skip to main content


Title: Snails across Scales: Local and Global Phase-mixing Structures as Probes of the Past and Future Milky Way
Abstract

Signatures of vertical disequilibrium have been observed across the Milky Way’s (MW’s) disk. These signatures manifest locally as unmixed phase spirals inzvzspace (“snails-in-phase”), and globally as nonzero meanzandvz, wrapping around the disk into physical spirals in thexyplane (“snails-in-space”). We explore the connection between these local and global spirals through the example of a satellite perturbing a test-particle MW-like disk. We anticipate our results to broadly apply to any vertical perturbation. Using azvzasymmetry metric, we demonstrate that in test-particle simulations: (a) multiple local phase-spiral morphologies appear when stars are binned by azimuthal actionJϕ, excited by a single event (in our case, a satellite disk crossing); (b) these distinct phase spirals are traced back to distinct disk locations; and (c) they are excited at distinct times. Thus, local phase spirals offer a global view of the MW’s perturbation history from multiple perspectives. Using a toy model for a Sagittarius (Sgr)–like satellite crossing the disk, we show that the full interaction takes place on timescales comparable to orbital periods of disk stars withinR≲ 10 kpc. Hence such perturbations have widespread influence, which peaks in distinct regions of the disk at different times. This leads us to examine the ongoing MW–Sgr interaction. While Sgr has not yet crossed the disk (currently,zSgr≈ −6 kpc,vz,Sgr≈ 210 km s−1), we demonstrate that the peak of the impact has already passed. Sgr’s pull over the past 150 Myr creates a globalvzsignature with amplitude ∝MSgr, which might be detectable in future spectroscopic surveys.

 
more » « less
Award ID(s):
1715582
NSF-PAR ID:
10364541
Author(s) / Creator(s):
; ; ; ; ;
Publisher / Repository:
DOI PREFIX: 10.3847
Date Published:
Journal Name:
The Astrophysical Journal
Volume:
928
Issue:
1
ISSN:
0004-637X
Format(s):
Medium: X Size: Article No. 80
Size(s):
["Article No. 80"]
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We develop a linear perturbative formalism to compute the response of an inhomogeneous stellar disk embedded in a nonresponsive dark matter (DM) halo to various perturbations like bars, spiral arms, and encounters with satellite galaxies. Without self-gravity to reinforce it, the response of a Fourier mode phase mixes away due to an intrinsic spread in the vertical (Ωz), radial (Ωr), and azimuthal (Ωϕ) frequencies, triggering local phase-space spirals. The detailed galactic potential dictates the shape of phase spirals: phase mixing occurs more slowly and thus phase spirals are more loosely wound in the outer disk and in the presence of an ambient DM halo. Collisional diffusion due to scattering of stars by structures like giant molecular clouds causes superexponential damping of the phase spiral amplitude. Thezvzphase spiral is one-armed (two-armed) for vertically antisymmetric (symmetric) bending (breathing) modes. Only transient perturbations with timescales (τP) comparable to the vertical oscillation period (τz∼ 1/Ωz) can trigger vertical phase spirals. Each (n,l,m) mode of the response to impulsive (τP<τ= 1/(nΩz+lΩr+mΩϕ)) perturbations is power-law (∼τP/τ) suppressed, but that to adiabatic (τP>τ) perturbations is exponentially weak (expτP/τα) except for resonant (τ→ ∞ ) modes. Slower (τP>τz) perturbations, e.g., distant encounters with satellite galaxies, induce stronger bending modes. Sagittarius (Sgr) dominates the solar neighborhood response of the Milky Way (MW) disk to satellite encounters. Thus, if the Gaia phase spiral was triggered by a MW satellite, Sgr is the leading contender. However, the survival of the phase spiral against collisional damping necessitates an impact ∼0.6–0.7 Gyr ago.

     
    more » « less
  2. Abstract

    Galactic disks are highly responsive systems that often undergo external perturbations and subsequent collisionless equilibration, predominantly via phase mixing. We use linear perturbation theory to study the response of infinite isothermal slab analogs of disks to perturbations with diverse spatiotemporal characteristics. Without self-gravity of the response, the dominant Fourier modes that get excited in a disk are the bending and breathing modes, which, due to vertical phase mixing, trigger local phase-space spirals that are one- and two-armed, respectively. We demonstrate how the lateral streaming motion of slab stars causes phase spirals to damp out over time. The ratio of the perturbation timescale (τP) to the local, vertical oscillation time (τz) ultimately decides which of the two modes is excited. Faster, more impulsive (τP<τz) and slower, more adiabatic (τP>τz) perturbations excite stronger breathing and bending modes, respectively, although the response to very slow perturbations is exponentially suppressed. For encounters with satellite galaxies, this translates to more distant and more perpendicular encounters triggering stronger bending modes. We compute the direct response of the Milky Way disk to several of its satellite galaxies and find that recent encounters with all of them excite bending modes in the solar neighborhood. The encounter with Sagittarius triggers a response that is at least 1–2 orders of magnitude larger than that due to any other satellite, including the Large Magellanic Cloud. We briefly discuss how ignoring the presence of a dark matter halo and the self-gravity of the response might impact our conclusions.

     
    more » « less
  3. ABSTRACT

    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.

     
    more » « less
  4. Abstract

    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 ± 106stat± 93syspc, 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 (VR,Vϕ,Vz) beyond the Galactic center. We show that theVRquadrupole 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 heightVZmap 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.

     
    more » « less
  5. ABSTRACT

    Stars born on near-circular orbits in spiral galaxies can subsequently migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars or spiral arms. This paper extends the study of migration to examine the role of external influences using the example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky Way (MW). We first make impulse approximation estimates to characterize the influence of Sgr disc passages. The tidal forcing from Sgr can produce changes in both guiding radius ΔRg and orbital eccentricity, as quantified by the maximum radial excursion ΔRmax. These changes follow a quadrupole-like pattern across the face of the disc, with amplitude increasing with Galactocentric radius. We next examine a collisionless N-body simulation of a Sgr-like satellite interacting with an MW-like galaxy and find that Sgr’s influence in the outer disc dominates the secular evolution of orbits between disc passages. Finally, we use the same simulation to explore possible observable signatures of Sgr-induced migration by painting the simulation with different age stellar populations. We find that following Sgr disc passages, the migration it induces manifests within an annulus as an approximate quadrupole in azimuthal metallicity variations (δ[Fe/H]), along with systematic variations in orbital eccentricity, ΔRmax. These systematic variations can persist for several rotational periods. We conclude that this combination of signatures may be used to distinguish between the different migration mechanisms shaping the chemical abundance patterns of the MW’s thin disc.

     
    more » « less