Abstract

The projected density distribution of type ab RR Lyrae (RRab) stars was characterised from the innermost regions of the Milky Way to the halo, with the aim of placing constraints on the Galaxy's evolution. The compiled sample (N-RRab=64 850) stems from fundamental mode RR Lyrae variables identified by the VVV, OGLE, and Gaia surveys. The distribution is well fitted by three power laws over three radial intervals. In the innermost region (R2.2) the distribution follows Sigma (RRab[1])proportional to R-0.94 +/- 0.051, while in the external region the distribution adheres to Sigma (RRab[2])proportional to R-1.50 +/- 0.019 for 2.2 degrees R 8.0 degrees and Sigma (RRab[3])proportional to R-2.43 +/- 0.043 for 8.0 degrees R30.0 degrees. Conversely, the cumulative distribution of red clump (RC) giants exhibits a more concentrated distribution in the mean, but in the central R2.2 the RRab population is more peaked, whereas globular clusters (GCs) follow a density power law (Sigma (GCs)proportional to R-1.59 +/- 0.060 for R30.0) similar to that of RRab stars, especially when considering a more metal-poor subsample ([Fe/H] -1.1 dex). The main conclusion emerging from the analysis is that the RRab distribution favours the star cluster infall and merger scenario for creating an important fraction (> 18%) of the central Galactic region. The radii containing half of the populations (half populations radii) are R-H RRab=6.8 degrees (0.99 kpc), R-H RC=4.2 degrees (0.61 kpc), and R-H GCs=11.9 degrees (1.75 kpc) for the RRab stars, RC giants, and GCs, respectively. Finally, merely similar to 1% of the stars have been actually discovered in the innermost region (R 35 pc) out of the expected (based on our considerations) total number of RRab therein: N1562. That deficit will be substantially ameliorated with future space missions like the Nancy Grace Roman Space Telescope (formerly WFIRST). QG1EJ 646: - FEB 4 2021 101 GLOBULAR-CLUSTER SYSTEM SPACE-TELESCOPE STARS GALAXY BULGE KINEMATICS VARIABLES DYNAMICS DISTANCE REGIONS