The inner Galactic bulge: evidence for a nuclear bar?

TL;DR

This study uses N-body simulations to analyze the structure of the Galactic bulge, providing evidence against the presence of a secondary nuclear bar and supporting a disk-origin for the bulge.

Contribution

The paper demonstrates that the observed bulge features can be explained by a single bar structure, challenging the idea of a nuclear bar in the Milky Way.

Findings

Simulation matches bulge longitude profiles for red clump stars.

Transition from elongated to axisymmetric contours explains slope change.

Nuclear star count map shows asymmetry similar to observations.

Abstract

Recent data from the VVV survey have strengthened evidence for a structural change in the Galactic bulge inwards of |l|<=4 deg. Here we show with an N-body barred galaxy simulation that a boxy bulge formed through the bar and buckling instabilities effortlessly matches measured bulge longitude profiles for red clump stars. The same simulation snapshot was earlier used to clarify the apparent boxy bulge - long bar dichotomy, for the same orientation and scaling. The change in the slope of the model longitude profiles in the inner few degrees is caused by a transition from highly elongated to more nearly axisymmetric isodensity contours in the inner boxy bulge. This transition is confined to a few degrees from the Galactic plane, thus the change of slope is predicted to disappear at higher Galactic latitudes. We also show that the nuclear star count map derived from this simulation snapshot displays a longitudinal asymmetry similar to that observed in the 2MASS data, but is less flattened to the Galactic plane than the 2MASS map. These results support the interpretation that the Galactic bulge originated from disk evolution, and question the evidence advanced from star count data for the existence of a secondary nuclear bar in the Milky Way.