A first glimpse at the line-of-sight structure of the Milky Way's nuclear stellar disc

TL;DR

This study uses red clump stars to analyze the kinematics, distance, and extinction of the Milky Way's nuclear stellar disc, revealing its axisymmetric structure and low internal extinction.

Contribution

It provides the first detailed kinematic and structural analysis of the NSD using proper motions and photometry of red clump stars, clarifying its morphology and extinction properties.

Findings

NSD stars show eastward and westward rotation with some contamination from bulge/bar stars.

Westward-moving stars are approximately 300 pc beyond eastward stars, indicating a similar line-of-sight structure.

Extinction within the NSD is low, less than 10% of total extinction for the farthest stars.

Abstract

The nuclear stellar disc (NSD) is a dense stellar structure at the centre of our Galaxy. Given its proximity, it constitutes a unique laboratory to understand other galactic nuclei. Nevertheless, the high crowding and extinction hamper its study, and even its morphology and kinematics are not yet totally clear. In this work we use NSD red clump stars, whose intrinsic properties are well known, to trace the kinematics of the NSD and to compute the distance and extinction towards the edges of the NSD. We used publicly available proper motion and photometric catalogues of the NSD to distinguish red clump stars by using a colour-magnitude diagram. We then applied a Gaussian mixture model to obtain the proper motion distribution, and computed the extinction and distance towards stars with different kinematics. We obtained that the proper motion distributions contain NSD stars rotating eastwards and westwards, plus some contamination from Galactic bulge/bar stars, in agreement with previous work. We computed the distance and extinction towards the eastward- and westward-moving stars and concluded that the latter are $\sim300$ pc beyond, indicating a similar structure along and across the line of sight, and consistent with an axisymmetric structure of the NSD. Moreover, we found that the extinction within the NSD is relatively low and accounts for less than 10 % of the total extinction of the stars belonging to the farthest edge of the NSD.