Listening to the Milky Way’s Oldest Stars: What RR Lyrae Reveal About the Galactic Halo
In this paper, Cabrera Garcia et al. investigate the structure and history of the Milky Way’s stellar halo using RR Lyrae stars, a special class of old (older than 10 billion years), pulsating variable stars. Because RR Lyrae stars have well-understood brightness variations, astronomers can use them as reliable distance indicators and chemical tracers. The authors assemble an exceptionally large and relatively unbiased sample of 135,873 RR Lyrae stars drawn from Gaia data, allowing them to probe the Galactic halo from the inner regions near the Galactic Centre out to distances of more than 100 kiloparsecs. Their central goal is to understand how the halo is chemically and dynamically structured, and what this reveals about the Milky Way’s formation history.
Data Set and Sample Construction
The study begins with a careful description of the data set and how it is refined. The authors use newly calibrated relations between RR Lyrae light-curve properties and metallicity, expressed as [Fe/H], to estimate chemical abundances, along with precise distance measurements. They then “clean” the initial sample by removing RR Lyrae stars associated with known substructures such as globular clusters, dwarf galaxies, the Sagittarius stream, and the Magellanic Clouds, as well as stars with problematic measurements. This process results in a Cleaned Sample of about 79,000 RR Lyrae stars that are intended to represent the diffuse stellar halo rather than obvious, already-identified components.
Metallicity Trends and the Dual-Halo Picture
With this Cleaned Sample, the paper next examines how metallicity varies throughout the halo. By looking at the metallicity distribution function (MDF) as a function of distance from the Galactic Centre and height above the Galactic plane, the authors find a clear trend: stars farther out and farther from the plane are, on average, more metal-poor. To quantify this, they apply a Gaussian mixture model that separates the halo into two components: an inner-halo population with a higher mean metallicity (around [Fe/H] ≈ −1.6) and an outer-halo population with a lower mean metallicity (around [Fe/H] ≈ −2.2). The relative contribution of the outer-halo population increases beyond roughly 20–30 kiloparsecs, supporting the long-standing “dual-halo” picture of the Milky Way.
Orbital Dynamics of Halo RR Lyrae Stars
The paper then turns to dynamics, focusing on a smaller subsample of about 5,300 RR Lyrae stars that also have measured radial velocities. Using a Galactic potential model and the AGAMA software, the authors compute orbital properties such as energies, actions, eccentricities, and maximum distances from the Galactic plane. A Toomre diagram shows that these stars occupy a wide range of orbits, with a mix of prograde and retrograde motions, and only a small fraction consistent with disc-like kinematics. This confirms that RR Lyrae stars are excellent tracers of the halo rather than the Milky Way’s disc.
Dynamically Tagged Groups and Accretion Signatures
One of the most novel aspects of the study is the identification of “dynamically tagged groups” (DTGs). Using the HDBSCAN clustering algorithm in energy–action space, the authors identify 97 DTGs, groups of stars that share similar orbital properties and are therefore likely to have originated from the same accreted system, such as a dwarf galaxy or globular cluster. Importantly, metallicity is not used in the clustering, yet the resulting groups often show small internal spreads in [Fe/H], suggesting common chemical histories and reinforcing the idea that these stars formed together.
Implications for the Milky Way’s Assembly History
Finally, the authors compare the identified DTGs with previously known Milky Way substructures. Many groups are associated with well-known merger remnants such as Gaia–Sausage–Enceladus, the Helmi stream, Sequoia, and the Sagittarius stream, while others appear to be new. The low metallicity dispersions within DTGs provide strong evidence that RR Lyrae stars preserve signatures of ancient accretion events. Overall, Cabrera Garcia et al. show that a large, homogeneous sample of RR Lyrae stars offers a powerful, relatively unbiased view of the Milky Way’s halo, confirming its dual nature and revealing a rich fossil record of the Galaxy’s assembly history.
Source: Cabrera Garcia
