The RR Lyrae projected density distribution from the Galactic centre to the halo

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 Galaxys 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 (R < 2.2 deg) the distribution follows Sigma_RRab[1] propto R ^(-0.94 +- 0.051), while in the external region the distribution adheres to Sigma_RRab[2] propto R^(-1.50 +- 0.019) for 2.2 deg< R <8.0 deg and Sigma_RRab[3] propto R ^(-2.43 +- 0.043) for 8.0 deg < R <30.0 deg. Conversely, the cumulative distribution of red clump (RC) giants exhibits a more concentrated distribution in the mean, but in the central R < 2.2 deg the RRab population is more peaked, whereas globular clusters (GCs) follow a density power law (Sigma_GCs propto R ^(-1.59 +- 0.060) for R<30.0 deg) 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=6.8 deg (0.99 kpc), R_H =4.2 deg (0.61 kpc), and R_H =11.9 deg (1.75 kpc) for the RRab stars, RC giants, and GCs, respectively. Finally, merely 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: N sim 1,562. That deficit will be substantially ameliorated with future space missions like the Nancy Grace Roman Space Telescope (formerly WFIRST).

11 pages, 9 figures and 2 tables. Accepted for publication in A&A

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• The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae stars, and Evidence for the Merger History
• An Intriguing Globular Cluster in the Galactic Bulge from the VVV Survey
• Globular Clusters in the Galactic Center Region: expected behavior in the infalling and merger scenario
• rrlfe: Software for Generating and Applying Metallicity Calibrations for RR Lyrae Variable Stars Across a Wide Range of Phases and Temperatures
• Confirmation Of Two New Galactic Bulge Globular Clusters: FSR 19 and FSR 25