. These perturbations have a variety of causes: close encounters with satellites, distant flybys of massive objects, accretion of misaligned cold gas from halo infall or from mergers. Tidally induced vertical patterns can be identified in both young and old stellar populations, whereas those originating from cold gas accretion are seen mainly in the younger populations. Galaxies with regular or at most weakly perturbed discs are usually, but not always, free from recent interactions with massive companions, although we have one case where an equilibrium compact disc reforms after a merger. [go to paper]