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We have used photometric data on almost 91 000 fundamental-mode RR Lyraestars (type RRab) detected by the OGLE survey to investigate properties of oldpopulations in the Milky Way. Based on their metallicity distributions, wedemonstrate that the Galaxy is built from three distinct old components: halo,bulge, and disk. The distributions reach their maxima at approximately[Fe/H]_J95 = -1.2, -1.0, and -0.6 dex on the Jurcsik's metallicity scale,respectively. We find that, very likely, the entire halo is formed frominfalling dwarf galaxies. It is evident that halo stars penetrate the innerregions of the Galactic bulge. We estimate that about one-third of all RR Lyrstars within the bulge area belong in fact to the halo population. The wholeold bulge is dominated by two populations, A and B, represented by a doublesequence in the period-amplitude (Bailey) diagram. The boundary in ironabundance between the halo and the disk population is at about [Fe/H]_J95 =-0.8 dex. Using Gaia DR2 for RRab stars in the disk area, we show that theobserved dispersion of proper motions along the Galactic latitude decreasessmoothly with the increasing metal content excluding a bump around [Fe/H]_J95 =-1.0 dex.

Properties of the Milky Way's Old Populations Based on Photometric Metallicities of the OGLE RR Lyrae Stars