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Session 59 - Milky Way, Center & Halo.
Oral session, Tuesday, January 16
Salon del Rey Central, Hilton
We present the first mid-infrared detection of a source coincident with black hole candidate Sgr A* in a deep image of the inner parsec of the Galaxy. The observations were obtained with SpectroCam-10 at the 200" Hale telescope at Palomar using a 1-\mum-wide filter centered at 8.7 \mum. The position of the infrared source agrees to within 0.25'' (one pixel) of the expected location of Sgr A* as determined by the radio offset from IRS 7, a supergiant observed in both the radio and infrared. Sgr A* is located on a steep gradient of warm dust emission. Recent near infrared obervations and accretion disk models indicate that Sgr A* is bluer than most of the other sources that are observable in the mid infrared. The wavelength was thus chosen in order to maximize the contrast between the expected flux from Sgr A* and the strong local diffuse dust emission, which is of order 500 mJy/(arcsec)^2 at 8.7 \mum. The source was detected on two independent observing runs in 1994 using different techniques: in June short (1 second) exposures were taken, and in July longer (10 second) exposures were taken with the aid of a tip-tilt adaptive optics secondary. The noise levels in the mosaiced raw images are less than 2 mJy/ (arcsec)^2 and the spatial resolution is 0.65''. A source at Sgr A* is weakly apparent in the raw mosaics, but is more evident after deconvolving to the diffraction-limited spatial resolution of 0.45''. The results of several different deconvolution techniques were similar, both in terms of flux and morphology. The mid-infrared source appears to be superposed on a thin ridge of emission oriented in the NW-SE direction. We estimate the flux above this ridge to be 25 mJy, based on a variety of methods. Extinction-corrected flux estimates of this source indicate that it is significantly brighter than predictions from accretion disk models for Sgr A*. It is likely that warm dust at the Galactic Center contributes to the 8.7 \mum emission.
Program listing for Tuesday