On the Use of Field RR Lyrae as Galactic Probes -- VIII. Early Formation of the Galactic Spheroid

We introduce a new photometric catalog of RR Lyrae variables (RRLs, $\sim$300,000) mainly based on data available in public datasets. We also present the largest and most homogeneous spectroscopic dataset of RRLs and Blue Horizontal Branch [BHB] stars ever collected. This includes radial velocity measurements ($\sim$16,000) and iron abundances ($Δ$S method for 8,140 RRLs, plus 547 from literature). Elemental abundances based on high-resolution spectra are provided for 487 RRLs and 64 BHB stars. We identified candidate RRLs associated to the main Galactic components and their iron distribution function (IDF) becomes more metal-rich when moving from the Halo ([Fe/H]=-1.56) to the Thick (TCD; [Fe/H]=-1.47) and Thin (TND; [Fe/H]=-0.73) disk. Furthermore, Halo RRLs and RRLs in retrograde orbits are $α$-enhanced ([$α$/Fe]=0.27, $σ$=0.18), while TCD RRLs are either $α$-enhanced ([Fe/H]$\le$-1.0) or $α$-poor ([Fe/H]$>$-1.0), and TND RRLs are mainly $α$-poor ([$α$/Fe]=-0.01, $σ$=0.20). We also identified RRLs associated to the main stellar streams (Gaia-Sausage-Enceladus [GSE]; Sequoia, Helmi, Sagittarius) and we found that their IDFs are quite similar to Halo RRLs. However, GSE RRLs lack the metal-poor/metal-rich tails and their $α$-element distribution is quite compact. The iron radial gradient in Galactocentric distance for TND, TCD and Halo RRLs is negative and it decreases from -0.026, to -0.010, and to -0.002 dex/kpc. The iron radial gradient based on dry Halo (Halo without substructures) RRLs is, within the errors, equal to the global Halo. We also found a strong similarity between iron and [$α$/Fe] radial gradients of Milky Way RRLs and M31 globular clusters throughout the full range of galactocentric distances covered by the two samples.

💡 Research Summary

This study presents the most extensive and homogeneous photometric and spectroscopic database of field RR Lyrae (RRL) variables and Blue Horizontal Branch (BHB) stars to date, with the goal of probing the early formation of the Milky Way’s spheroidal components. By combining public time‑domain surveys (Gaia, OGLE, ZTF, ASAS‑SN, etc.) with dedicated observations, the authors assembled the Photometric Rome RRL Catalog (PR3C) containing roughly 300,000 RRLs. Distances and reddening were derived with a uniform pipeline, achieving 3–5 % distance precision.

Spectroscopically, the Spectroscopic Rome RRL Catalog (SR3C) comprises about 16,000 radial‑velocity measurements, iron abundances for 8,140 RRLs obtained via the classic ΔS method (plus 547 literature values), and high‑resolution (R≈30,000–60,000) elemental abundances for 487 RRLs and 64 BHB stars. The high‑resolution spectra provide both Fe and α‑element (Mg, Si, Ca, Ti) abundances, enabling a consistent chemical scale across low‑ and high‑resolution data.

Using Gaia DR3 astrometry, the authors computed full 6‑D phase‑space information and derived orbital parameters (energy, angular momentum, eccentricity, pericenter, apocenter). They then kinematically separated the sample into the canonical Galactic components—Halo, Thick Disk (TCD), Thin Disk (TND)—and identified retrograde Halo RRLs as a distinct sub‑population. Additionally, they flagged members of the major stellar streams Gaia‑Sausage‑Enceladus (GSE), Sequoia, Helmi, and Sagittarius.

The iron distribution functions (IDFs) show a clear progression: Halo RRLs peak at ⟨