ApJ, 2009, 703, 1323

High Angular Resolution Integral-Field Spectroscopy of the Galaxy's Nuclear Cluster: A Missing Stellar Cusp?

T. Do, A. M. Ghez, M. R. Morris, J. R. Lu, K. Matthews, S. Yelda, J. Larkin

Abstract

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. In the survey region, we classified 60 stars as early-type (22 newly identified) and 74 stars as late-type (61 newly identified). 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, Σ(R) ~ R^(-Γ), is flat, with Γ = -0.27 +- 0.19. Monte Carlo simulations of the possible de-projected volume density profile, n(r) ~ -γ, show that γ is less than 1.0 at the 99.7% confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than that predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey—stellar collisions, mass segregation from stellar remnants, or a recent merger event—mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions.

Figure Caption

(a): Surface number density vs. projected distance from Sgr A* in the plane of the sky for different populations, old (late type, red), young (early type, blue) and total number counts from K' imaging.
(b, Left): Broken power-law density profiles with break radius, rbreak = 8'' and outer power law γ2 = 2.0, and varying inner power laws γ1. (b, Right): The projected surface number density profile of each of the broken power laws. The fitted inner surface density power law Γ is flat for γ1 <= 0.5.