Non-Parametric Density Reconstruction of the Galactic Bulge Area using Red Clump Stars in the VVV Survey

Abstract

Studies of the red clump giant population in the inner Milky Way suggest the Galactic bulge/bar has a boxy/peanut/X-shaped structure as predicted by its formation via a disc buckling instability. We used a non-parametric method of estimating the Galactic bulge morphology that is based on maximum entropy regularisation. This enabled us to extract the three-dimensional distribution of the red giant stars in the bulge from deep photometric catalogues of the VISTA Variables in the Via Lactea (VVV) survey. Our high-resolution reconstruction confirms the well-known boxy/peanut/X-shaped structure of the bulge. We also find spiral arm structures that extend to around three kpc in front of and behind the bulge and are on different sides of the bulge major axis. We show that the detection of these structures is robust to the uncertainties in the luminosity function.