Predicted Stellar Kinematics of a Kiloparsec-Scale Nuclear Disc (or Ring) in the Milky Way

TL;DR

This paper predicts the kinematic signatures of a kiloparsec-scale nuclear disc or ring in the Milky Way, focusing on line-of-sight velocities and proper motions to distinguish between these structures and their age implications.

Contribution

It provides new predictions for the kinematic properties of nuclear discs and rings, including proper motion signatures, and discusses their longevity and presence in external galaxies.

Findings

Nuclear discs produce distinctive LOSVD peaks at positive and negative longitudes.

Proper motion signatures can differentiate between nuclear discs and rings.

Old nuclear discs of similar size are observed in external galaxies.

Abstract

In Debattista et al. (2015), we proposed that a kiloparsec-scale nuclear disc is responsible for the high-velocity secondary peak in the stellar line-of-sight velocity distributions (LOSVDs) seen at positive longitudes in the bulge by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Here, we make further qualitative but distinctive predictions of the kinematic properties of a nuclear disc, including for the LOSVDs at negative longitudes (which APOGEE-2 will observe) and examine the proper motions throughout the disc. Since a nuclear ring is also able to produce similar high-velocity LOSVD peaks, we present predictions for the proper motion signatures which distinguish between a nuclear disc and a nuclear ring. We also demonstrate that the stars in a nuclear disc, which would be on x2 orbits perpendicular to the bar, can remain on these orbits for a long time and can therefore be old. We show that such (old) nuclear discs of comparable size exist in external galaxies.