ABSTRACT
Studies of the ages, abundances, and motions of individual stars in the Milky Way provide one of the best ways to study the evolution of disc galaxies over cosmic time. The formation of the ...Milky Way’s barred inner region in particular is a crucial piece of the puzzle of disc galaxy evolution. Using data from APOGEE and Gaia, we present maps of the kinematics, elemental abundances, and age of the Milky Way bulge and disc that show the barred structure of the inner Milky Way in unprecedented detail. The kinematic maps allow a direct, purely kinematic determination of the bar’s pattern speed of $41\pm 3\, \mathrm{km\, s}^{-1}\, \mathrm{kpc}^{-1}$ and of its shape and radial profile. We find the bar’s age, metallicity, and abundance ratios to be the same as those of the oldest stars in the disc that are formed in its turbulent beginnings, while stars in the bulge outside of the bar are younger and more metal-rich. This implies that the bar likely formed ${\approx}8\, \mathrm{Gyr}$ ago, when the decrease in turbulence in the gas disc allowed a thin disc to form that quickly became bar-unstable. The bar’s formation therefore stands as a crucial epoch in the evolution of the Milky Way, a picture that is in line with the evolutionary path that emerges from observations of the gas kinematics in external disc galaxies over the last ${\approx}10\, \mathrm{Gyr}$.
ABSTRACT
The stellar mass in the halo of the Milky Way is notoriously difficult to determine, owing to the paucity of its stars in the solar neighbourhood. With tentative evidence from Gaia that the ...nearby stellar halo is dominated by a massive accretion event – referred to as Gaia-Enceladus or Sausage – these constraints are now increasingly urgent. We measure the mass in kinematically selected mono-abundance populations (MAPs) of the stellar halo between −3 < Fe/H < −1 and 0.0 < Mg/Fe < 0.4 using red giant star counts from APOGEE DR14. We find that MAPs are well fit by single power laws on triaxial ellipsoidal surfaces, and we show that that the power-law slope α changes such that high Mg/Fe populations have α ∼ 4, whereas low Mg/Fe MAPs are more extended with shallow slopes, α ∼ 2. We estimate the total stellar mass to be $M_{*,\mathrm{tot}} = 1.3^{+0.3}_{-0.2}\times 10^{9}\ \mathrm{M_{\odot}}$, of which we estimate ${\sim}0.9^{+0.2}_{-0.1} \times 10^{9}\ \mathrm{M_{\odot}}$ to be accreted. We estimate that the mass of accreted stars with e > 0.7 is M*,accreted, e > 0.7 = 3 ± 1 (stat.) ± 1 (syst.) × 108 M⊙, or ${\sim}30{-}50{{\ \rm per\ cent}}$ of the accreted halo mass. If the majority of these stars are the progeny of a massive accreted dwarf, this places an upper limit on its stellar mass, and implies a halo mass for the progenitor of ∼1010.2 ± 0.2 M⊙. This constraint not only shows that the Gaia-Enceladus/Sausage progenitor may not be as massive as originally suggested, but that the majority of the Milky Way stellar halo was accreted. These measurements are an important step towards fully reconstructing the assembly history of the Milky Way.
Orbital parameters, such as eccentricity and maximum vertical excursion, of stars in the Milky Way are an important tool for understanding its dynamics and evolution, but calculation of such ...parameters usually relies on computationally expensive numerical orbit integration. We present and test a fast method for estimating these parameters using an application of the Stäckel fudge, used previously for the estimation of action-angle variables. We show that the method is highly accurate, to a level of <1% in eccentricity, over a large range of relevant orbits and in different Milky Way-like potentials, and demonstrate its validity by estimating the eccentricity distribution of the RAVE-TGAS data set and comparing it with that from orbit integration. Using this method, the orbital characteristics of the ∼7 million Gaia DR2 stars with radial velocity measurements are computed with Monte Carlo sampled errors in ∼116 hours of parallelized cpu time, at a speed that we estimate to be ∼3 to 4 orders of magnitude faster than using numerical orbit integration. We demonstrate using this catalog that Gaia DR2 samples a large range of orbits in the solar vicinity, down to those with rap 2.5 kpc, and out to rperi 13 kpc. We also show that many of the features present in orbital parameter space have a low mean zmax, suggesting that they likely result from disk dynamical effects.
ABSTRACT
Spectroscopic surveys of the Galaxy reveal that its disc stars exhibit a spread in α/Fe at fixed Fe/H, manifest at some locations as a bimodality. The origin of these diverse, and possibly ...distinct, stellar populations in the Galactic disc is not well understood. We examine the Fe and α-element evolution of 133 Milky Way-like galaxies from the EAGLE simulation, to investigate the origin and diversity of their α/Fe–Fe/H distributions. We find that bimodal α/Fe distributions arise in galaxies whose gas accretion histories exhibit episodes of significant infall at both early and late times, with the former fostering more intense star formation than the latter. The shorter characteristic consumption time-scale of gas accreted in the earlier episode suppresses its enrichment with iron synthesized by Type Ia SNe, resulting in the formation of a high-α/Fe sequence. We find that bimodality in α/Fe similar to that seen in the Galaxy is rare, appearing in approximately 5 per cent of galaxies in our sample. We posit that this is a consequence of an early gas accretion episode requiring the mass accretion history of a galaxy’s dark matter halo to exhibit a phase of atypically rapid growth at early epochs. The scarcity of EAGLE galaxies exhibiting distinct sequences in the α/Fe–Fe/H plane may therefore indicate that the Milky Way’s elemental abundance patterns, and its accretion history, are not representative of the broader population of ∼L⋆ disc galaxies.
Abstract
The kinematics of the Milky Way disc as a function of age are well measured at the solar radius, but have not been studied over a wider range of Galactocentric radii. Here, we measure the ...kinematics of mono-age, mono-Fe/H populations in the low and high α/Fe discs between 4 ≲ R ≲ 13 kpc and |z| ≲ 2 kpc using 65 719 stars in common between APOGEE DR14 and Gaia DR2 for which we estimate ages using a Bayesian neural network model trained on asteroseismic ages. We determine the vertical and radial velocity dispersions, finding that the low and high α/Fe discs display markedly different age–velocity dispersion relations (AVRs) and shapes σz/σR. The high α/Fe disc has roughly flat AVRs and constant σz/σR = 0.64 ± 0.04, whereas the low α/Fe disc has large variations in this ratio that positively correlate with the mean orbital radius of the population at fixed age. The high α/Fe disc component’s flat AVRs and constant σz/σR clearly indicate an entirely different heating history. Outer disc populations also have flatter radial AVRs than those in the inner disc, likely due to the waning effect of spiral arms. Our detailed measurements of AVRs and σz/σR across the disc indicate that low α/Fe, inner disc ($R \lesssim 10\, \mathrm{kpc}$) stellar populations are likely dynamically heated by both giant molecular clouds and spiral arms, while the observed trends for outer disc populations require a significant contribution from another heating mechanism such as satellite perturbations. We also find that outer disc populations have slightly positive mean vertical and radial velocities likely because they are part of the warped disc.
Abstract
The measurement of the structure of stellar populations in the Milky Way disc places fundamental constraints on models of galaxy formation and evolution. Previously, the disc's structure has ...been studied in terms of populations defined geometrically and/or chemically, but a decomposition based on stellar ages provides a more direct connection to the history of the disc, and stronger constraint on theory. Here, we use positions, abundances and ages for 31 244 red giant branch stars from the Sloan Digital Sky Survey (SDSS)-APOGEE survey, spanning 3 < R
gc < 15 kpc, to dissect the disc into mono-age and mono-Fe/H populations at low and high
$\mathrm{ \alpha \mathrm{/Fe}}$
. For each population, with Δage < 2 Gyr and ΔFe/H < 0.1 dex, we measure the structure and surface-mass density contribution. We find that low
$\mathrm{ \alpha \mathrm{/Fe}}$
mono-age populations are fit well by a broken exponential, which increases to a peak radius and decreases thereafter. We show that this profile becomes broader with age, interpreted here as a new signal of disc heating and radial migration. High
$\mathrm{ \alpha \mathrm{/Fe}}$
populations are well fit as single exponentials within the radial range considered, with an average scalelength of 1.9 ± 0.1 kpc. We find that the relative contribution of high to low
$\mathrm{ \alpha \mathrm{/Fe}}$
populations at R
0 is
$f_\Sigma = 18\hbox{ per cent} \pm 5\hbox{ per cent}$
; high
$\mathrm{ \alpha \mathrm{/Fe}}$
contributes most of the mass at old ages, and low
$\mathrm{ \alpha \mathrm{/Fe}}$
at young ages. The low and high
$\mathrm{ \alpha \mathrm{/Fe}}$
populations overlap in age at intermediate Fe/H, although both contribute mass at R
0 across the full range of Fe/H. The mass-weighted scaleheight hZ
distribution is a smoothly declining exponential function. High
$\mathrm{ \alpha \mathrm{/Fe}}$
populations are thicker than low
$\mathrm{ \alpha \mathrm{/Fe}}$
, and the average hZ
increases steadily with age, between 200 and 600 pc.
ABSTRACT
Constraints on the formation and evolution of the Milky Way Galaxy require multidimensional measurements of kinematics, abundances, and ages for a large population of stars. Ages for ...luminous giants, which can be seen to large distances, are an essential component of studies of the Milky Way, but they are traditionally very difficult to estimate precisely for a large data set and often require careful analysis on a star-by-star basis in asteroseismology. Because spectra are easier to obtain for large samples, being able to determine precise ages from spectra allows for large age samples to be constructed, but spectroscopic ages are often imprecise and contaminated by abundance correlations. Here we present an application of a variational encoder–decoder on cross-domain astronomical data to solve these issues. The model is trained on pairs of observations from APOGEE and Kepler of the same star in order to reduce the dimensionality of the APOGEE spectra in a latent space while removing abundance information. The low dimensional latent representation of these spectra can then be trained to predict age with just ∼1000 precise seismic ages. We demonstrate that this model produces more precise spectroscopic ages ($\sim 22~{{\ \rm per\ cent}}$ overall, $\sim 11~{{\ \rm per\ cent}}$ for red-clump stars) than previous data-driven spectroscopic ages while being less contaminated by abundance information (in particular, our ages do not depend on α/M). We create a public age catalogue for the APOGEE DR17 data set and use it to map the age distribution and the age-Fe/H-α/M distribution across the radial range of the Galactic disc.
ABSTRACT
Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of ...the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only α (mostly Si and Mg) elements and Fe. Our results show that (i) the loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) the chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low-metallicity end, the distinction between accreted and in situ populations is blurred; (iv) regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si–Fe plane suggests an in situ origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ.
ABSTRACT
We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within ∼4 kpc of the Galactic Centre. Characterized by a chemical composition resembling those ...of low-mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of ∼5 × 108 M⊙, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:in situ ratio within our metal-poor (Fe/H < –0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.
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