Chemically-dissected rotation curves of the Galactic Bulge from Main Sequence proper motions

TL;DR

This study uses Hubble Space Telescope proper motions and photometric indices to analyze the rotation curves of the Galactic bulge for metal-rich and metal-poor Main Sequence stars, revealing significant differences that inform models of Galactic evolution.

Contribution

It introduces a new method combining proper motions and photometric indices to study Galactic bulge kinematics using Main Sequence stars, complementing traditional giant branch tracers.

Findings

Metal-rich and metal-poor samples show discrepant rotation curves.

Metal-rich stars exhibit greater rotation amplitude and steeper gradients.

Instrumental effects are unlikely to explain the observed differences.

Abstract

We report results from an exploratory study implementing a new probe of Galactic evolution using archival Hubble Space Telescope imaging observations. Precise proper motions are combined with photometric relative metallicity and temperature indices, to produce the proper motion rotation curves of the Galactic bulge separately for metal-poor and metal-rich Main Sequence samples. This provides a "pencil-beam" complement to large-scale wide-field surveys, which to-date have focused on the more traditional bright Giant Branch tracers. We find strong evidence that the Galactic bulge rotation curves drawn from "Metal-rich" and "Metal-poor" samples are indeed discrepant. The "Metal-rich" sample shows greater rotation amplitude and a steeper gradient against line of sight distance, as possibly a stronger central concentration along the line of sight. This may represent a new detection of differing orbital anisotropy between metal-rich and metal-poor bulge objects. We also investigate selection effects that would be implied for the longitudinal proper motion cut often used to isolate a "pure-bulge" sample. Extensive investigation of synthetic stellar populations suggest that instrumental and observational artifacts are unlikely to account for the observed rotation curve differences. Thus, proper motion-based rotation curves can be used to probe chemo-dynamical correlations for Main Sequence tracer stars, which are orders of magnitude more numerous in the Galactic Bulge than the bright Giant Branch tracers. We discuss briefly the prospect of using this new tool to constrain detailed models of Galactic formation and evolution.