Taking the Milky Way for a spin: disc formation in the ARTEMIS simulations

We investigate the formation (spin-up) of galactic discs in the ARTEMIS simulations of Milky Way-mass galaxies. In almost all galaxies discs spin up at higher [Fe/H] than the Milky Way (MW). Those that contain an analogue of the Gaia Sausage-Enceladus (GSE) spin up at a lower average metallicity than those without. We identify six galaxies with spin-up metallicity similar to that of the MW, which form their discs $\sim 8-11$ Gyr ago. Five of these experience a merger similar to the GSE. The spin-up times correlate with the halo masses at early times: galaxies with early spin-up have larger virial masses at a lookback time $t_L=12$ Gyr. The fraction of stars accreted from outside the host galaxy is smaller in galaxies with earlier spin-ups. Accreted fractions small enough to be comparable to the MW are only found in galaxies with the earliest disc formation and large initial virial masses ($M_\mathrm{200c} \approx2\times10^{11}M_\odot$ at $t_L=12$ Gyr). We find that discs form when the halo's virial mass reaches a threshold of $M_\mathrm{200c}\approx(6\pm3)\times10^{11}M_\odot$, independent of the spin-up time. We also find that discs form when the central potential is not particularly steep. Our results indicate that the MW assembled its mass and formed its disc earlier than the average galaxy of a similar mass.Comment: 8 pages, 6 figures, submitted to MNRA

Stellar halo striations from assumptions of axisymmetry

Motivated by the LMC's impact on the integral of motion space of the stellar halo, we run an $N$-body merger simulation to produce a population of halo-like stars. We subsequently move to a test particle simulation, in which the LMC perturbs this debris. When an axisymmetric potential is assumed for the final snapshot of the $N$-body merger remnant, a series of vertical striations in $(L_z, E)$ space form as the LMC approaches its pericentre. These result from the formation of overdensities in angular momentum owing to a relationship between the precession rate of near radial orbits and the torquing of these orbits by the LMC. This effect is heavily dependent on the shape of the inner potential. If a quadrupole component of the potential is included these striations become significantly less apparent due to the difference in precession rate between the two potentials. The absence of these features in data, and the dramatic change in orbital plane precession rate, discourages the use of an axisymmetric potential for highly eccentric orbits accreted from a massive GSE-like merger. Given the link between appearance of these striations and the shape of the potential, this effect may provide a new method of constraining the axisymmetry of the halo.Comment: 14 pages, 17 figures, submitted to MNRA

Merger-induced galaxy transformations in the ARTEMIS simulations

Using the ARTEMIS set of 45 high-resolution cosmological simulations, we investigate a range of merger-induced dynamical transformations of Milky Way-like galaxies. We first identify populations of accreted stars on highly radial orbits, similar to the 'Gaia Sausage' in the Milky Way. We show that $\approx1/3$ of the ARTEMIS galaxies contain a similar feature, and confirm that they usually comprise stellar debris from the most massive accreted satellite. Selecting these 15 galaxies, we study their changes around the times of the GS-like mergers. Dark matter haloes of many of these exhibit global changes in shape and orientation. Focusing on the galaxies themselves, we find multiple examples of stellar discs whose angular momentum (AM) axes change orientation at rapid rates of $\sim60$ degrees Gyr$^{-1}$. By calculating the orbital angular momentum axes of the satellites before they are accreted, we show that there is a tendency for the disc's AM to become more aligned with this axis after the merger. We also investigate the origin of in situ retrograde stars, analogous to the 'Splash' in the Milky Way. Tracing them back to earlier snapshots, we demonstrate that they were often disrupted onto their extreme orbits by multiple early mergers. We also find that the total mass of these stars outside the central regions positively correlates with the total accreted stellar mass. Finally, we conduct a brief investigation into whether bars form soon after the mergers. In a few galaxies we find a bar-like feature whose emergence coincides with a significant merger.Comment: 20 pages, 18 figures, submitted to MNRA

Parallelized Hybrid Monte Carlo Simulation of Stress-Induced Texture Evolution

A parallelized hybrid Monte Carlo (HMC) methodology is devised to quantify the microstructural evolution of polycrystalline material under elastic loading. The approach combines a time explicit material point method (MPM) for the mechanical stresses with a calibrated Monte Carlo (cMC) model for grain boundary kinetics. The computed elastic stress generates an additional driving force for grain boundary migration. The paradigm is developed, tested, and subsequently used to quantify the effect of elastic stress on the evolution of texture in nickel polycrystals. As expected, elastic loading favors grains which appear softer with respect to the loading direction. The rate of texture evolution is also quantified, and an internal variable rate equation is constructed which predicts the time evolution of the distribution of orientations.Comment: 20 pages, 8 figure

Taking the Milky Way for a spin: disc formation in the ARTEMIS simulations

We investigate the formation (spin-up) of galactic discs in the artemis simulations of Milky Way (MW)-mass galaxies. In almost all galaxies, discs spin up at higher [Fe/H] than the MW. Those galaxies that contain an analogue of the Gaia Sausage-Enceladus (GSE) spin up at a lower average metallicity than those without. We identify six galaxies with spin-up metallicity similar to that of the MW, which formed their discs ∼8-11 Gyr ago. Five of these experience a merger similar to the GSE. The spin-up times correlate with the halo masses at early times: galaxies with early spin-up have larger virial masses at a lookback time tL = 12 Gyr. The fraction of stars accreted from outside the host galaxy is smaller in galaxies with earlier spin-ups. Accreted fractions small enough to be comparable to the MW are only found in galaxies with the earliest disc formation and large initial virial masses (M200c ≈ 2 × 1011 M⊙ at tL = 12 Gyr). We find that discs form when the halo's virial mass reaches a threshold of M200c ≈ (6 ± 3) × 1011 M⊙, independent of the spin-up time. However, the failure to form a disc in other galaxies appears to be instead related to mergers at early times. We also find that discs form when the central potential is not particularly steep. Our results indicate that the MW assembled its mass and formed its disc earlier than the average galaxy of a similar mass

Merger-induced galaxy transformations in the ARTEMIS simulations

Using the ARTEMIS set of 45 high-resolution cosmological simulations, we investigate a range of merger-induced dynamical transformations of Milky Way-like galaxies. We first identify populations of accreted stars on highly radial orbits, similar to the ‘Gaia Sausage’ in the Milky Way. We show that ≈1/3 of the ARTEMIS galaxies contain a similar feature, and confirm that they usually comprise stellar debris from the most massive accreted satellite. Selecting 15 galaxies with discs at the present-day, we study their changes around the times of the GS-like mergers. Dark matter haloes of many of these exhibit global changes in shape and orientation, with almost half becoming significantly more spherical when the mergers occur. Focusing on the galaxies themselves, we find that 4/15 have stellar discs which experience large changes in the orientation of their angular momentum (AM) axes, at rates of up to ∼60 degrees Gyr−1. By calculating the orbital angular momentum axes of the satellites before they are accreted, we show that there is a tendency for the disc’s AM to become more aligned with this axis after the merger. We also investigate the origin of in situ retrograde stars, analogous to the ‘Splash’ in the Milky Way. Tracing them back to earlier snapshots, we demonstrate that they were often disrupted on to their extreme orbits by multiple early mergers. We also find that the total mass of these stars outside the central regions positively correlates with the total accreted stellar mass

A correlation between accreted stellar kinematics and dark matter halo spin in the ARTEMIS simulations

We report a correlation between the presence of a Gaia-Sausage-Enceladus (GSE) analogue and dark matter halo spin in the ARTEMIS simulations of Milky Way-like galaxies. The haloes which contain a large population of accreted stars on highly radial orbits (like the GSE) have lower spin on average than their counterparts with more isotropic stellar velocity distributions. The median modified spin parameters λ′ differ by a factor of ∼1.7 at the present-day, with a similar value when the haloes far from virial equilibrium are removed. We also show that accreted stars make up a smaller proportion of the stellar populations in haloes containing a GSE analogue, and are stripped from satellites with stellar masses typically ∼4 times smaller. Our findings suggest that the higher spin of DM haloes without a GSE-like feature is due to mergers with large satellites of stellar mass ∼1010M⊙, which do not result in prominent radially anisotropic features like the GSE

Ultrasonic Monitoring of Recrystallization Textures in Aluminum

The present paper is an attempt to use ultrasonic velocity measurements to characterize the texture of an aluminum-magnesium alloy (Al 5xxx) and to compare the results with orientation imaging microscopy (OIM) results. The results are characterized in terms of three orientation distribution coefficients (ODC’s), W400, W420, and W440, each of which describes a particular forming anisotropy, and each of which has significant impact on the final products

Energy wrinkles and phase-space folds of the last major merger

Relying on the dramatic increase in the number of stars with full 6D phase-space information provided by the Gaia Data Release 3, we resolve the distribution of the stellar halo around the Sun to uncover signatures of incomplete phase-mixing. We show that, for the stars likely belonging to the last massive merger, the (vr, r) distribution contains a series of long and thin chevron-like overdensities. These phase-space substructures have been predicted to emerge following the dissolution of a satellite, when its tidal debris is given time to wind up, thin out, and fold. Such chevrons have been spotted in external galaxies before; here, we report the first detection in our own Milky Way. We also show that the observed angular momentum Lz distribution appears more prograde at high energies, possibly revealing the original orbital angular momentum of the in-falling galaxy. The energy distribution of the debris is strongly asymmetric with a peak at low E – which, we surmise, may be evidence of the dwarf’s rapid sinking – and riddled with wrinkles and bumps. We demonstrate that similar phase-space and (E, Lz) substructures are present in numerical simulations of galaxy interactions, both in bespoke N-body runs and in cosmological hydrodynamical zoom-in suites. The remnant traces of the progenitor’s disruption and the signatures of the on-going phase-mixing discovered here will not only help to constrain the properties of our Galaxy’s most important interaction, but also can be used as a novel tool to map out the Milky Way’s current gravitational potential and its perturbations