Fabry-Perot Absorption Line Spectroscopy of the Galactic Bar. I. Kinematics

TL;DR

This study uses Fabry-Perot spectroscopy to analyze stellar velocities and metallicities in the Milky Way's bar, revealing kinematic structures, stellar streams, and metallicity gradients.

Contribution

It provides detailed velocity distributions and metallicity measurements of stars in the Galactic bar using Fabry-Perot absorption line spectroscopy, a novel application for this purpose.

Findings

Red clump giants have symmetric, flat-topped velocity distributions.

Detected stellar streams with >30 km/s velocity difference.

Metallicity gradient observed with higher metallicity in Baade's Window.

Abstract

We use Fabry-Perot absorption line imaging spectroscopy to measure radial velocities using the Ca II 8542 line in 3360 stars towards three lines of sight in the Milky Way's bar: Baade's Window and offset position at (l,b) ~ (+-5.0, -3.5). This sample includes 2488 bar red clump giants, 339 bar M/K-giants, and 318 disk main sequence stars. We measure the first four moments of the stellar velocity distribution of the red clump giants, and find it to be symmetric and flat-topped. We also measure the line-of-sight average velocity and dispersion of the red clump giants as a function of distance in the bar. We detect stellar streams at the near and far side of the bar with velocity difference > 30 km/s at l = +-5, but we do not detect two separate streams in Baade's Window. Our M-giants kinematics agree well with previous studies, but have dispersions systematically lower than those of the red clump giants by ~ 10 km/s. For the disk main sequence stars we measure a velocity dispersion of ~ 45 km/s for all three lines-of-sight, placing a majority of them in the thin disk within 3.5 kpc of the Sun, associated with the Sagittarius spiral arm. We measure the equivalent widths of the Ca II 8542 line that can be used to infer metallicities. We find indications of a metallicity gradient with Galactic longitude, with greater metallicity in Baade's Window. We find the bulge to be metal-rich, consistent with some previous studies.