Analysing the structure of the bulge with Mira variables

Abstract

The Galactic bulge at latitude $4<|b|$(deg.)$<10$ was claimed to show an X-shape, which means that stellar density distributions along the line of sight have a double peak. However, this double peak is only observed with the red-clump population, and doubt has been cast on its use as a perfect standard candle.\ As such, a boxy bulge without an X-shape is not discarded. We aim to constrain the shape of the bulge making use of a different population: Mira variables from the new Optical Gravitational Lensing Experiment data release, OGLE-IV, with an average age of 9 Gyr. We analysed an area of the bulge far from the plane, where we fitted the density of the Miras with boxy bulge and X-shaped bulge models and calculated the probability of each model. We find that the probability of a boxy bulge fitting the data is $p=0.19$, whereas the probability for the X-shaped bulge is only $p=2.85 \cdot 10^{-6}$ (equivalent to a tension of the model with the data of a 4.7$\sigma $ level). Therefore, the boxy bulge model seems to be more appropriate for describing the Galactic bulge, although we cannot exclude any model with complete certainty.