Physics – Optics
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21531004d&link_type=abstract
American Astronomical Society, AAS Meeting #215, #310.04; Bulletin of the American Astronomical Society, Vol. 42, p.308
Physics
Optics
Scientific paper
I 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 and kinematics 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. Based on the late-type stars alone, the surface stellar number density profile is consistent with being flat. We place constraints on the volume density profile using Monte Carlo simulations of the de-projected surface density profile; these simulations show that the power law index of the volume density (n(r)) is less than 1.0 at the 99.7% confidence level. Using 3D velocity and acceleration measurements, we are able to further constrain the true physical distribution of these stars. 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.
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