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Register a new userThe building blocks of the Milky Way halo using APOGEE and Gaia -- or -- Is the Galaxy a typical galaxy?
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Added by
Ricardo P. Schiavon
Publication date
2020
fields
Physics
and research's language is
English
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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.
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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.
I would like to review recent efforts of detailed chemical abundance measurements for field Milky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a several kpc from the Sun, large samples of field halo stars with detailed chemical measurements are continuously expanding. Combination of the chemical information and full six dimensional phase-space information is now recognized as a powerful tool to identify cosmological accretion events that have built a sizable fraction of the present-day stellar halo. Future observational prospects with wide-field spectroscopic surveys and theoretical prospects with supernova nucleosynthetic yields are also discussed.
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.
We investigate the properties of the double sequences of the Milky Way discs visible in the [$alpha$/Fe] vs [Fe/H] diagram. In the framework of Galactic formation and evolution, we discuss the complex relationships between age, metallicity, [$alpha$/Fe], and the velocity components. We study stars with measured chemical, seismic and astrometric properties from the APOGEE survey, the Kepler and Gaia satellites, respectively. We separate the [$alpha$/Fe]-[Fe/H] diagram into 3 stellar populations: the thin disc, the high-$alpha$ metal-poor thick disc and the high-$alpha$ metal-rich thick disc and characterise each of these in the age-chemo-kinematics parameter space. We compare results obtained from different APOGEE data releases and using two recent age determinations. We use the Besanc{c}on Galaxy model (BGM) to highlight selection biases and mechanisms not included in the model. The thin disc exhibits a flat age-metallicity relation while [$alpha$/Fe] increases with stellar age. We confirm no correlation between radial and vertical velocities with [Fe/H], [$alpha$/Fe] and age for each stellar population. Considering both samples, V$_varphi$ decreases with age for the thin disc, while it increases with age for the h$alpha$mp thick disc. Although the age distribution of the h$alpha$mr thick disc is very close to that of the h$alpha$mp thick disc between 7 and 14 Gyr, its kinematics seems to follow that of the thin disc. This feature, not predicted by the hypotheses included in the BGM, suggests a different origin and history for this population. Finally, we show that there is a maximum dispersion of the vertical velocity, $sigma_Z$, with age for the h$alpha$mp thick disc around 8 Gyr. The comparisons with the BGM simulations suggest a more complex chemo-dynamical scheme to explain this feature, most likely including mergers and radial migration effects
Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 dataset using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. (2020). We detect LMS-1 as a $60deg$ long stream to the north of the Galactic bulge, at a distance of $sim 20$ kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from LAMOST, SDSS and APOGEE, we infer that the stream is very metal poor (${rm langle [Fe/H]rangle =-2.1}$) with a significant metallicity dispersion ($sigma_{rm [Fe/H]}=0.4$), and it possesses a large radial velocity dispersion (${rm sigma_v=20 pm 4,km,s^{-1}}$). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of $75deg$ to the Galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024 and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.
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