Distant Echoes of the Milky Way’s Last Major Merger

Abstract

The majority of the Milky Way’s stellar halo consists of debris from our galaxy’s last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from the GSE have been kinematically and chemically studied in the inner 30 kpc of our galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to 100 kpc. We obtain follow-up spectra of stars in two strong overdensities—including the previously identified outer Virgo Overdensity—and find them to be relatively metal rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming 60–90 kpc counterparts to the 15–20 kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from 50 to 100 kpc, in the same plane as the Sagittarius Stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our galaxy’s stellar halo.

Discovery of the Magellanic Stellar Stream Out to 100 kpc

“…We first assemble a sample of plausible RGB stars using Gaia DR3 astrometry and unWISE infrared photometry (Mainzer et al 2014;Schlafly et al 2019;Brown et al 2021;Lindegren et al 2021;Gaia Collaboration et al 2023). The selection procedure for our parent sample is described in Chandra et al (2023). Briefly, we initially query the Gaia DR3 catalog for stars with ϖ < 0.4 mas, μ < 5 mas yr -1 , and |b| > 20°to remove obvious nearby dwarfs and mask out the MW disk.…”

“…We derive a significance statistic χ ϖ that encodes how many standard deviations lie between the observed Gaia parallax and the predicted parallax for a dwarf with the same apparent magnitude and color. Using stars from the H3 Spectroscopic Survey (Conroy et al 2019a) as a guide, Chandra et al (2023) find that removing stars with χ ϖ < 2 results in a ≈90% pure sample of stars with g log 3.5 < . We further purify this sample by applying a broad color cut in the BP RP, RP W1 --(…”

“…Since these stars lie closer than our main d > 100 kpc sample, all are brighter than G < 17.65 and consequently have lowresolution "XP" spectra available from Gaia DR3 (De Angeli et al 2023;Gaia Collaboration et al 2023;Montegriffo et al 2023). We utilized the Gaia XP K giant catalog of Chandra et al (2023) to select stars in the Plume, since the XP metallicities enable more precise distance estimates than the color-isochrone methodology outlined above. There were a total of ≈100 stars in this Pisces Plume selection, of which we randomly observed 45 stars.…”

“…Broader results from this data set-along with a public data release-will be presented in forthcoming publications. For the present work, we select 191 stars with spectroscopic d > 50 kpc, targeted from both selections described in Section 2.1: photometry + parallax selected stars estimated to lie beyond ≈100 kpc, and stars from Chandra et al (2023) beyond ≈50 kpc that lie within the Pisces Plume overdensity.…”

“…While updated simulations that reproduce both the MS and MSS will provide some intuition about any expected offsets between the gas and stars, it would be desirable to measure the distance to the gas directly. One promising avenue is a more densely sampled spectroscopic survey of the outer halo, using the efficient techniques outlined here to select distant K giants (see also Conroy et al 2018;Chandra et al 2023). Searching for MS absorption along lines of sight to giants at different distances would enable us to tomographically reconstruct the MS gas distribution in 3D, mapping its full spatial extent.…”