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M. Morris, A.M. Tanner, A.M. Ghez (UCLA), E.E. Becklin (UCLA,USRA), A. Cotera (UAz), M.W. Werner, M.E. Ressler (JPL)
Recent proper motion studies show that the compact radio source Sgr A* is coincident to within 0.01 pc with a concentration of ~3 x 106 M\sun of dark matter, thus cementing the notion that this source corresponds to an accreting supermassive black hole. A variety of emission mechanisms have been proposed to account for the radio and X-ray emission from SgrA* in the context of an accreting black hole model. The details of the spectrum provide essential clues for assessing and constraining the proposed mechanisms. When compared with an extrapolation of measured radio/submm fluxes to higher frequencies, existing upper limits in the mid- and far-infrared, after correction for extinction, indicate a spectral turnover between 10 and 300 microns. This could be indicative of a cutoff in the electron energy distribution, or to a synchrotron cutoff. The luminosity of Sgr A* depends strongly on where this cutoff occurs. We have used the MIRLIN camera on the Keck Telescope to obtain diffraction-limited imaging of Sgr A* at 8.8, 12.5, 20.8, and 24.5 microns in order to improve on previous limits and thereby constrain the spectrum and luminosity of SgrA*. The limit we are able to achieve is determined not by instrumental sensitivity, but by the spatial fluctuations in the local dust emission background. As suggested by previous observations, Sgr A* sits on a ridge of weak emission. There is no evidence for (or against) an association of this ridge with the radio source. The flux limits for a point source coincident with Sgr A* will be presented, and compared to existing models for the spectrum of Sgr A*.