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Register a new userFormation history, structure and dynamics of discs and spheroids in simulated Milky Way mass galaxies
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Cecilia Scannapieco
Publication date
2011
fields
Physics
and research's language is
English
Authors
Cecilia Scannapieco
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We study the stellar discs and spheroids in eight simulations of galaxy formation within Milky Way-mass haloes in a Lambda Cold Dark Matter cosmology. A first paper in this series concentrated on disc properties. Here, we extend this analysis to study how the formation history, structure and dynamics of discs and spheroids relate to the assembly history and structure of their haloes. We find that discs are generally young, with stars spanning a wide range in stellar age: the youngest stars define thin discs and have near-circular orbits, while the oldest stars form thicker discs which rotate ~2 times slower than the thin components, and have 2-3 times larger velocity dispersions. Unlike the discs, spheroids form early and on short time-scales, and are dominated by velocity dispersion. We find great variety in their structure. The inner regions are bar- or bulge-like, while the extended outer haloes are rich in complex non-equilibrium structures such as stellar streams, shells and clumps. Our discs have very high in-situ fractions, i.e. most of their stars formed in the disc itself. Nevertheless, there is a non-negligible contribution (~15 percent) from satellites that are accreted on nearly coplanar orbits. The inner regions of spheroids also have relatively high in-situ fractions, but 65-85 percent of their outer stellar population is accreted. We analyse the circular velocities, rotation velocities and velocity dispersions of our discs and spheroids, both for gas and stars, showing that the dynamical structure is complex as a result of the non-trivial interplay between cooling and SN heating.
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We simulate an isolated, magnetised Milky Way-like disc galaxy using a self-consistent model of unresolved star formation and feedback, evolving the system until it reaches statistical steady state. We show that the quasi-steady-state structure is distinctly layered in galactocentric height $z$, with an innermost region having comparable gas and magnetic pressures (plasma beta $beta sim 1$), an outermost region having dominant gas pressures ($beta gg 1$), and an intermediate region between $300$ pc $lesssim |z| lesssim 3$ kpc that is dynamically dominated by magnetic fields ($beta ll 1$). We find field strengths, gas surface densities, and star formation rates that agree well with those observed both in the Galactic centre and in the Solar neighbourhood. The most significant dynamical effect of magnetic fields on the global properties of the disc is a reduction of the star formation rate by a factor of 1.5-2 with respect to an unmagnetised control simulation. At fixed star formation rate, there is no significant difference in the mass outflow rates or profiles between the magnetised and non-magnetised simulations. Our results for the global structure of the magnetic field have significant implications for models of cosmic ray-driven winds and cosmic-ray propagation in the Galaxy, and can be tested against observations with the forthcoming Square Kilometre Array and other facilities. Finally, we report the discovery of a physical error in the implementation of neutral gas heating and cooling in the popular GIZMO code, which may lead to qualitatively incorrect phase structures if not corrected.
Spectroscopic surveys of the Milky Ways stars have revealed spatial, chemical and kinematical structures that encode its history. In this work, we study their origins using a cosmological zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at $zsim 1.5$, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-[$alpha$/Fe] gas disc around the inner, metal-rich galaxy containing the old high-[$alpha$/Fe] stars. This event leads to a bimodality in [$alpha$/Fe] over a range of [Fe/H]. A detailed analysis of how the galaxy evolves since $zsim 1$ is presented. We demonstrate the way in which inside-out growth shapes the radial surface density and metallicity profile and how radial migration preferentially relocates stars from the inner to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scaleheights with stellar age, which together with disc flaring explains several trends observed in the Milky Way, including shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy formation scenario imprints non-trivial mappings between structural associations (i.e. thick and thin discs), velocity dispersions, $alpha$-enhancements, and ages of stars, e.g. the most metal-poor stars in the low-[$alpha$/Fe] sequence are found to have a scaleheight comparable to old high-[$alpha$/Fe] stars. Finally, we illustrate how at low spatial resolution, comparable to the thickness of the galaxy, the proposed pathway to distinct sequences in [$alpha$/Fe]-[Fe/H] cannot be captured.
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We study the formation of stellar haloes in three Milky Way-mass galaxies using cosmological smoothed particle hydrodynamics simulations, focusing on the subset of halo stars that form in situ, as opposed to those accreted from satellites. In situ stars in our simulations dominate the stellar halo out to 20 kpc and account for 30-40 per cent of its total mass. We separate in situ halo stars into three straightforward, physically distinct categories according to their origin: stars scattered from the disc of the main galaxy (heated disc), stars formed from gas smoothly accreted on to the halo (smooth gas) and stars formed in streams of gas stripped from infalling satellites (stripped gas). We find that most belong to the stripped gas category. Those originating in smooth gas outside the disc tend to form at the same time and place as the stripped-gas population, suggesting that their formation is associated with the same gas-rich accretion events. The scattered disc star contribution is negligible overall but significant in the Solar neighbourhood, where ~90 per cent of stars on eccentric orbits once belonged to the disc. However, the distinction between halo and thick disc in this region is highly ambiguous. The chemical and kinematic properties of the different components are very similar at the present day, but the global properties of the in situ halo differ substantially between the three galaxies in our study. In our simulations, the hierarchical buildup of structure is the driving force behind not only the accreted stellar halo, but also those halo stars formed in situ.
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