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

TL;DR

This study uses high-resolution VVV survey data to map the Galactic bar's structure via red clump stars, revealing the influence of spiral arms and emphasizing the importance of accounting for these features in Galactic models.

Contribution

It provides a detailed reddening map with twice the resolution of previous maps and clarifies the nature of the secondary peak in the luminosity function as spiral arm over-density, not just bulge features.

Findings

Secondary peak linked to spiral arm over-density

Reddening maps with doubled spatial resolution

Implications for bulge red-giant branch studies

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 V\'ia L\'actea survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes $-10^{\circ}<l<10^{\circ}$ and latitudes $-1.5^{\circ}<b<1.5^{\circ}$. We use these de-reddened catalogues to construct the $K_{s}$ 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 characterise the bulge red-giant branch bump should avoid low galactic latitudes ($|b|< 2^{\circ}$), 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