Analysing the structure of the bulge with Mira variables

TL;DR

This study uses Mira variables from OGLE-IV data to analyze the Galactic bulge's shape, providing evidence that supports a boxy bulge structure over an X-shaped one, based on statistical model fitting.

Contribution

It introduces the use of Mira variables as an alternative population to constrain the bulge shape, challenging previous claims based on red-clump stars.

Findings

Boxy bulge model has a higher probability fit (p=0.19) than X-shaped model (p=2.85e-6).

The X-shaped bulge is statistically disfavored at a 4.7σ level.

Results suggest the Galactic bulge is more likely boxy than X-shaped.

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.