Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Evidence for multiple crossings and stripping of Gaia-Enceladus/Sausage across the Milky Way

L. Berni, M. Palla, L. Magrini, L. Spina

TL;DR

This work tests the hypothesis that Gaia-Enceladus/Sausage (GES) consisted of multiple accretion passages by identifying two chemo-dynamical subpopulations within GES using APOGEE DR17 data and a CREEK-based membership algorithm. Through KDE analyses and non-parametric statistical tests, the authors demonstrate significant differences in metallicity and alpha-element patterns between the two populations, consistent with inner, more chemically enriched and outer, less enriched regions of the progenitor. Galactic chemical evolution models, scanned over parameters controlling gas inflow, star formation efficiency, and outflow onset, reproduce the observed distributions with Pop1 (inner) having a shorter infall timescale and Pop2 (outer) a longer one, supporting an inside-out growth and a multi-passage accretion history. The results bolster a complex assembly scenario for the MW halo, where the GES progenitor’s core and periphery were stripped in distinct disc-crossing events, leaving behind chemically distinct imprints that future high-resolution surveys can further resolve.

Abstract

The accretion of Gaia-Enceladus/Sausage (GES) onto the Milky Way (MW) is one of the most prominent features of the Galactic halo revealed by the combination of the Gaia satellite and large spectroscopic surveys. This massive accretion largely contributes to the local stellar halo mass and was significant enough to alter the formation history and the morphology of the MW. In this work, we aim to analyse the selection of stars previously identified as belonging to GES with different kinematics and chemical properties to test the hypothesis of a two-phase accretion event. We apply several statistical tests to assess the significance of the separation between the two populations in GES. We then employ galactic chemical evolution models to investigate the origin of the chemical differences encountered in the analysis. We confirm the presence of two distinct populations, with consistently different dynamical and chemical properties. The low energy population seems to show higher overall abundances, whereas the high-energy one may be more metal-poor. We attribute this difference to the presence of at least two separate populations of stars within Gaia-Enceladus, likely associated with the innermost (low-energy) and outermost (high-energy) regions of the progenitor. The adopted models successfully reproduce the patterns in metallicity and [alpha/M] distributions in an inside-out scenario. Our analysis supports the presence of a former metallicity gradient in Gaia-Enceladus, and reinforces the interpretation of its accretion as a multi-passage event through the Milky Way disc.

Evidence for multiple crossings and stripping of Gaia-Enceladus/Sausage across the Milky Way

TL;DR

This work tests the hypothesis that Gaia-Enceladus/Sausage (GES) consisted of multiple accretion passages by identifying two chemo-dynamical subpopulations within GES using APOGEE DR17 data and a CREEK-based membership algorithm. Through KDE analyses and non-parametric statistical tests, the authors demonstrate significant differences in metallicity and alpha-element patterns between the two populations, consistent with inner, more chemically enriched and outer, less enriched regions of the progenitor. Galactic chemical evolution models, scanned over parameters controlling gas inflow, star formation efficiency, and outflow onset, reproduce the observed distributions with Pop1 (inner) having a shorter infall timescale and Pop2 (outer) a longer one, supporting an inside-out growth and a multi-passage accretion history. The results bolster a complex assembly scenario for the MW halo, where the GES progenitor’s core and periphery were stripped in distinct disc-crossing events, leaving behind chemically distinct imprints that future high-resolution surveys can further resolve.

Abstract

The accretion of Gaia-Enceladus/Sausage (GES) onto the Milky Way (MW) is one of the most prominent features of the Galactic halo revealed by the combination of the Gaia satellite and large spectroscopic surveys. This massive accretion largely contributes to the local stellar halo mass and was significant enough to alter the formation history and the morphology of the MW. In this work, we aim to analyse the selection of stars previously identified as belonging to GES with different kinematics and chemical properties to test the hypothesis of a two-phase accretion event. We apply several statistical tests to assess the significance of the separation between the two populations in GES. We then employ galactic chemical evolution models to investigate the origin of the chemical differences encountered in the analysis. We confirm the presence of two distinct populations, with consistently different dynamical and chemical properties. The low energy population seems to show higher overall abundances, whereas the high-energy one may be more metal-poor. We attribute this difference to the presence of at least two separate populations of stars within Gaia-Enceladus, likely associated with the innermost (low-energy) and outermost (high-energy) regions of the progenitor. The adopted models successfully reproduce the patterns in metallicity and [alpha/M] distributions in an inside-out scenario. Our analysis supports the presence of a former metallicity gradient in Gaia-Enceladus, and reinforces the interpretation of its accretion as a multi-passage event through the Milky Way disc.
Paper Structure (8 sections, 4 equations, 6 figures, 4 tables)

This paper contains 8 sections, 4 equations, 6 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Dataset and chemical characterisation
  3. Sample selection
  4. Chemical characterisation and statistical tests
  5. Origin of the two populations: insights from chemical evolution models
  6. Ingredients and parameters of GCE models
  7. Comparison between observed and predicted populations
  8. Summary and conclusions

Figures (6)

  • Figure 1: Left panel: [Al/Fe] versus [M/H] for the whole APOGEE dataset, inclusive of disc (blue contours). Pop 1 and Pop 2 from this work are highlighted in green and orange and individual stars in globular clusters are represented as light blue and yellow stars for the two populations respectively. Right panel: Energy versus angular momentum for the APOGEE halo (blue contours). Pop 1 and Pop 2 from this work are highlighted in green and orange and individual stars in globular clusters are represented as light blue and yellow stars for the two populations respectively.
  • Figure 2: Left panel: [Al/Fe] versus [Fe/H] for Pop 1 of Gaia Enceladus. Globular clusters are colour-coded according to the legend. Right panel: [Al/Fe] versus [Fe/H] for Pop 2 of Gaia Enceladus. Globular clusters are colour-coded according to the legend
  • Figure 3: Left panel: Metallicity distribution function (MDF) for the two populations of Gaia-Enceladus. Right panel: histogram of [$\alpha$/M] for the two populations of Gaia-Enceladus. Continuous lines represent KDEs for the two distributions.
  • Figure 4: Panels a to f show [Si/Fe], [Al/Fe], [Mg/Fe], [O/Fe], [Ca/Fe], and [Ce/Fe] as a function of [M/H] respectively. The green and orange points correspond to Pop 1 and Pop 2, while the dashed lines indicate the linear fits to the mean abundance values in bins of metallicity.
  • Figure 5: [$\alpha$/M] versus [M/H] for Pop 1 and Pop 2 as discussed in Section \ref{['sec:dataset']} (green and orange filled circles, respectively) as compared with predicted abundance ratios for the correspondent best models (green and orange solid lines, see Section \ref{['ss:GCE_results']}) .
  • ...and 1 more figures