A2A: 21,000 bulge stars from the ARGOS survey with stellar parameters on the APOGEE scale

TL;DR

This study calibrates ARGOS bulge survey data onto the APOGEE scale using The Cannon, revealing detailed chemical and kinematic structures of the Galactic bulge that suggest different disk origins for various stellar populations.

Contribution

It introduces a data-driven calibration method to unify ARGOS and APOGEE surveys, enabling detailed analysis of bulge stellar populations and their origins.

Findings

X-shaped abundance distributions indicate disk origin.

Inner bulge shows flattened gradients near the plane.

Distinct kinematic structures for different metallicity groups.

Abstract

We use the data-driven method, The Cannon, to bring 21,000 stars from the ARGOS bulge survey, including 10,000 red clump stars, onto the parameter and abundance scales of the cross-Galactic survey, APOGEE, obtaining rms precisions of 0.10 dex, 0.07 dex, 74 K, and 0.18 dex for [Fe/H], [Mg/Fe], Teff, and log(g), respectively. The re-calibrated ARGOS survey - which we refer to as the A2A survey - is combined with the APOGEE survey to investigate the abundance structure of the Galactic bulge. We find X-shaped [Fe/H] and [Mg/Fe] distributions in the bulge that are more pinched than the bulge density, a signature of its disk origin. The mean abundance along the major axis of the bar varies such that the stars are more [Fe/H]-poor and [Mg/Fe]-rich near the Galactic center than in the long bar/outer bulge region. The vertical [Fe/H] and [Mg/Fe] gradients vary between the inner bulge and long bar with the inner bulge showing a flattening near the plane that is absent in the long bar. The [Fe/H]-[Mg/Fe] distribution shows two main maxima, an ``[Fe/H]-poor [Mg/Fe]- rich'' maximum and an ``[Fe/H]-rich [Mg/Fe]-poor'' maximum, that vary in strength with position in the bulge. In particular, the outer long bar close to the Galactic plane is dominated by super-solar [Fe/H], [Mg/Fe]-normal stars. Stars composing the [Fe/H]-rich maximum show little kinematic dependence on [Fe/H], but for lower [Fe/H] the rotation and dispersion of the bulge increase slowly. Stars with [Fe/H]<-1 dex have a very different kinematic structure than stars with higher [Fe/H]. Comparing with recent models for the Galactic boxy-peanut bulge, the abundance gradients and distribution, and the relation between [Fe/H] and kinematics suggest that the stars comprising each maximum have separate disk origins with the ``[Fe/H]-poor [Mg/Fe]-rich'' stars originating from a thicker disk than the ``[Fe/H]-rich [Mg/Fe]-poor'' stars.