The morphology and luminosity function of the Galactic bulge from TMGS star counts

TL;DR

This study uses K-band star counts from the TMGS to analyze the Galactic bulge's morphology and luminosity function, revealing a triaxial shape with specific orientation and density distribution.

Contribution

It provides the first detailed triaxial model of the Galactic bulge based on star counts, including its shape, orientation, and luminosity function.

Findings

The bulge is triaxial with a 12° major axis angle.

Luminosity function shows a sharp decrease brighter than M_K=-8.0.

Stellar density follows a specific ellipsoidal distribution.

Abstract

The bulge of the Galaxy is analysed by inverting K-band star counts from the Two-Micron Galactic Survey in a number of off-plane regions. A total area of about 75 square degrees of sky is analysed. Assuming a non-variable luminosity function within the bulge, we derive the top end of the K-band luminosity function and the stellar density function, whose morphology is fitted to triaxial ellipsoids. The luminosity function shows a sharp decrease brighter than $M_K=-8.0$ when compared with the disc population. By fitting ellipsoids, we find that the bulge is triaxial with the major axis in the plane at an angle with line of sight to the Galactic centre of $12^\circ$ in the first quadrant. The axial ratios are $1:0.54:0.33$ and the distance of the Sun from the centre of the triaxial ellipsoid is 7860 pc. The best fit for the stellar density, assuming an ellipsoidal distribution, is $D(t)=1.17(t/2180)^{-1.8}\exp(- (t/2180)^{1.8}) stars pc^{-3}$, for $1300<t<3000$, where $t$ is the distance along the major axis of the ellipsoid in parsecs.