About 5% of the stars in the bulge have metallicities < –1.0. The ARGOS survey showed that the fraction of these stars, which they associated with the Galactic halo population, increases with height above the plane. This population also rotates more slowly, at about 50% of the speed of more metal rich stars (Ness et al. 2013b). It also has a high and relatively latitude-independent velocity dispersion as seen in Fig. 4. Kunder et al. (2015) have also suggested that the halo population in the bulge is not insignificant, even within 1 kpc of the Galactic center. Bensby et al. (2013) measured the individual abundances of microlensed dwarf bulge stars and found that the stars with [Fe/H] ≈ –1.0 have very similar chemical abundance trends to the nearby thick disk stars.
The bulge RR Lyrae population, whose MDF peaks around [Fe/H] = −1.0 (Pietrukowicz et al. 2014), has been used to trace the distribution of the old metal-poor population of stars in the bulge. The RR Lyrae stars are ideal tracers of structure because their distances can be accurately estimated. Although Dékány et al. (2013) reported the spatial distribution of the RR Lyrae to be spheroidal with only a slight elongation, the survey of Pietrukowicz et al. (2014) showed that the metal-poor RR Lyrae very closely trace the barred structure of intermediate-age red clump giants. Pietrukowicz et al. (2014) show that the spatial distribution of the RR Lyrae population is barred but that these stars are not part of the X-shape structure. They also report two sequences of RR Lyrae with marginally different metallicities, proposing evidence for the role of mergers in the initial formation of the bulge. Although they may follow the elongated distribution of the bar, the RR Lyrae stars may still be part of the halo but which has been pulled into a bar shape by the potential of the other stars (e.g. Saha and Gerhard 2013).
The high-resolution detailed chemical abundance analysis of a handful of the most metal-poor bulge stars with [Fe/H] < −2.0 by Howes et al. (2014) and García Pérez et al. (2013) reported similar individual element enhancement and alpha-enhancement trends to halo stars of the same metallicity (although Howes et al. (2014) noted an abundance scatter that may be larger). From the analysis of the dynamics and detailed chemical abundances of three bulge stars with [Fe/H] < −2.7, Casey and Schlaufman (2015) report that these stars follow the abundance trends identified previously for metal-poor halo stars, except for scandium. Simulations (e.g. Brook et al. 2007; Diemand et al. 2008; Tumlinson 2010) predict that the oldest stars in the Galaxy will be concentrated to the bulge. These very metal poor stars found by Casey and Schlaufman (2015) may well be members of the oldest populations of stars in the Milky Way, formed at redshift z > 10. The abundance patterns of this rare stellar population which is concentrated to the bulge could provide critical insight into the composition of the universe at the highest redshift and the conditions at the beginning of the Milky Way's formation.