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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 Far Side of the Galactic Bar/Bulge Revealed through Semi-regular Variables

   https://doi.org/10.3847/1538-3881/ad01bf  

   Hey, Daniel R. ; Huber, Daniel ; Shappee, Benjamin J. ; Bland-Hawthorn, Joss ; Tepper-García, Thor ; Sanderson, Robyn ; Chakrabarti, Sukanya ; Saunders, Nicholas ; Hunt, Jason A. S. ; Bedding, Timothy R. ; et al ( November 2023 , The Astronomical Journal)

   The Galactic bulge is critical to our understanding of the Milky Way. However, due to the lack of reliable stellar distances, the structure and kinematics of the bulge/bar beyond the Galactic center have remained largely unexplored. Here, we present a method to measure distances of luminous red giants using a period–amplitude–luminosity relation anchored to the Large Magellanic Cloud, with random uncertainties of 10%–15% and systematic errors below 1%–2%. We apply this method to data from the Optical Gravitational Lensing Experiment to measure distances to 190,302 stars in the Galactic bulge and beyond out to 20 kpc. Using this sample, we measure a distance to the Galactic center ofR₀= 8108 ± 106_stat± 93_syspc, consistent with direct measurements of stars orbiting Sgr A*. We cross-match our distance catalog with Gaia DR3 and use the subset of 39,566 overlapping stars to provide the first constraints on the Milky Way’s velocity field (V_R,V_ϕ,V_z) beyond the Galactic center. We show that theV_Rquadrupole from the bar’s near side is reflected with respect to the Galactic center, indicating that the bar is bisymmetric and aligned with the inner disk. We also find that the vertical heightV_Zmap has no major structure in the region of the Galactic bulge, which is inconsistent with a current episode of bar buckling. Finally, we demonstrate withN-body simulations that distance uncertainty plays a factor in the alignment of the major and kinematic axes of the bar, necessitating caution when interpreting results for distant stars.

    

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3. The power of co-ordinate transformations in dynamical interpretations of Galactic structure

   https://doi.org/10.1093/mnras/staa1987  

   Hunt, Jason A ; Johnston, Kathryn V ; Pettitt, Alex R ; Cunningham, Emily C ; Kawata, Daisuke ; Hogg, David W ( July 2020 , Monthly Notices of the Royal Astronomical Society)

   Abstract Gaia DR2 has provided an unprecedented wealth of information about the positions and motions of stars in our Galaxy, and has highlighted the degree of disequilibria in the disc. As we collect data over a wider area of the disc it becomes increasingly appealing to start analysing stellar actions and angles, which specifically label orbit space, instead of their current phase space location. Conceptually, while $\bar{x}$ and $\bar{v}$ tell us about the potential and local interactions, grouping in action puts together stars that have similar frequencies and hence similar responses to dynamical effects occurring over several orbits. Grouping in actions and angles refines this further to isolate stars which are travelling together through space and hence have shared histories. Mixing these coordinate systems can confuse the interpretation. For example, it has been suggested that by moving stars to their guiding radius, the Milky Way spiral structure is visible as ridge-like overdensities in the Gaia data (Khoperskov et al. 2020). However, in this work, we show that these features are in fact the known kinematic moving groups, both in the Lz − φ and the vR − vφ planes. Using simulations we show how this distinction will become even more important as we move to a global view of the Milky Way. As an example, we show that the radial velocity wave seen in the Galactic disc in Gaia and APOGEE should become stronger in the action-angle frame, and that it can be reproduced by transient spiral structure. 

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4. Radial phase spirals in the Solar neighbourhood

   https://doi.org/10.1093/mnras/stad3918  

   Hunt, Jason A. S. ; Price-Whelan, Adrian M. ; Johnston, Kathryn V. ; McClure, Rachel L. ; Filion, Carrie ; Cassese, Ben ; Horta, Danny ( December 2023 , Monthly Notices of the Royal Astronomical Society)

   The second data release of ESA’s Gaia mission revealed numerous signatures of disequilibrium in the Milky Way’s disc. These signatures are seen in the planar kinematics of stars, which manifest as ridges and ripples in R–vϕ, and in vertical kinematics, where a prominent spiral is seen in the z–vz phase space. In this work, we show an equivalent ΔR–vR phase spiral forms following a perturbation to the disc. We demonstrate the behaviour of the ΔR–vR phase spirals in both a toy model and a high-resolution N-body simulation of a satellite interaction. We then confront these models with the data, where we find partial ΔR–vR phase spirals in the Solar neighbourhood using the most recent data from Gaia DR3. This structure indicates ongoing radial phase mixing in the Galactic disc, suggesting a history of recent perturbations, either through internal or external (e.g. satellite) processes. Future work modelling the z–vz and ΔR–vR phase spirals in tandem may help break degeneracy’s between possible origins of the perturbation.

    

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5. The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). V. The Structure of M33 in Resolved Stellar Populations

   https://doi.org/10.3847/1538-4357/acf3e8  

   Smercina, Adam ; Dalcanton, Julianne J. ; Williams, Benjamin F. ; Durbin, Meredith J. ; Lazzarini, Margaret ; Bell, Eric F. ; Choi, Yumi ; Dolphin, Andrew ; Gilbert, Karoline ; Guhathakurta, Puragra ; et al ( October 2023 , The Astrophysical Journal)

   We present a detailed analysis of the structure of the Local Group flocculent spiral galaxy M33, as measured using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region (PHATTER) survey. Leveraging the multiwavelength coverage of PHATTER, we find that the oldest populations are dominated by a smooth exponential disk with two distinct spiral arms and a classical central bar—completely distinct from what is seen in broadband optical imaging, and the first-ever confirmation of a bar in M33. We estimate a bar extent of ∼1 kpc. The two spiral arms are asymmetric in orientation and strength, and likely represent the innermost impact of the recent tidal interaction responsible for M33's warp at larger scales. The flocculent multiarmed morphology for which M33 is known is only visible in the young upper main-sequence population, which closely tracks the morphology of the interstellar medium. We investigate the stability of M33's disk, findingQ∼ 1 over the majority of the disk. We fit multiple components to the old stellar density distribution and find that, when considering recent stellar kinematics, M33's bulk structure favors the inclusion of an accreted halo component, modeled as a broken power law. The best-fit halo has an outer power-law index of −3 and accurately describes observational evidence of M33's stellar halo from both resolved stellar spectroscopy in the disk and its stellar populations at large radius. Integrating this profile yields a total halo stellar mass of ∼5 × 10⁸M_⊙, for a stellar halo mass fraction of 16%, most of which resides in the innermost 2.5 kpc.

    

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