On the Correlation Between Metallicity and the X-Shaped Morphology of the Milky Way Bulge

TL;DR

This paper investigates how stellar evolution biases affect the observed correlation between metallicity and the X-shaped structure of the Milky Way bulge, suggesting previous findings may have overestimated this relationship.

Contribution

It highlights the importance of accounting for stellar evolution effects when analyzing the metallicity dependence of the bulge's X-shape, providing a more nuanced interpretation of observational data.

Findings

Metallicity influences the color and ratio of red clump to red giant stars.

Red giant branch ascent duration and red clump lifetime vary with metallicity.

Red giant branch bump stars can mimic signals of a bimodal distance distribution.

Abstract

We demonstrate that failure to properly account for stellar evolution can bias results in determinations of the spatial morphology of Galactic bulge stars, focusing on the question of whether or not the X-shape is more pronounced among the more metal-rich stars than among the metal-poor stars. We argue that this trend, a result recently claimed by three separate groups, may have been overestimated as it is relatively easier to detect a bimodality in the distance distribution function at higher metallicities. This is due to three factors. First, the intrinsic colour of red clump and red giant stars vary with metallicity, at the level d(V-I)_{RC}/d\[M/H] ~ 0.25 mag dex^{-1}, and thus the ratio of red clump to red giant stars within a spectroscopic sample will depend on the photometric selection of any investigation. Second, the duration of ascent of the red giant branch goes down and the red clump lifetime goes up as metallicity increases, which has the effect of increasing the ratio of red clump to red giant stars by as much as ~33% over the range of the bulge metallicity-distribution function. Finally, over the same metallicity interval, the effective number of red giant branch bump stars is predicted to increase by ~200%, and their presence becomes degenerate with the observational parameters of the two red clumps, creating an illusory increase in signal-to-noise for a second peak in the distance modulus distribution.