Metallicity Properties of the Galactic Bulge Stars Near and Far: Expectations from the Auriga Simulation

Using the high-resolution Milky Way–like model from Auriga simulation we study the chemical properties of the Galactic bulge, focusing on the metallicity difference between stars on the near side (in front of the Galactic center) and the far side (behind the Galactic center). In general, along certain sight lines the near side is more metal-rich than the far side, consistent with the negative vertical metallicity gradient of the disk, since the far side is located higher above the disk plane than the near side. However, at the region l < 0° and ∣ b ∣ ≤ 6°, the near side is even more metal-poor than the far side, and their difference changes with the Galactic longitude. This is mainly due to the fact that stars near the minor axis of the bar are more metal-poor than that around the major axis. Since the bar is tilted, in the negative longitude region, the near side is mainly populated by stars close to the minor-axis region rather than the far side resulting in such a metallicity difference. We extract stars in the X-shape structure by identifying the overdensities in the near and far sides. Their metallicity properties are consistent with the results of the whole Galactic bulge. The boxy/peanut-shaped bulge can naturally explain the metallicity difference of the double red clump stars in the observation. There is no need to involve a classical bulge component with different stellar populations.