Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way

Abstract

We use the Gaia eDR3 data and legacy spectroscopic surveys to map the Milky Way disc substructure towards the Galactic Anticenter at heliocentric distances $d\ge 10\, \rm {kpc}$. We report the discovery of multiple previously undetected new filaments embedded in the outer disc in highly extincted regions. Stars in these over-densities have distance gradients expected for disc material and move on disc-like orbits with $v_{\phi }\sim 170-230\, \rm {km\, s^{-1}}$, showing small spreads in energy. Such a morphology argues against a quiescently growing Galactic thin disc. Some of these structures are interpreted as excited outer disc material, kicked up by satellite impacts and currently undergoing phase-mixing (disc streams, ‘feathers’). Due to the long timescale in the outer disc regions, these structures can stay coherent in configuration space over several Gyrs. We nevertheless note that some of these structures could also be folds in the perturbed disc seen in projection from the Sun’s location. A full 6D phase-space characterization and age dating of these structure should help distinguish between the two possible morphologies.

A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. I. Phase Spirals in an Infinite, Isothermal Slab

“…In the case of the Milky Way (MW) disk, which can be studied in much greater detail than any other system, recent data from astrometric and radial velocity surveys such as SEGUE (Yanny et al 2009), RAVE (Steinmetz et al 2006), GALAH (Bland-Hawthorn et al 2019), LAMOST (Cui et al 2012), and above all Gaia (Gaia Collaboration et al 2016, 2018a, 2018b have revealed a variety of additional vertical distortions. At large galactocentric radii (>10 kpc) this includes, among others, oscillations and corrugations (Xu et al 2015;Schönrich & Dehnen 2018), and streams of stars kicked up from the disk that undergo phase mixing, sometimes referred to as "feathers" (e.g., Price-Whelan et al 2015;Thomas et al 2019;Laporte et al 2022). Similar oscillations and vertical asymmetries have also been reported in the solar vicinity (e.g., Widrow et al 2012;Williams et al 2013;Yanny & Gardner 2013;Gaia Collaboration et al 2018b;Quillen et al 2018;Bennett & Bovy 2019;Carrillo et al 2019).…”

The High-Velocity Clouds Above the Disk of the Outer Milky Way: Misty Precipitating Gas in a Region Roiled by Stellar Streams

“…porte et al. 2018, 2022), which could persist for significant amounts of time (Laporte et al 2022). Laporte et al (2018) favor the Sagittarius dwarf galaxy (Ibata et al 1994;Law & Majewski 2010) as the main perturber, but the Large Magellanic Cloud could also play a role (see also Weinberg & Blitz 2006).…”

Overview of the DESI Milky Way Survey

“…A combination of dynamical and chemical data on thick disk stars is required to distinguish between these various scenarios. In particular, the long dynamical times, especially in the outer disk, result in a persistent memory of past perturbations, either secular or external, which have created lasting Galactic warps and flares (Momany et al 2006;Minchev et al 2015;Laporte et al 2021).…”