Energy wrinkles and phase-space folds of the last major merger

Abstract

Relying on the dramatic increase in the number of stars with full 6D phase-space information provided by the Gaia Data Release 3, we resolve the distribution of the stellar halo around the Sun to uncover signatures of incomplete phase-mixing. We show that for the stars likely belonging to the last massive merger, the (vr, r) distribution contains a series of long and thin chevron-like overdensities. These phase-space sub-structures have been predicted to emerge following the dissolution of a satellite, when its tidal debris is given time to wind up, thin out and fold. Such chevrons have been spotted in external galaxies before, here we report the first detection in our own Milky Way. We also show that the observed angular momentum Lz distribution appears more prograde at high energies, possibly revealing the original orbital angular momentum of the in-falling galaxy. The energy distribution of the debris is strongly asymmetric with a peak at low E – which, we surmise, may be evidence of the dwarf’s rapid sinking – and riddled with wrinkles and bumps. We demonstrate that similar phase-space and (E, Lz) sub-structures are present in numerical simulations of galaxy interactions, both in bespoke N-body runs and in cosmological hydrodynamical zoom-in suites. The remnant traces of the progenitor’s disruption and the signatures of the on-going phase-mixing discovered here will not only help to constrain the properties of our Galaxy’s most important interaction, but also can be used as a novel tool to map out the Milky Way’s current gravitational potential and its perturbations.

DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of Our Nearest Neighbor

“…M31 is remarkably similar to the Milky Way in that the inner halos of both galaxies are dominated by stars from a single accretion event. Indeed, a recent study of the kinematics of Milky Way stars near the Sun reports chevron-shaped kinematic substructures (Belokurov et al 2023) that are reminiscent of those reported here. If the progenitor of the M31 shell system studied here is 10 10 M e , M31 may provide a close analog to what our own galaxy looked like several gigayears ago.…”

Dark matter substructures affect dark matter-electron scattering in xenon-based direct detection experiments

“…However, for relatively recent mergers, there will not be sufficient time for virialization, resulting in plenty of substructures both in the stellar and in the DM component [85][86][87][88][89][90][91][92][93][94][95][96][97]. The presence of such additional stellar substructures (beyond the MW stars) have been detected by different sky-surveys like Gaia [83,90,[97][98][99][100], SDSS [90], LAMOST [101,102], etc., and have also been predicted in various N-body simulations [103][104][105][106][107][108][109][110][111].…”

Hunting for C-rich long-period variable stars in the Milky Way's bar-bulge using unsupervised classification of Gaia BP/RP spectra

“…The spectra are provided for ∼ 220 million stars in Gaia with 𝐺 < 17.65. Despite their low resolution, several studies have demonstrated that the information content is sufficient to measure bulk spectroscopic parameters (𝑇 eff , log 𝑔, [M/H], Liu et al 2012;Witten et al 2022;Xylakis-Dornbusch et al 2022;Andrae et al 2022;Fouesneau et al 2022;Creevey et al 2022;Belokurov et al 2022;Rix et al 2022) although detailed chemical abundances are likely too challenging (Gavel et al 2021). However, particularly in cooler stars, the presence of molecular features in these spectra is detectable (Lucey et al 2022) enabling more accurate metallicity determinations and detailed carbon abundances.…”