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