Hubble Space Telescope Observations of the Stellar Distribution near Sgr A*

Abstract

Sgr A* is embedded within the nuclear cluster, which consists of a mixture of evolved and young populations of stars dominating the light over a wide range of angular scales. Here we present Hubble Space Telescope/NICMOS data to study the surface brightness distribution of stellar light within the inner 10'' of Sgr A* at 1.45 μm, 1.7 μm, and 1.9 μm. We use these data to independently examine the surface brightness distribution that had been measured previously with NICMOS and to determine whether there is a drop in the surface density of stars very near Sgr A*. Our analysis confirms that a previously reported drop in the surface brightness within 0farcs8 of Sgr A* is an artifact of bright and massive stars near that radius. We also show that the surface brightness profile within 5'' or ~0.2 pc of Sgr A* can be fitted with broken power laws. The power laws are consistent with previous measurements, in that the profile becomes shallower at small radii. For radii >0farcs7, the slope is β = -0.34 ± 0.04, where Σ is vpropr ^β and becomes flatter at smaller radii with β = -0.13 ± 0.04. Modeling of the surface brightness profile gives a stellar density that increases roughly as r ^-1 within the inner 1'' of Sgr A*. This slope confirms earlier measurements in that it is not consistent with that expected from an old, dynamically relaxed stellar cluster with a central supermassive black hole. Assuming that the diffuse emission is not contaminated by a faint population of young stars down to the 17.1 mag limit of our imaging data at 1.70μm, the shallow cusp profile is not consistent with a decline in stellar density in the inner arcsecond. In addition, converting our measured diffuse light profile to a stellar mass profile, with the assumption that the light is dominated by K0 dwarfs, the enclosed stellar mass within radius r <~ 0.1 pc of Sgr A* is ≈3.2 × 10⁴ M _⊙(r/0.1 pc)^2.1.