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

G. Bono, V. F. Braga, M. Fabrizio, M. Tantalo, K. Baeza-Villagra, J. Crestani, V. D'Orazi, M. Dall'Ora, M. Di Criscienzo, G. Fiorentino, M. Gholami, M. Marengo, C. E. Martínez-Vázquez, M. Monelli, J. P. Mullen, A. Nunnari, V. D. Pipwala, Z. Prudil, C. Sneden, G. Altavilla, M. Bergemann, G. Böcek Topcu, R. Buonanno, A. Calamida, E. Carretta, G. Ceci, B. Chaboyer, M. Correnti, R. da Silva, I. Ferraro, F. A. Gómez, G. Iannicola, R. -P. Kudritzki, A. Kunder, S. Kwak, M. Marconi, S. Marinoni, N. Matsunaga, F. Matteucci, A. Monachesi, I. Musella, M. G. Navarro Ovando, G. W. Preston, V. Ripepi, M. Salaris, M. Sánchez-Benavente, E. Spitoni, P. B. Stetson, F. Thévenin, I. B. Thompson, P. B. Tissera, T. Tsujimoto, E. Valenti, A. K. Vivas, A. R. Walker, M. Zoccali, A. Zocchi

TL;DR

This work delivers the largest, most homogeneous photometric (PR3C) and spectroscopic (SR3C) catalogs for RR Lyrae and related old stellar tracers, enabling a comprehensive chemo-dynamical study of the Milky Way’s old spheroid. By combining Gaia DR3 astrometry with LR/MR/HR spectroscopy and a consistent metallicity scale (including ΔS and HR abundances), the authors map the distribution, kinematics, and metallicity gradients across the Halo, Thick Disk, Thin Disk, and major streams (GSE, Sequoia, Helmi, Sgr). They find a smooth iron abundance progression from Halo to Thin Disk, with negative radial gradients that are steeper in the Disk components and milder in the Halo, and only mild gradients in the stellar streams, suggesting a complex but coherent early assembly. The MW gradients are then contrasted with those in M31, revealing broadly similar trends and reinforcing the view that old stellar tracers preserve signatures of early chemical enrichment; the results have important implications for hierarchical assembly and inside-out formation scenarios of galactic spheroids.

Abstract

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.

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

TL;DR

This work delivers the largest, most homogeneous photometric (PR3C) and spectroscopic (SR3C) catalogs for RR Lyrae and related old stellar tracers, enabling a comprehensive chemo-dynamical study of the Milky Way’s old spheroid. By combining Gaia DR3 astrometry with LR/MR/HR spectroscopy and a consistent metallicity scale (including ΔS and HR abundances), the authors map the distribution, kinematics, and metallicity gradients across the Halo, Thick Disk, Thin Disk, and major streams (GSE, Sequoia, Helmi, Sgr). They find a smooth iron abundance progression from Halo to Thin Disk, with negative radial gradients that are steeper in the Disk components and milder in the Halo, and only mild gradients in the stellar streams, suggesting a complex but coherent early assembly. The MW gradients are then contrasted with those in M31, revealing broadly similar trends and reinforcing the view that old stellar tracers preserve signatures of early chemical enrichment; the results have important implications for hierarchical assembly and inside-out formation scenarios of galactic spheroids.

Abstract

We introduce a new photometric catalog of RR Lyrae variables (RRLs, 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 (16,000) and iron abundances (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 -enhanced ([/Fe]=0.27, =0.18), while TCD RRLs are either -enhanced ([Fe/H]-1.0) or -poor ([Fe/H]-1.0), and TND RRLs are mainly -poor ([/Fe]=-0.01, =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 -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 [/Fe] radial gradients of Milky Way RRLs and M31 globular clusters throughout the full range of galactocentric distances covered by the two samples.
Paper Structure (24 sections, 2 equations, 29 figures, 10 tables)

This paper contains 24 sections, 2 equations, 29 figures, 10 tables.

Table of Contents

  1. Introduction
  2. Photometric and spectroscopic datasets
  3. Photometric datasets
  4. Spectroscopic datasets
  5. Individual radial velocities
  6. Individual iron abundances
  7. Individual $\alpha$-element abundances
  8. Kinematic selection
  9. Identification of the main Galactic components
  10. Identification of stellar streams
  11. Comparison with different stellar tracers
  12. Radial metallicity gradient
  13. RRLs in Galactic components
  14. RRLs in Stellar streams
  15. Comparison with MW metallicity gradients
  16. ...and 9 more sections

Figures (29)

  • Figure 1: Aitoff projection in Galactic coordinates of the RRLs in the PR3C. Isolated over--densities are associated either to the Magellanic Clouds, or to nearby stellar systems or to stellar streams.
  • Figure 2: Period distribution of the RRLs in the PR3C. Fundamental (RRab), first overtone (RRc), and mixed mode (RRd) RRLs are plotted with different colors (see labels).
  • Figure 3: Selected magnitude distributions for RRLs in the PR3C. From top to bottom the histograms display the mid-infrared $W1$-band, the near-infrared $K$-band, and the Gaia optical $R_P$-band magnitudes.
  • Figure 4: Aitoff projection in Galactic coordinates of the RRLs in the SR3C. Yellow and blue dots mark RRLs and HB stars for which are available either HR or LR spectra, respectively. The number of RRLs and HBs with HR and LR spectra is labeled.
  • Figure 5: Top: V-band luminosity amplitude vs period (Bailey diagram) for RRLs in SR3C. RRLs that are pulsating in the fundamental mode (RRab) are highlighted in purple, those pulsating in the first overtone (RRc) in red and the RRLs that are simultaneously pulsating in the first two radial modes (RRd, mixed mode) in dark green. The numbers close to the acronyms show the sample size. Middle: Same as the top, but for the period distribution. Bottom: Same as the top, but for the mean $R_P$-band magnitude distribution.
  • ...and 24 more figures