Tracing Ancient Mergers in the Heart of the Milky Way: RR Lyrae Stars and the Gaia-Enceladus/Sausage in the Inner Stellar Halo
Andrea Kunder and collaborators investigate the oldest parts of our Milky Way by looking at stars in its innermost regions. They aim to understand how much of the Galaxy’s central stellar halo, an ancient, faint cloud of stars, is made up of remnants from an ancient galaxy merger called Gaia-Enceladus/Sausage (GES). To do this, they study special pulsating stars known as RR Lyrae, which act as reliable beacons of distance and age.
Introduction: Why Study the Inner Halo?
Galaxies, including the Milky Way, are surrounded by halos of dark matter and stars. These halos are shaped over billions of years through a series of mergers and collisions with smaller galaxies. Some of the debris from these events ends up in the halo, especially in its outer parts where it is easier to detect because of lower crowding from other stars. But the inner halo, deep inside the Galaxy near the bulge, is much harder to study because it overlaps with the bright and dense central bulge and bar. The authors focus on this inner region to see if they can still find evidence of the GES merger, thought to have occurred 8–11 billion years ago, leaving behind metal-poor stars with distinct motions. RR Lyrae stars are ideal tracers for this purpose: they are old, metal-poor, and have predictable brightness, making them easy to identify and measure.
Selecting Halo Stars from the Bulge
The team starts with a catalog of 8,457 RR Lyrae stars from previous work, which includes precise measurements of their motions and distances. Using criteria based on their orbits, specifically how far above or below the Galactic plane they travel (between 3–20 kpc), they identify a sample of 281 RR Lyrae stars likely belonging to the inner halo rather than the bulge. These stars are more metal-poor and have period distributions consistent with having been accreted from another galaxy rather than forming within the Milky Way itself. Comparing these halo RR Lyrae to the bulge and disk RR Lyrae reveals they are distinct in both metallicity and pulsation properties.
Looking for GES Signatures in Motion
One way to identify GES stars is through their unique orbits. When plotted in a space defined by their energy and angular momentum, quantities that describe the shape and size of their orbits, GES stars tend to occupy a specific region: they have slightly retrograde motions and high eccentricities (long, narrow orbits). The authors divide their 281 inner halo RR Lyrae into two groups: those with motion characteristics consistent with GES and those without. They find 110 RR Lyrae in the GES-like region, which also tend to have higher eccentricities and lower metallicities compared to the rest. This suggests some of these stars may indeed be remnants of the GES merger.
Simulations to the Rescue: Testing the Hypothesis
To estimate how much of their sample truly comes from GES rather than from other mergers or in-situ (formed here) stars, the team compares their observations with a detailed simulation of a Milky Way–like galaxy called Auriga-18. This simulation includes a GES-like event (called GES-18) and shows how its stars are distributed today. In the simulation, GES-18 stars stand out for their high eccentricities and specific energy–momentum patterns, matching what is seen in the RR Lyrae data. The authors find that the excess of RR Lyrae on highly eccentric orbits (e > 0.85) suggests about 9% of the inner halo RR Lyrae originated from GES.
Examining Metallicity and Other Clues
Next, the authors check if the GES debris shows a metallicity gradient (a change in metal content with distance from the Galactic center) as seen in some other galaxies. Both the observations and the Auriga simulation show no significant gradient in the inner halo. They also find that at lower metallicities ([Fe/H] ~ -1.5 or below), more stars tend to come from mergers in general. However, even among metal-poor stars, only about 6–9% of the RR Lyrae in the inner halo can be confidently attributed to GES.
Discussion: Comparing to Other Parts of the Galaxy
In the solar neighborhood (where the Sun is), previous studies estimate that 25-30% of RR Lyrae come from GES. But in the crowded inner halo, the fraction is much lower. This is consistent with predictions from the Auriga simulation: while the inner regions of the halo contain more ancient debris overall, much of it comes from earlier, smaller mergers rather than the massive GES event, which dominates more at intermediate distances (~5-10 kpc). The findings support the idea that GES stars are less concentrated toward the Galactic center than other, earlier accreted populations.
Conclusion: A Small but Distinct GES Presence
Kunder and colleagues conclude that the inner halo of the Milky Way does contain a measurable fraction of stars from GES, but this fraction is modest, only about 6-9% of the RR Lyrae sample shows strong evidence of belonging to this ancient merger. This is smaller than in the solar neighborhood but matches expectations from simulations. With upcoming data from the Gaia mission and spectroscopic surveys, much larger samples of RR Lyrae will allow for even better maps of the Milky Way’s merger history, potentially disentangling overlapping debris from different ancient galaxies.
Source: Kunder
