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Register a new userDynamical heating across the Milky Way disc using APOGEE and $it{Gaia}$
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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-$mathrm{[Fe/H]}$ populations in the low and high $mathrm{[alpha/Fe]}$ discs between $4 lesssim R lesssim 13$ kpc and $|z| lesssim 2$ kpc using 65,719 stars in common between APOGEE DR14 and $it{Gaia}$ 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 $mathrm{[alpha/Fe]}$ discs display markedly different age--velocity-dispersion relations (AVRs) and shapes $sigma_z/sigma_R$. The high $mathrm{[alpha/Fe]}$ disc has roughly flat AVRs and constant $sigma_z/sigma_R = 0.64pm 0.04$, whereas the low $mathrm{[alpha/Fe]}$ disc has large variations in this ratio which positively correlate with the mean orbital radius of the population at fixed age. The high $mathrm{[alpha/Fe]}$ disc components flat AVRs and constant $sigma_z/sigma_R$ clearly indicates 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 $sigma_z/sigma_R$ across the disc indicate that low $mathrm{[alpha/Fe]}$, 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.
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We construct a dynamical model of the Milky Way disk from a data set, which combines Gaia EDR3 and APOGEE data throughout Galactocentric radii between $5.0leq Rleq19.5$ kpc. We make use of the spherically-aligned Jeans Anisotropic Method to model the stellar velocities and their velocity dispersions. Building upon our previous work, our model now is fitted to kinematic maps that have been extended to larger Galactocentric radii due to the expansion of our data set, probing the outer regions of the Galactic disk. Our best-fitting dynamical model suggests a logarithmic density slope of $alpha_{rm DM}=-1.602pm0.079_{rm syst}$ for the dark matter halo and a dark matter density of $rho_{rm DM}(R_{odot})=(8.92pm0.56_{rm syst})times 10^{-3}$ M$_{odot}$ pc$^{-3}$ ($0.339pm0.022_{rm syst}$ GeV cm$^{3}$). We estimate a circular velocity at the solar radius of $v_{rm circ}=(234.7pm1.7_{rm syst})$ km s$^{-1}$ with a decline towards larger radii. The total mass density is $rho_{rm tot}(R_{odot})$=$(0.0672pm0.0015_{rm syst})$ M$_{odot}$ pc$^{-3}$ with a slope of $alpha_{rm tot}$=$-2.367pm0.047_{rm syst}$ for $5leq Rleq19.5$ kpc and the total surface density is $Sigma(R{_odot}, |z|leq$ 1.1 kpc)=$(55.5pm1.7_{rm syst})$ M$_{odot}$ pc$^{-2}$. While the statistical errors are small, the error budget of the derived quantities is dominated by the 3 to 7 times larger systematic uncertainties. These values are consistent with our previous determination, but systematic uncertainties are reduced due to the extended data set covering a larger spatial extent of the Milky Way disk. Furthermore, we test the influence of non-axisymmetric features on our resulting model and analyze how a flaring disk model would change our findings.
Chemically tagging groups of stars born in the same birth cluster is a major goal of spectroscopic surveys. To investigate the feasibility of such strong chemical tagging, we perform a blind chemical tagging experiment on abundances measured from APOGEE survey spectra. We apply a density-based clustering algorithm to the eight dimensional chemical space defined by [Mg/Fe], [Al/Fe], [Si/Fe], [K/Fe], [Ti/Fe], [Mn/Fe], [Fe/H], and [Ni/Fe], abundances ratios which together span multiple nucleosynthetic channels. In a high quality sample of 182,538 giant stars, we detect twenty-one candidate clusters with more than fifteen members. Our candidate clusters are more chemically homogeneous than a population of non-member stars with similar [Mg/Fe] and [Fe/H], even in abundances not used for tagging. Group members are consistent with having the same age and fall along a single stellar-population track in logg vs. Teff space. Each groups members are distributed over multiple kpc, and the spread in their radial and azimuthal actions increases with age. We qualitatively reproduce this increase using N-body simulations of cluster dissolution in Galactic potentials that include transient winding spiral arms. Observing our candidate birth clusters with high-resolution spectroscopy in other wavebands to investigate their chemical homogeneity in other nucleosynthetic groups will be essential to confirming the efficacy of strong chemical tagging. Our initially spatially-compact but now widely dispersed candidate clusters will provide novel limits on chemical evolution and orbital diffusion in the Galactic disc, and constraints on star formation in loosely-bound groups.
To illustrate the potential of GDR2, we provide a first look at the kinematics of the Milky Way disc, within a radius of several kiloparsecs around the Sun. We benefit for the first time from a sample of 6.4 million F-G-K stars with full 6D phase-space coordinates, precise parallaxes, and precise Galactic cylindrical velocities . From this sample, we extracted a sub-sample of 3.2 million giant stars to map the velocity field of the Galactic disc from $sim$5~kpc to $sim$13~kpc from the Galactic centre and up to 2~kpc above and below the plane. We also study the distribution of 0.3 million solar neighbourhood stars ($r < 200$~pc), with median velocity uncertainties of 0.4~km/s, in velocity space and use the full sample to examine how the over-densities evolve in more distant regions. GDR2 allows us to draw 3D maps of the Galactocentric median velocities and velocity dispersions with unprecedented accuracy, precision, and spatial resolution. The maps show the complexity and richness of the velocity field of the galactic disc. We observe streaming motions in all the components of the velocities as well as patterns in the velocity dispersions. For example, we confirm the previously reported negative and positive galactocentric radial velocity gradients in the inner and outer disc, respectively. Here, we see them as part of a non-axisymmetric kinematic oscillation, and we map its azimuthal and vertical behaviour. We also witness a new global arrangement of stars in the velocity plane of the solar neighbourhood and in distant regions in which stars are organised in thin substructures with the shape of circular arches that are oriented approximately along the horizontal direction in the $U-V$ plane. Moreover, in distant regions, we see variations in the velocity substructures more clearly than ever before, in particular, variations in the velocity of the Hercules stream. (abridged)
We summarise recent results from analysis of APOGEE/Gaia data for stellar populations in the Galactic halo, disk, and bulge, leading to constraints on the contribution of dwarf galaxies and globular clusters to the stellar content of the Milky Way halo. Interpretation of the extant data in light of cosmological numerical simulations suggests that the Milky Way has been subject to an unusually intense accretion history at z >~ 1.5.
We present the analysis of a suite of simulations run with different particle-and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion - age relation for disc stars at z=0 and find that four of the simulations are more consistent with observations by Holmberg et al. (2008) in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett (2001) observations that suggest a saturation in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Ways stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger - i.e. once the disc has formed. All of the simulations are significantly hotter than the Milky Way disc; on top of the effects of mergers, we find a floor in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical codes.
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