A case against an X-shaped structure in the Milky Way young bulge

TL;DR

This study investigates the presence of an X-shaped structure in the Milky Way's bulge using young stellar populations, finding no evidence of such a structure contrary to previous claims based on red clump stars.

Contribution

It provides a novel analysis using young main-sequence stars to test the X-shape hypothesis, offering an alternative perspective to prior red clump star studies.

Findings

No double peak in stellar density along lines of sight.

Results challenge the existence of an X-shaped bulge in young stars.

Photometric uncertainties are insufficient to explain the lack of X-shape evidence.

Abstract

CONTEXT. A number of recent papers have claimed the discovery of an X-shape structure in the bulge of our Galaxy in the population of the red clumps. AIMS. We endeavor to analyze the stellar density of bulge stars in the same regions using a different stellar population that is characteristic of the young bulge ($\lesssim 5$ Gyr). Particularly, we use F0-F5 main-sequence stars with distances derived through photometric parallax. METHODS. We extract these stars from extinction-corrected color-magnitude diagrams in the near-infrared of VISTA-VVV data in some bulge regions and calculate the densities along the line of sight. We take the uncertaintity in the photometric parallax and the contamination of other sources into account, and we see that these errors do not avoid the detection of a possible double peak along some lines of sight as expected for a X-shape bulge if it existed. RESULTS. Only a single peak in the density distribution along the line of sight is observed, so apparently there is no X-shape structure for this population of stars. Nonetheless, the effects of the dispersion of absolute magnitudes in the selected population might be an alternative explanation, although in principle these effects are insufficient to explain this lack of double peak according to our calculations. CONCLUSIONS. The results of the present paper do not demonstrate that previous claims of X-shaped bulge using only red clump stars are incorrect, but there are apparently some puzzling questions if we want to maintain the validity of both the red-clump results and the results of this paper.