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Register a new userChemodynamics of a Simulated Disc Galaxy: Initial Mass Functions and SNIa Progenitors
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We trace the formation and advection of several elements within a cosmological adaptive mesh refinement simulation of an L* galaxy. We use nine realisations of the same initial conditions with different stellar Initial Mass Functions (IMFs), mass limits for type-II and type-Ia supernovae (SNII, SNIa) and stellar lifetimes to constrain these sub-grid phenomena. Our code includes self-gravity, hydrodynamics, star formation, radiative cooling and feedback from multiple sources within a cosmological framework. Under our assumptions of nucleosynthesis we find that SNII with progenitor masses of up to 100 Msun are required to match low metallicity gas oxygen abundances. Tardy SNIa are necessary to reproduce the classical chemical evolution knee in [O/Fe]-[Fe/H]: more prompt SNIa delayed time distributions do not reproduce this feature. Within our framework of hydrodynamical mixing of metals and galaxy mergers we find that chemical evolution is sensitive to the shape of the IMF and that there exists a degeneracy with the mass range of SNII. We look at the abundance plane and present the properties of different regions of the plot, noting the distinct chemical properties of satellites and a series of nested discs that have greater velocity dispersions, are more alpha-rich and metal poor with age.
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Using 22 hydrodynamical simulated galaxies in a LCDM cosmological context we recover not only the observed baryonic Tully-Fisher relation, but also the observed mass discrepancy--acceleration relation, which reflects the distribution of the main components of the galaxies throughout their disks. This implies that the simulations, which span the range 52 < V$_{rm flat}$ < 222 km/s where V$_{rm flat}$ is the circular velocity at the flat part of the rotation curve, and match galaxy scaling relations, are able to recover the observed relations between the distributions of stars, gas and dark matter over the radial range for which we have observational rotation curve data. Furthermore, we explicitly match the observed baryonic to halo mass relation for the first time with simulated galaxies. We discuss our results in the context of the baryon cycle that is inherent in these simulations, and with regards to the effect of baryonic processes on the distribution of dark matter.
Massive early-type galaxies commonly have gas discs which are kinematically misaligned with the stellar component. These discs feel a torque from the stars and the angular momentum vectors are expected to align quickly. We present results on the evolution of a misaligned gas disc in a cosmological simulation of a massive early-type galaxy from the Feedback In Realistic Environments project. This galaxy experiences a merger which, together with a strong galactic wind, removes most of the original gas disc. The galaxy subsequently reforms a gas disc through accretion of cold gas, but it is initially 120 degrees misaligned with the stellar rotation axis. This misalignment persists for about 2 Gyr before the gas-star misalignment angle drops below 20 degrees. The time it takes for the gaseous and stellar components to align is much longer than previously thought, because the gas disc is accreting a significant amount of mass for about 1.5 Gyr after the merger, during which the angular momentum change induced by accreted gas dominates over that induced by stellar torques. Once the gas accretion rate has decreased sufficiently, the gas disc decouples from the surrounding halo gas and realigns with the stellar component in about 6 dynamical times. During the late evolution of the misaligned gas disc, the centre aligns faster than the outskirts, resulting in a warped disc. We discuss the observational consequences of the long survival of our misaligned gas disc and how our results can be used to calibrate merger rate estimates from observed gas misalignments.
Within a cosmological hydrodynamical simulation, we form a disc galaxy with sub- components which can be assigned to a thin stellar disc, thick disk, and a low mass stellar halo via a chemical decomposition. The thin and thick disc populations so selected are distinct in their ages, kinematics, and metallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity dispersion ({sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The thick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by sim21 km/s, possess higher velocity dispersion ({sigma}U,V,W = 49, 44, 35 km/s), relatively low [Fe/H] and high [O/Fe]. The halo component comprises less than 4% of stars in the solar annulus of the simulation, has low metallicity, a velocity ellipsoid defined by ({sigma}U,V,W = 62, 46, 45 km/s) and is formed primarily in-situ during an early merger epoch. Gas-rich mergers during this epoch play a major role in fuelling the formation of the old disc stars (the thick disc). This is consistent with studies which show that cold accretion is the main source of a disc galaxys baryons. Our simulation initially forms a relatively short (scalelength sim1.7 kpc at z=1) and kinematically hot disc, primarily from gas accreted during the galaxys merger epoch. Far from being a competing formation scenario, migration is crucial for reconciling the short, hot, discs which form at high redshift in {Lambda}CDM, with the properties of the thick disc at z=0. The thick disc, as defined by its abundances maintains its relatively short scale-length at z = 0 (2.31 kpc) compared with the total disc scale-length of 2.73 kpc. The inside-out nature of disc growth is imprinted the evolution of abundances such that the metal poor {alpha}-young population has a larger scale-length (4.07 kpc) than the more chemically evolved metal rich {alpha}-young population (2.74 kpc).
We study the structure, age and metallicity gradients, and dynamical evolution using a cosmological zoom-in simulation of a Milky Way-mass galaxy from the Feedback in Realistic Environments project. In the simulation, stars older than 6 Gyr were formed in a chaotic, bursty mode and have the largest vertical scale heights (1.5-2.5 kpc) by z=0, while stars younger than 6 Gyr were formed in a relatively calm, stable disk. The vertical scale height increases with stellar age at all radii, because (1) stars that formed earlier were thicker at birth, and (2) stars were kinematically heated to an even thicker distribution after formation. Stars of the same age are thicker in the outer disk than in the inner disk (flaring). These lead to positive vertical age gradients and negative radial age gradients. The radial metallicity gradient is neg- ative at the mid-plane, flattens at larger disk height |Z|, and turns positive above |Z|~1.5kpc. The vertical metallicity gradient is negative at all radii, but is steeper at smaller radii. These trends broadly agree with observations in the Milky Way and can be naturally understood from the age gradients. The vertical stellar density profile can be well-described by two components, with scale heights 200-500 pc and 1-1.5 kpc, respectively. The thick component is a mix of stars older than 4 Gyr which formed through a combination of several mechanisms. Our results also demonstrate that it is possible to form a thin disk in cosmological simulations even with strong stellar feedback.
We use multi-band imagery data from the Sloan Digital Sky Survey (SDSS) to measure projected distances of 302 supernova type Ia (SNIa) from the centre of their host galaxies, normalized to the galaxys brightness scale length, with a Bayesian approach. We test the hypothesis that SNIas further away from the centre of their host galaxy are less subject to dust contamination (as the dust column density in their environment is smaller) and/or come from a more homogeneous environment. Using the Mann-Whitney U test, we find a statistically significant difference in the observed colour correction distribution between SNIas that are near and those that are far from the centre of their host. The local p-value is 3 x 10^{-3}, which is significant at the 5 per cent level after look-elsewhere effect correction. We estimate the residual scatter of the two subgroups to be 0.073 +/- 0.018 for the far SNIas, compared to 0.114 +/- 0.009 for the near SNIas -- an improvement of 30 per cent, albeit with a low statistical significance of 2sigma. This confirms the importance of host galaxy properties in correctly interpreting SNIa observations for cosmological inference.
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