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%arXiv/0908.0311

\begin{document}

\title{High angular resolution integral-field spectroscopy of the 
Galaxy's nuclear cluster: a missing stellar cusp?}

\author{T. Do\altaffilmark{1}, A. M. Ghez\altaffilmark{1},  M. R. 
Morris\altaffilmark{1}, J. R. Lu\altaffilmark{2}, K. 
Matthews\altaffilmark{2}, S. Yelda\altaffilmark{1}, J. 
Larkin\altaffilmark{1}}
     Los Angeles, CA 90095-1547}

\email{tdo@astro.ucla.edu}

\begin{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 ($\sim$ 1 Gyr) 
stars, which are presumed to be dynamically relaxed, and the unrelaxed 
young ($\sim$ 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 (23 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$^\prime < 15.5$) within the central arcsecond.  Based on the 
late-type stars alone, the surface stellar number density profile, 
$\Sigma(R) \propto R^{-\Gamma}$, is flat, with $\Gamma = -0.27\pm0.19$. 
Monte Carlo simulations of the possible de-projected volume density 
profile, n(r) $\propto r^{-\gamma}$, show that $\gamma$ is less than 1.0 
at the 99.73 \% confidence level. 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, 
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.

\end{abstract}

\end{document}