Testing stellar cusp formation theories with observations of the Milky Way nuclear star cluster

TL;DR

This study measures the stellar density profile of the Milky Way's nuclear star cluster, finding a flat, core-like distribution that challenges existing cusp formation theories near the black hole.

Contribution

It provides the first detailed observational analysis distinguishing between young and old stars to test stellar cusp formation models in the Galactic center.

Findings

Late-type stars show a flat density profile with a slope of -0.26±0.24.

Young stars are more than twice as numerous as old stars within the central arcsecond.

Results suggest the absence of a stellar cusp, favoring a core profile over cusp models.

Abstract

(Abridged) We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type (~ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young (~ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, is flat, with a projected power law slope of -0.26+-0.24. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories. Here, we also review the methods for further constraining the true three-dimensional radial profile using kinematic measurements. Precise acceleration measurements in the plane of the sky as well as along the line of sight has the potential to directly measure the density profile to establish whether there is a "hole" in the distribution of late-type stars in the inner 0.1 pc.