The structure behind the Galactic bar traced by red clump stars in the VVV survey

Abstract

ABSTRACT Red clump stars are commonly used to map the reddening and morphology of the inner regions of the Milky Way. We use the new photometric catalogues of the VISTA Variables in the Vía Láctea survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes $\rm -10^{\circ }\lt l\lt 10^{\circ }$ and latitudes $\rm -1.5^{\circ }\lt b\lt 1.5^{\circ }$. We use these de-reddened catalogues to construct the $\rm K_{s}$ luminosity function around the red clump in the Galactic plane. We show that the secondary peak (fainter than the red clump) detected in these regions does not correspond to the bulge red-giant branch bump alone, as previously interpreted. Instead, this fainter clump corresponds largely to the over-density of red clump stars tracing the spiral arm structure behind the Galactic bar. This result suggests that studies aiming to characterize the bulge red-giant branch bump should avoid low galactic latitudes ($\rm |b|\lt 2^{\circ }$), where the background red clump population contributes significant contamination. It furthermore highlights the need to include this structural component in future modelling of the Galactic bar.