Cosmological Simulations of Stellar Halos with Gaia Sausage-Enceladus Analogues: Two Sausages, One Bun?

TL;DR

The paper investigates whether the Milky Way's Gaia Sausage–Enceladus (GSE) debris could originate from more than one progenitor by analyzing 98 MW-like halos in the IllustrisTNG50 simulation. It reconstructs each analogue’s accretion history, tagging ex situ stars to specific mergers, and defines GSE-like debris as radially biased ex situ material with $f>0.5$ and velocity anisotropy $eta>0.5$, distinguishing single-merger from two-merger GSEs via star formation histories and chemical abundances. It finds that about a third of GSE-like debris comes from two mergers, with single-merger GSEs accreted later and RA pairs generally having briefer, earlier SFHs and distinct chemical signatures, including lower [Fe/H] and higher [Mg/Fe] in some cases. These results imply that chemodynamical information is essential for disentangling halo assembly histories and that the MW’s GSE could plausibly be a composite of multiple accretion events, informing models of galaxy formation and halo evolution.

Abstract

Observations of the Milky Way's stellar halo find that it is predominantly comprised of a radially-biased population of stars, dubbed the Gaia Sausage--Enceladus, or GSE. These stars are thought to be debris from dwarf galaxy accretion early in the Milky Way's history. Though typically considered to be from a single merger, it is possible that the GSE debris has multiple sources. To investigate this possibility, we use the IllustrisTNG50 simulation to identify stellar accretion histories in 98 Milky Way analogues -- the largest sample for which such an identification has been performed -- and find GSE-like debris in 32, with two-merger GSEs accounting for a third of these cases. Distinguishing single-merger GSEs from two-merger GSEs is difficult in common kinematic spaces, but differences are more evident through chemical abundances and star formation histories. This is because single-merger GSEs are typically accreted more recently than the galaxies in two-merger GSEs: the median infall times (with 16th and 84th percentiles) are $5.9^{+3.3}_{-2.0}$ and $10.7^{+1.2}_{-3.7}$ Gyr ago for these scenarios, respectively. The systematic shifts in abundances and ages which occur as a result suggest that efforts in modeling these aspects of the stellar halo prove ever-important in understanding its assembly.

Figures (47)

• Figure 1: (Left:) The composition of the ex situ stellar halo for a particular MW analogue (ID 543729) as a function of galactocentric distance $r$. The five largest galaxies contributing to the ex situ stellar population are shown as individual colored bands, sorted by the contribution within 40 kpc. The galaxy which contributes the most ex situ stellar mass is colored in blue, the second in purple, the third in magenta, the fourth in orange, and the fifth in gold. Stars contributed by all other mergers are collected into the light gray band at the top. This MW analogue's ex situ population is primarily from one large merger with infall $M_\mathrm{dyn} \sim 8\times10^{10}~\mathrm{M}_\odot$. All other mergers contribute fewer stars to the accreted population. The horizontal line indicates 50%, the threshold used throughout the work to denote significant mergers. (Right:) The same as the left panel, but for ID 516760. This halo hosts a pair of mergers (blue and purple) that, taken together, comprise more than 50% of the ex situ stellar material within 40 kpc.
• Figure 2: (Left:) The number of mergers required to reach the threshold of 50% of ex situ stars within 40 kpc of the galactic center. Approximately 60% of the MW analogues have a single merger which contributes over half of the ex situ stars within 40 kpc, and an additional $\sim 30\%$ reach this threshold with two mergers. It is therefore not unreasonable to expect that an ex situ halo could be predominantly comprised of two mergers, though typically there is a single large merger which comprises the majority of the inner halo. (Right:) The velocity anisotropy $\beta$ of ex situ stellar debris is shown for all large mergers (those which contribute over 50% of their host's ex situ stellar population within 40 kpc) in red. Those which are considered radially-anisotropic (RA) debris lie in the shaded large-$\beta$ region. Each pair of mergers which does sum to $f > 50\%$ is shown in purple. There are occasionally multiple choices of pairs of mergers in the same halo that satisfy the RA debris criteria (i.e., that fall in the shaded region). There are 21 single-halo RA mergers, and there are 16 RA pairs of mergers selected from 11 MW analogues (5 MW analogues have multiple choices of RA pair).
• Figure 3: The infall times $t_\mathrm{infall}$ and masses $M_\mathrm{dyn}$ of all mergers across the TNG MW analogues. The RA mergers passing the selection criteria established in \ref{['sec:3.2']} are shown in red, while RA pairs are shown in purple (for the larger-$f$ merger) and pink (for the smaller-$f$ merger), with marker shapes indicating the MW analogue to which they belong. Other (non-RA) mergers are shown in black, with the opacity set by $f$. The corresponding marginal distributions are shown above and to the right of the main panel. Generally, RA mergers are more recently accreted and larger in mass than those comprising RA pairs. For the RA pairs, the larger-$f$ mergers tend to be more recently accreted and to have slightly larger mass.
• Figure 4: The distribution of spherical velocities $v_r$ and $v_\phi$ for RA merger debris in four example MW analogues. The grayscale histogram shows the full distribution of RA merger debris. The top row shows these distributions for single RA mergers, with 68% of the stars enclosed within the red contour. The bottom row shows the distributions for RA pairs, with 68% of stars from the high-$f$ (low-$f$) merger enclosed within the purple (pink) contour. In each bottom panel, the fractions $f$ are provided in the lower-left corner and an icon unique to the MW analogue is provided in the lower-right corner, consistent with \ref{['fig:3']}. The examples chosen here highlight the diversity in kinematic features observed in MW's with RA debris. Versions of this figure for each choice of RA debris are included in \ref{['app:C']}.
• Figure 5: Total energy shown against the $z$-component of angular momentum for the same MW analogues as in \ref{['fig:4']}, with identical formatting, except as noted. Each axis has been normalized to an energy and angular momentum scale characteristic of the RA debris to emphasize the shapes of these distributions. Specifically, $\bar{E}$ is the median energy of RA debris, while $\Delta E$ is the difference between the 2nd and 98th percentile. Similarly, $\Delta L_{z}$ is the difference between the 2nd and 98th percentile values of RA debris' $L_z$. As in the $v_r - v_\phi$ plane, these distributions exhibit varying degrees of complexity irregardless of whether they correspond to single or pairs of mergers. Versions of this figure for each choice of RA debris are included in \ref{['app:C']}.