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1. Mapping the Galactic disc with the LAMOST and Gaia red clump sample: II. 3D asymmetrical kinematics of mono-age populations in the disc between 6–14  kpc

   https://doi.org/10.1093/mnras/stz3113  

   Wang, H-F ; López-Corredoira, M ; Huang, Y ; Carlin, J L ; Chen, B-Q ; Wang, C ; Chang, J ; Zhang, H-W ; Xiang, M-S ; Yuan, H-B ; et al ( January 2020 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We perform analysis of the 3D kinematics of Milky Way disc stars in mono-age populations. We focus on stars between Galactocentric distances of R = 6 and 14  kpc, selected from the combined LAMOST Data Release 4 (DR4) red clump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3D asymmetrical motions of recent works and provide time tagging of the Galactic outer disc asymmetrical motions near the anticentre direction out to Galactocentric distances of 14 kpc. Radial Galactocentric motions reach values up to 10 km s−1, depending on the age of the population, and present a north–south asymmetry in the region corresponding to density and velocity substructures that were sensitive to the perturbations in the early 6  Gyr. After that time, the disc stars in this asymmetrical structure have become kinematically hotter, and are thus not sensitive to perturbations, and we find the structure is a relatively younger population. With quantitative analysis, we find stars both above and below the plane at R ≳ 9 kpc that exhibit bending mode motions of which the sensitive duration is around 8  Gyr. We speculate that the in-plane asymmetries might not be mainly caused by a fast rotating bar, intrinsically elliptical outer disc, secular expansion of the disc, or streams. Spiral arm dynamics, out-of-equilibrium models, minor mergers or others are important contributors. Vertical motions might be dominated by bending and breathing modes induced by complicated inner or external perturbers. It is likely that many of these mechanisms are coupled together. 

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2. The surface mass density of the Milky Way: does the traditional KZ approach work in the context of new surveys?

   https://doi.org/10.1093/mnras/stad3013  

   Cheng, Xinlun ; Anguiano, Borja ; Majewski, Steven R. ; Arras, Phil ( October 2023 , Monthly Notices of the Royal Astronomical Society)

   We revisit the classical KZ problem – determination of the vertical force and implied total mass density distribution of the Milky Way disc – for a wide range of Galactocentric radius and vertical height using chemically selected thin and thick disc samples based on Apache Point Observatory Galactic Evolution Experiment spectroscopy combined with the Gaia astrometry. We derived the velocity dispersion profiles in Galactic cylindrical coordinates, and solved the Jeans equation for the two samples separately. The result is surprising that the total surface mass density as a function of vertical height as derived for these two chemically distinguished populations is different. The discrepancies are larger in the inner compared to the outer Galaxy, with the density calculated from thick disc being larger, independent of the Galactic radius. Furthermore, while there is an overall good agreement between the total mass density derived for the thick disc population and the standard halo model for vertical heights larger than 1 kpc, close to the mid-plane the mass density observed using the thick disc population is larger than that predicted from the standard halo model. We explore various implications of these discrepancies, and speculate their sources, including problems associated with the assumed density laws, velocity dispersion profiles, and the Galactic rotation curve, potential non-equilibrium of the Galactic disc, or a failure of the Navarro-Frenk-White (NFW) dark matter halo profile for the Milky Way. We conclude that the growing detail in hand on the chemodynamical distributions of Milky Way stars challenges traditional analytical treatments of the KZ problem.

    

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3. Chemical cartography with LAMOST and Gaia reveal azimuthal and spiral structure in the Galactic disc

   https://doi.org/10.1093/mnras/stad1244  

   Hawkins, Keith ( April 2023 , Monthly Notices of the Royal Astronomical Society)

   Chemical Cartography, or mapping, of our Galaxy has the potential to fully transform our view of its structure and formation. In this work, we use chemical cartography to explore the metallicity distribution of OBAF-type disc stars from the LAMOST survey and a complementary sample of disc giant stars from Gaia DR3. We use these samples to constrain the radial and vertical metallicity gradients across the Galactic disc. We also explore whether there are detectable azimuthal variations in the metallicity distribution on top of the radial gradient. For the OBAF-type star sample from LAMOST, we find a radial metallicity gradient of Δ[Fe/H]/ΔR ∼−0.078 ± 0.001 dex kpc−1 in the plane of the disc and a vertical metallicity gradient of Δ[Fe/H]/ΔZ ∼−0.15 ± 0.01 dex kpc−1 in the solar neighbourhood. The radial gradient becomes shallower with increasing vertical height, while the vertical gradient becomes shallower with increasing Galactocentric radius, consistent with other studies. We also find detectable spatially dependent azimuthal variations on top of the radial metallicity gradient at the level of ∼0.10 dex. Interestingly, the azimuthal variations appear be close to the Galactic spiral arms in one data set (Gaia DR3) but not the other (LAMOST). These results suggest that there is azimuthal structure in the Galactic metallicity distribution and that in some cases it is co-located with spiral arms.

    

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4. Quantifying radial migration in the Milky Way: inefficient over short time-scales but essential to the very outer disc beyond ∼15 kpc

   https://doi.org/10.1093/mnras/stac479  

   Lian, Jianhui ; Zasowski, Gail ; Hasselquist, Sten ; Holtzman, Jon A. ; Boardman, Nicholas ; Cunha, Katia ; Fernández-Trincado, José G. ; Frinchaboy, Peter M. ; Garcia-Hernandez, D. A. ; Nitschelm, Christian ; et al ( February 2022 , Monthly Notices of the Royal Astronomical Society)

   Stellar radial migration plays an important role in reshaping a galaxy’s structure and the radial distribution of stellar population properties. In this work, we revisit reported observational evidence for radial migration and quantify its strength using the age–[Fe/H] distribution of stars across the Milky Way with APOGEE data. We find a broken age–[Fe/H] relation in the Galactic disc at r > 6 kpc, with a more pronounced break at larger radii. To quantify the strength of radial migration, we assume stars born at each radius have a unique age and metallicity, and then decompose the metallicity distribution function (MDF) of mono-age young populations into different Gaussian components that originated from various birth radii at rbirth < 13 kpc. We find that, at ages of 2 and 3 Gyr, roughly half the stars were formed within 1 kpc of their present radius, and very few stars (<5 per cent) were formed more than 4 kpc away from their present radius. These results suggest limited short-distance radial migration and inefficient long-distance migration in the Milky Way during the last 3 Gyr. In the very outer disc beyond 15 kpc, the observed age–[Fe/H] distribution is consistent with the prediction of pure radial migration from smaller radii, suggesting a migration origin of the very outer disc. We also estimate intrinsic metallicity gradients at ages of 2 and 3 Gyr of −0.061 and −0.063 dex kpc−1, respectively.

    

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5. Our Galaxy’s youngest disc

   https://doi.org/10.1093/mnras/stac1788  

   Li, Chengdong ; Binney, James ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   We investigate the structure of our Galaxy’s young stellar disc by fitting the distribution functions (DFs) of a new family to 5D Gaia data for a sample of $47\, 000$ OB stars. Tests of the fitting procedure show that the young disc’s DF would be strongly constrained by Gaia data if the distribution of Galactic dust were accurately known. The DF that best fits the real data accurately predicts the kinematics of stars at their observed locations, but it predicts the spatial distribution of stars poorly, almost certainly on account of errors in the best-available dust map. We argue that dust models could be greatly improved by modifying the dust model until the spatial distribution of stars predicted by a DF agreed with the data. The surface density of OB stars is predicted to peak at $R\simeq 5.5\, \mathrm{kpc}$, slightly outside the reported peak in the surface density of molecular gas; we suggest that the latter radius may have been underestimated through the use of poor kinematic distances. The velocity distributions predicted by the best-fitting DF for stars with measured line-of-sight velocities v∥ reveal that the outer disc is disturbed at the level of $10\, \mathrm{km\, s}^{-1}$ in agreement with earlier studies, and that the measured values of v∥ have significant contributions from the orbital velocities of binaries. Hence the outer disc is colder than it is sometimes reported to be.

    

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