ABSTRACT
We characterize the 3D spatial variations of Fe/H, Mg/H, and Mg/Fe in stars at the time of their formation, across 11 simulated Milky Way (MW)- and M31-mass galaxies in the FIRE-2 ...simulations, to inform initial conditions for chemical tagging. The overall scatter in Fe/H within a galaxy decreased with time until $\approx 7 \, \rm {Gyr}$ ago, after which it increased to today: this arises from a competition between a reduction of azimuthal scatter and a steepening of the radial gradient in abundance over time. The radial gradient is generally negative, and it steepened over time from an initially flat gradient $\gtrsim 12 \, \rm {Gyr}$ ago. The strength of the present-day abundance gradient does not correlate with when the disc ‘settled’; instead, it best correlates with the radial velocity dispersion within the galaxy. The strength of azimuthal variation is nearly independent of radius, and the 360 deg scatter decreased over time, from $\lesssim 0.17 \, \rm {dex}$ at $t_{\rm lb} = 11.6 \, \rm {Gyr}$ to $\sim 0.04 \, \rm {dex}$ at present-day. Consequently, stars at $t_{\rm lb} \gtrsim 8 \, \rm {Gyr}$ formed in a disc with primarily azimuthal scatter in abundances. All stars formed in a vertically homogeneous disc, ΔFe/H$\le 0.02 \, \rm {dex}$ within $1 \, \rm {kpc}$ of the galactic mid-plane, with the exception of the young stars in the inner $\approx 4 \, \rm {kpc}$ at z ∼ 0. These results generally agree with our previous analysis of gas-phase elemental abundances, which reinforces the importance of cosmological disc evolution and azimuthal scatter in the context of stellar chemical tagging. We provide analytic fits to our results for use in chemical-tagging analyses.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted) We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 147 stellar targets along the ...line of sight to the newly discovered "ultrafaint" stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru 1). Based on simultaneous estimates of line of sight velocity and stellar-atmospheric parameters, we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of ... km s super(-1) about a mean of ... km s super(-1)(solar rest frame), and to estimate a mean metallicity of Fe/H = ... These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies, suggesting a dominant dark matter component with dynamical mass ... M sub(middot in circle) enclosed within the central ~160 pc, and dynamical mass-to-light ratio ... M sub(middot in circle)/Lv .middot in circle. For Gru 1 we estimate a mean velocity of ... km s super(-1) and a mean metallicity of Fe/H = ... but our sample does not resolve Gru 1's velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within a distance of <, ~ 300 kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.
We present evidence that isolated growing discs, subject to internal spiral perturbations, thicken due to both heating and radial migration. We show this by demonstrating that the thickness and ...vertical velocity dispersions of coeval stars depend on their age as well as the change in their radii. While the disc thickens due to internal processes, we find that this induces only a minor amount of flaring. We further demonstrate the consequences of such thickening on the structural properties of stellar populations and find that they qualitatively agree with recent studies of the Milky Way disc.
With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing ...observations to simulations of Milky Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and use it to generate a suite of synthetic surveys resembling Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments project, which feature self-consistent clustering of star formation in dense molecular clouds and thin stellar/gaseous disks in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We sample synthetic stars by assuming each star particle (of mass 7070 M ) represents a single stellar population. At each viewpoint, we compute dust extinction from the simulated gas metallicity distribution and apply a simple error model to produce a synthetic Gaia-like survey that includes both observational properties and a pointer to the generating star particle. We provide the complete simulation snapshot at z = 0 for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data.
ABSTRACT
Kinematic studies of disc galaxies, using individual stars in the Milky Way or statistical studies of global disc kinematics over time, provide insight into how discs form and evolve. We use ...a high-resolution, cosmological zoom-simulation of a Milky Way-mass disc galaxy (h277) to tie together local disc kinematics and the evolution of the disc over time. The present-day stellar age–velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighbourhood measurement in the Milky Way. A crucial element of this success is the simulation’s dynamically cold multiphase ISM, which allows young stars to form with a low velocity dispersion (σbirth$\sim \!6 - 8 \ \mathrm{km\, s}^{-1}$) at late times. Older stars are born kinematically hotter (i.e. the disc settles over time in an ‘upside-down’ formation scenario), and are subsequently heated after birth. The disc also grows ‘inside-out’, and many of the older stars in the present-day solar neighbourhood are present because of radial mixing. We demonstrate that the evolution of σbirth in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disc galaxies from z ∼ 2–3 to the present-day, in which the disc evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way’s AVR with observed kinematics of high z disc galaxies.
We compare the spatial, kinematic, and metallicity distributions of stars in the Milky Way disk, as observed by the Sloan Digital Sky Survey and Geneva-Copenhagen Survey, to predictions made by ...N-body simulations that naturally include radial migration as proposed by Sellwood & Binney. In these simulations, stars that migrate radially outward feel a decreased restoring force, consequentially they reach larger heights above the mid-plane. We find that this model is in qualitative agreement with observational data and can explain the disk's double-exponential vertical structure and other characteristics as due to internal evolution. In particular, the model reproduces observations of stars in the transition region between exponential components, which do not show a strong correlation between rotational velocity and metallicity. Although such a correlation is present in young stars because of epicyclic motions, radial migration efficiently mixes older stars and weakens the correlation. Classifying stars as members of the thin or thick disk by either velocity or metallicity leads to an apparent separation in the other property, as observed. We find a much stronger separation when using Delta *a/Fe, which is a good proxy for stellar age. The model success is remarkable because the simulation was not tuned to reproduce the Galaxy, hinting that the thick disk may be a ubiquitous Galactic feature generated by stellar migration. Nonetheless, we cannot exclude the possibility that some fraction of the thick disk is a fossil of a more violent history, nor can radial migration explain thick disks in all galaxies, most strikingly those which counterrotate with respect to the thin disk.
ABSTRACT
The disc structure of the Milky Way is marked by a chemical dichotomy, with high-α and low-α abundance sequences, traditionally identified with the geometric thick and thin discs. This ...identification is aided by the old ages of the high-α stars, and lower average ages of the low-α ones. Recent large-scale surveys such as APOGEE have provided a wealth of data on this chemical structure, including showing that an identification of chemical and geometric thick discs is not exact, but the origin of the chemical dichotomy has remained unclear. Here we demonstrate that a dichotomy arises naturally if the early gas-rich disc fragments, leading to some fraction of the star formation occuring in clumps of the type observed in high-redshift galaxies. These clumps have high star formation rate density. They therefore enrich rapidly, moving from the low-α to the high-α sequence, while more distributed star formation produces the low-α sequence. We demonstrate that this model produces a chemically defined thick disc that has many of the properties of the Milky Way’s thick disc. Because clump formation is common in high-redshift galaxies, we predict that chemical bimodalities are common in massive galaxies.
ABSTRACT
We develop a hybrid model of galactic chemical evolution that combines a multiring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution ...of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc’s abundance structure: (i) the dependence of the O/Fe–Fe/H distribution on radius Rgal and mid-plane distance |z|; (ii) the changing shapes of the O/H and Fe/H distributions with Rgal and |z|; (iii) a broad distribution of O/Fe at sub-solar metallicity and changes in the O/Fe distribution with Rgal, |z|, and Fe/H; (iv) a tight correlation between O/Fe and stellar age for O/Fe > 0.1; (v) a population of young and intermediate-age α-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age–O/H and age–Fe/H relations, with large scatter and a median age of ∼4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate ∼2 Gyr ago improve agreement with the observed age–Fe/H and age–O/H relations, but worsen agreement with the observed age–O/Fe relation. None of our models predict an O/Fe distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice).
ABSTRACT
We present the first measurement of the lifetimes of giant molecular clouds (GMCs) in cosmological simulations at z = 0, using the Latte suite of FIRE-2 simulations of Milky Way (MW) mass ...galaxies. We track GMCs with total gas mass ≳105 M⊙ at high spatial (∼1 pc), mass (7100 M⊙), and temporal (1 Myr) resolution. Our simulated GMCs are consistent with the distribution of masses for massive GMCs in the MW and nearby galaxies. We find GMC lifetimes of 5–7 Myr, or 1–2 freefall times, on average, with less than 2 per cent of clouds living longer than 20 Myr. We find decreasing GMC lifetimes with increasing virial parameter, and weakly increasing GMC lifetimes with galactocentric radius, implying that environment affects the evolutionary cycle of GMCs. However, our GMC lifetimes show no systematic dependence on GMC mass or amount of star formation. These results are broadly consistent with inferences from the literature and provide an initial investigation into ultimately understanding the physical processes that govern GMC lifetimes in a cosmological setting.
ABSTRACT
We use FIRE-2 simulations to examine 3D variations of gas-phase elemental abundances of O/H, Fe/H, and N/H in 11 MW and M31-mass galaxies across their formation histories at z ≤ 1.5 ($t_{\rm ...lookback} \le 9.4 \, \rm {Gyr}$), motivated by characterizing the initial conditions of stars for chemical tagging. Gas within $1 \, \rm {kpc}$ of the disc mid-plane is vertically homogeneous to $\lesssim 0.008 \, \rm {dex}$ at all z ≤ 1.5. We find negative radial gradients (metallicity decreases with galactocentric radius) at all times, which steepen over time from $\approx \! -0.01 \, \rm {dex}\, \rm {kpc}^{-1}$ at z = 1 ($t_{\rm lookback} = 7.8 \, \rm {Gyr}$) to $\approx \! -0.03 \, \rm {dex}\, \rm {kpc}^{-1}$ at z = 0, and which broadly agree with observations of the MW, M31, and nearby MW/M31-mass galaxies. Azimuthal variations at fixed radius are typically $0.14 \, \rm {dex}$ at z = 1, reducing to $0.05 \, \rm {dex}$ at z = 0. Thus, over time radial gradients become steeper while azimuthal variations become weaker (more homogeneous). As a result, azimuthal variations were larger than radial variations at z ≳ 0.8 ($t_{\rm lookback} \gtrsim 6.9 \, \rm {Gyr}$). Furthermore, elemental abundances are measurably homogeneous (to ≲0.05 dex) across a radial range of $\Delta R \approx 3.5 \, \rm {kpc}$ at z ≳ 1 and $\Delta R \approx 1.7 \, \rm {kpc}$ at z = 0. We also measure full distributions of elemental abundances, finding typically negatively skewed normal distributions at z ≳ 1 that evolve to typically Gaussian distributions by z = 0. Our results on gas abundances inform the initial conditions for stars, including the spatial and temporal scales for applying chemical tagging to understand stellar birth in the MW.
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