Spectroscopic Observations of Obscured Populations in the Inner Galaxy: 2MASS-GC02, Terzan 4, and the 200 km/s stellar peak

TL;DR

This study uses spectroscopic data to analyze obscured stellar populations in the inner Galaxy, revealing detailed chemical compositions of clusters and challenging the existence of a proposed high-velocity stellar structure.

Contribution

First chemical abundance analysis of stars in the poorly studied clusters 2MASS-GC02 and Terzan 4, and investigation of the high-velocity stellar peak in the Galactic bulge.

Findings

Confirmed multiple populations in metal-rich clusters.

Revised radial velocity for 2MASS-GC02 significantly different from previous data.

No evidence found for the proposed high-velocity nuclear disk or ring.

Abstract

The interpretation of potentially new and already known stellar structures located at low-latitudes is hindered by the presence of dense gas and dust, as observations toward these sight-lines are limited. We have identified APOGEE stars belonging to the low-latitude globular clusters 2MASS-GC02 and Terzan 4, presenting the first chemical element abundances of stars residing in these poorly studied clusters. As expected, the signature of multiple populations co-existing in these metal-rich clusters is evident. We redetermine the radial velocity of 2MASS-GC02 to be -87 +- 7 km/s, finding that this cluster's heliocentric radial velocity is offset by more than 150 km/s from the literature value. We investigate a potentially new low-latitude stellar structure, a kiloparsec-scale nuclear disk (or ring) which has been put forward to explain a high-velocity (V_{GSR} ~200 km/s) peak reported in several Galactic bulge fields based on the APOGEE commissioning observations. New radial velocities of field stars at (l,b)=(-6,0) are presented and combined with the APOGEE observations at negative longitudes to carry out this search. Unfortunately no prominent -200 km/s peak at negative longitudes along the plane of the Milky Way is apparent, as predicted for the signature of a nuclear feature. The distances and Gaia EDR3 proper motions of the high-V_{GSR} stars do not support the current models of stars on bar-supporting orbits as an explanation of the +200 km/s peak.