The vertical phase-space spirals in the Milky Way are clear evidence of disequilibrium. However, they are challenging to study because phase mixing signals evolve under the influence of many different dynamical processes and can be driven by many sources of disequilibrium. We characterize phase spirals in two simulations—one test particle and one N-body—with basis function expansions, using these to derive winding times (Tfit). We find that phase spirals in the test-particle simulation wind up as expected from pure phase mixing theory while those in the self-consistent simulation do not. Specifically, in the N-body simulation we find that (i) the onset of winding is delayed, (ii) the winding rate is slowed, and (iii) the rate of winding oscillates with time. The extent of these effects depends on the azimuthal action Jϕ of the phase-spiral region. We build some physical intuition for these effects through 1D toy models, which follow a group of comoving stars traveling through several different evolving potentials. We find that phase spiral winding can be delayed until the group no longer moves coherently with the midplane of the (perturbed) potential and oscillates with time as the group experiences (e.g., a breathing mode traveling through the disk). The modifications to winding are strongest in the inner Galaxy where the disk potential dominates. We conclude that in the Milky Way, all calculations of the winding time should be interpreted as lower limits and that the most trustworthy winding times are likely in the outer disk.

The phase spiral is a perturbation to the vertical phase-space distribution of stars in the Milky Way disk. We study the phase spiral’s properties and how they vary with spatial position, in order to constrain its origin and evolution, as well as properties of the disk itself. We produce high-resolution maps using two complementary data processing schemes: (a) we bin the Gaia proper-motion sample in a disk-parallel spatial grid, reaching distances up to 4 kpc; and (b) we bin the spatially nearby line-of-sight velocity sample in terms of disk-parallel orbital parameters. We find complex structure, most significantly with respect to Galactocentric radius and guiding radius, but also in Galactic azimuth and epicyclic action and phase. We find that spiral winding and rotation phase vary smoothly across the disk, with close-to-flat radial profiles. This uniform structure, in particular for the rotation phase, disfavors small-scale perturbations as the primary source of the phase spiral, for example, from giant molecular clouds or dark matter subhalos. Curiously, these close-to-flat profiles also imply that the winding time has a strong slope with respect to Galactocentric radius, with low values for the inner disk. A uniform perturbation time cannot be reconciled by self-gravity effects that act only in the disk’s vertical dimension, because this would delay winding but not the evolution of the rotation phase. This suggests that more complex self-gravity effects, acting also in dimensions parallel to the disk plane, are important for the spiral’s formation and evolution.

We develop a novel approach to performing precision inference on tidally perturbed dwarf galaxies. We use a Bayesian inference framework of implicit likelihood inference, previously applied mainly in the field of cosmology, based on forward simulation, data compression, and likelihood emulation with neural density estimators. We consider the case of NGC 205, a satellite of M31. NGC 205 exhibits an S-shape in the mean line-of-sight velocity along its semimajor spatial axis, suggestive of tidal perturbation. We demonstrate that this velocity profile can be qualitatively reproduced even if NGC 205 was in a spherically symmetric and isotropic state before its most recent pericenter passage. We apply our inference method to mock data and show that the precise shape of a perturbed satellite’s sky-projected internal velocity field, mapped across its entire face, can be highly informative of both its orbit and total mass density profile, even in the absence of proper motion information. For NGC 205 specifically, our method is hampered because the available data only covers a line along its semimajor axis. This shortcoming could be addressed with another round of observations.

We applied the vertical Jeans equation to the Milky Way disk in order to study non-axisymmetric variations in the thin disk surface density. We divided the disk plane into area cells with a 100 pc grid spacing and used four separate subsets of the Gaia DR3 stars, defined by cuts in absolute magnitude, that reach distances up to 3 kpc. The vertical Jeans equation is informed by the stellar number density field and the vertical velocity field; for the former, we used maps produced via Gaussian process regression; for the latter, we used Bayesian neural network radial velocity predictions, which allowed us to utilise the full power of the Gaia DR3 proper motion sample. For the first time, we find evidence of a spiral arm in the form of an over-density in the dynamically measured disk surface density, detected in all four data samples, which agrees very well with the spiral arm as traced by stellar age and chemistry. We fitted a simple spiral arm model to this feature and infer a relative over-density of roughly 20% and a width of roughly 400 pc. We also infer a thin disk surface density scale length of 3.3–4.2 kpc when restricting the analysis to stars within a distance of 2 kpc.

Fuzzy dark matter (FDM) has dynamical properties that differ significantly from cold dark matter (CDM). These dynamical differences are strongly manifested on the spatial scale of dwarf spheroidal galaxies (dSphs), which roughly corresponds to the de Broglie wavelength of a canonical mass FDM particle. We study simulations of a dSph satellite which is tidally perturbed by its host galaxy, in order to identify dynamical signatures that are unique to FDM, and to quantify the imprints of such perturbations on an observable stellar tracer population. We find that a perturbed FDM soliton develops a long-standing breathing mode, whereas for CDM such a breathing mode quickly phase-mixes and disappears. We also demonstrate that such signatures become imprinted on the dynamics of a stellar tracer population, making them observable with sufficiently precise astrometric measurements.

Recent studies indicate that thermally produced dark matter will form highly concentrated, low-mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected 𝛾-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi-LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic 𝛾-ray background, excluding with 95% confidence dark matter annihilation to 𝑏⁢¯𝑏 final states for dark matter masses below 120 GeV.

In an earlier work, we demonstrated the effectiveness of Bayesian neural networks in estimating the missing line-of-sight velocities of Gaia stars, and published an accompanying catalogue of blind predictions for the line-of-sight velocities of stars in Gaia DR3. These were not merely point predictions, but probability distributions reflecting our state of knowledge about each star. Here, we verify that these predictions were highly accurate: the DR3 measurements were statistically consistent with our prediction distributions, with an approximate error rate of 1.5  per cent. We use this same technique to produce a publicly available catalogue of predictive probability distributions for the 185 million stars up to a G-band magnitude of 17.5 still missing line-of-sight velocities in Gaia DR3. Validation tests demonstrate that the predictions are reliable for stars within approximately 7 kpc from the Sun and with distance precisions better than around 20 per cent. For such stars, the typical prediction uncertainty is 25–30 km s⁻¹. We invite the community to use these radial velocities in analyses of stellar kinematics and dynamics, and give an example of such an application.

The density of dark matter near the Sun, ρ_{DM, ⊙}, is important for experiments hunting for dark matter particles in the laboratory, and for constraining the local shape of the Milky Way’s dark matter halo. Estimates to date have typically assumed that the Milky Way’s stellar disc is axisymmetric and in a steady-state. Yet the Milky Way disc is neither, exhibiting prominent spiral arms and a bar, and vertical and radial oscillations. We assess the impact of these assumptions on determinations of ρ_{DM, ⊙} by applying a free-form, steady-state, Jeans method to two different N-body simulations of Milky Way-like galaxies. In one, the galaxy has experienced an ancient major merger, similar to the hypothesized Gaia–Sausage–Enceladus; in the other, the galaxy is perturbed more recently by the repeated passage and slow merger of a Sagittarius-like dwarf galaxy. We assess the impact of each of the terms in the Jeans–Poisson equations on our ability to correctly extract ρ_{DM, ⊙} from the simulated data. We find that common approximations employed in the literature – axisymmetry and a locally flat rotation curve – can lead to significant systematic errors of up to a factor ∼1.5 in the recovered surface mass density ∼2 kpc above the disc plane, implying a fractional error on ρ_{DM, ⊙} of the order of unity. However, once we add in the tilt term and the rotation curve term in our models, we obtain an unbiased estimate of ρ_{DM, ⊙}, consistent with the true value within our 95 per cent confidence intervals for realistic 20 per cent uncertainties on the baryonic surface density of the disc. Other terms – the axial tilt, 2nd Poisson and time-dependent terms – contribute less than 10 per cent to ρ_{DM, ⊙} (given current data) and can be safely neglected for now. In the future, as more data become available, these terms will need to be included in the analysis.

This report summarises progress made in estimating the local density of dark matter (ρ_DM,⊙), a quantity that is especially important for dark matter direct detection experiments. We outline and compare the most common methods to estimate ρ_DM,⊙ and the results from recent studies, including those that have benefited from the observations of the ESA/Gaia satellite. The result of most local analyses coincide within a range of , while a slightly lower range of is preferred by most global studies. In light of recent discoveries, we discuss the importance of going beyond the approximations of what we define as the ideal Galaxy (a steady-state Galaxy with axisymmetric shape and a mirror symmetry across the mid-plane) in order to improve the precision of ρ_DM,⊙ measurements. In particular, we review the growing evidence for local disequilibrium and broken symmetries in the present configuration of the Milky Way, as well as uncertainties associated with the galactic distribution of baryons. Finally, we comment on new ideas that have been proposed to further constrain the value of ρ_DM,⊙, most of which would benefit from Gaia's final data release.

We present a new method for inferring the gravitational potential of the Galactic disk, using the time-varying structure of a phase-space spiral in the (z, w)-plane (where z and w represent vertical position and vertical velocity). Our method of inference extracts information from the shape of the spiral and disregards the bulk density distribution that is usually used to perform dynamical mass measurements. In this manner, it is complementary to traditional methods that are based on the assumption of a steady state. Our method consists of fitting an analytical model for the phase-space spiral to data, where the spiral is seen as a perturbation of the stellar number density in the (z, w)-plane. We tested our method on one-dimensional simulations, which were initiated in a steady state and then perturbed by an external force similar to that of a passing satellite. We were able to retrieve the true gravitational potentials of the simulations with high accuracy. The gravitational potential at 400-500 parsec distances from the disk mid-plane was inferred with an error of only a few percent. This is the first paper of a series in which we plan to test and refine our method on more complex simulations, as well as apply our method to Gaia data.