On the correlation between metallicity and the X-shaped morphology of the Milky Way bulge

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 metalpoor 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 varies 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. Secondly, 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 per cent 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 per cent, 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.