The Apache Point Observatory Galactic Evolution Experiment: First Detection of High Velocity Milky Way Bar Stars

TL;DR

This study presents the first detection of high-velocity stars associated with the Milky Way's bar using high-resolution near-infrared spectroscopy of thousands of bulge stars, revealing a significant high-velocity component.

Contribution

It provides the largest NIR high-resolution spectroscopic sample of bulge giants and identifies a high-velocity peak linked to the Galactic bar, a novel discovery in Galactic dynamics.

Findings

Detection of a high-velocity peak at ~+200 km/s in many bulge fields

High-velocity stars constitute about 10% of the sample in several fields

High velocities are consistent with stars orbiting in the Galactic bar potential

Abstract

Commissioning observations with the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, have produced radial velocities (RVs) for ~4700 K/M-giant stars in the Milky Way bulge. These high-resolution (R \sim 22,500), high-S/N (>100 per resolution element), near-infrared (1.51-1.70 um; NIR) spectra provide accurate RVs (epsilon_v~0.2 km/s) for the sample of stars in 18 Galactic bulge fields spanning -1<l<20 deg, |b|<20 deg, and dec>-32 deg. This represents the largest NIR high-resolution spectroscopic sample of giant stars ever assembled in this region of the Galaxy. A cold (sigma_v~30 km/s), high-velocity peak (V_GSR \sim +200 km/s) is found to comprise a significant fraction (~10%) of stars in many of these fields. These high RVs have not been detected in previous MW surveys and are not expected for a simple, circularly rotating disk. Preliminary distance estimates rule out an origin from the background Sagittarius tidal stream or a new stream in the MW disk. Comparison to various Galactic models suggests that these high RVs are best explained by stars in orbits of the Galactic bar potential, although some observational features remain unexplained.