We assembled the largest photometric and spectroscopic catalog of field RR Lyrae. These variable stars (the luminosity changes on a time scale ranging from a few hours to less than one day) are very useful to trace properties of stellar populations in the Milky Way and in nearby galaxies, because they are very accurate standard candles and they belong to stellar populations (older than 10 Gyrs). RRL variables are for Astrophysicists like beacons for sailors. Once you identify them, you know with great accuracy their position and their distance. Sailors use the lighthouses to estimate their position and distance from the shore, so we use the distribution of the RRLs across the sky to trace the 3D structure of the Milky Way.

The RRL spectra, the light from the RRLs goes through the telescope and a prisma and produces spectra that are used to estimate their radial velocities (the velocity along the line of sight) and their iron abundances. These information together with proper motions (tangential velocity components) and distances provided by the astrometric satellite Gaia allowed us to perform a detailed 6D+chemistry investigation of the early formation of the Milky Way. The position, the velocity and the distance allow us to investigate their origin. In particular, we can trace their movements back in time and in space at the epochin which the Milky Way was forming. This means that they are fundamental stellar tracers because they provide direct evidence of these  past events. We succeeded in the identification of RRLs belonging to different Galactic components (halo, thin/thick disk) and we found that they have similar properties, thus suggesting, that they formed on a relatively short timescale. The main difference is in the metallicity distribution that becomes systematically more metal-rich when moving from the Halo to the Thick and to the Thin disk. The very first stellar populations (those formed soon after the Big Bang) were metal-free. All the metals were formed by stars, and subsequent stellar populations become systematically more metal-rich. The chemical enrichment history is a complex phenomenon affected by different astrophysical ingredients and by the environment.

The results of our paper indicate that the environment played a key role in their chemical enrichment history. We also identified the most populous stellar streams (GSE, Sequoia, Sagittarius, Helmi) and they show evidence of substructures that need to be investigated in more detail. The main implication of the current findings is that Halo and disks formed at very similar epochs. Moreover, the comparison between the variation of the iron abundance as a function of the Galactocentric distance in our Galaxy (based on RRLs) and in M31 (based on globular glusters and red giants) are quite similar. This means that these two galaxies, in spite of the difference in the total mass, and the significant difference in their accretion history experienced very similar early formation processes. The most popular theories concerning the MW formation date back to the sexties suggesting a dissipative collapse (monolithic collapse) and to the end of the seventies suggesting a dissipation-less mechanism (coalescence of 
smaller progenitors). The latter scenario is supported by Cold Dark Matter cosmological simulations suggesting that the Halo formed from the aggregation of protogalactic fragments (dwarf galaxies).

The discovery of stellar streams and the merging of the Sagittarius dwarf galaxy further supported this hierarchical mechanism. More recently, the astrometric satellite Gaia provided new signatures of major/minor mergers (Gaia Sausage Enceladus, Sequoia, Helmi). To constrain the different models of galaxy formation we need firm constraints on the fraction of stars that formed "in situ", in our own galaxy, from those that have been "accreted" as a consequence of a galaxy merger.  The results of our paper open the path for constraining this crucial parameter.

For the first time we succeeded in using a primary distance indicator to trace old stellar populations in three Galactic components (Halo, thick/thin disk) and in the most populous stellar streams. We adopted two different Galactic potentials and different selection criteria to identify the Galactic components and the stellar streams. Moreover, we also performed a homogeneous comparison of their metallicity (iron, alpha elements) distributions including also RRLs that are in retrograde orbits.

Furthermore, we performed for the first time a detailed comparison of the iron radial gradient in our Galaxy and in M31 by using old stellar tracers. There is the common believe that the work done by Astrophysicists share several methodologies in common with archeologists/paleontologists. Indeed, this field of modern Astrophysics is called "Galactic Archeology". In particular, we investigate the physical mechanisms and the physical conditions driving the early formation of cosmic structures by using fossil records. The RRLs can be considered "living relics" of these early epochs. Seventy-five years after the early evidence for a revision of the cosmic distancescale brought forward by Baade, based on the non-detection of RRLs in thefield of M31 in blue and red photographic plates collected with the Palomartelescope, and the seminal discovery of stellar populations, Galactic RRLs andM31 appear to be once again crucial to improve our knowledge of evolutionary andpulsation properties of old stellar populations and galaxy formation.

Link to the preview of the paper:

"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