The principal focus of the SMA for the study of the Galactic Center region is to investigate the structure, time variability and polarization of the Sgr A*, the compact radio source associated with the black hole in center of our galaxy.

Precise astrometric observations over the past 15 years at near infrared wavelength observations of stars orbiting Sgr A* show there is a central mass of 3.6 x 10⁷ M_⊙ within a volume of 10^-7 pc³. Since the lifetime of any stellar cluster of this density is rather short, the central mass is most probably a black hole.

Direct evidence of an accretion envelope around this black hole can be obtained by measurement of the rotation of the plane of polarization of a linearly polarized wave traversing the envelope due to the effect of Faraday rotation. The rotation is proportional to the product of the longitudinal magnetic field strength, the electron density, the path length and the wavelength squared. The SMA was first instrument to definitively measure the Faraday rotation of Sgr A* from observations at different wavelengths simultaneously. The so called rotation measure was found to be 5 x 10⁵ rad m^-2. Based on equipartition of energy arguments and other assumptions, the accretion rate corresponding to this rotation measure is about
10^-8 M_⊙ yr^-1. This low accretion rate supports the idea that the black hole is in a quiescent period with highly sub-Eddington luminosity.

The emission fluctuates at all wavelengths, and coordinated observations have been made with the SMA and telescopes at longer radio, IR and X-Ray wavelengths. Some flare "events" have been observed, which provide limits on expanding plasmoid models of synchrotron emission and the SSC radiation mechanism.

The SMA shows that although the rotation measure of the accretion envelope stays relatively constant, the polarization of the Sgr A* radio source fluctuates on a sub-hourly basis. Some quasi-periodic events have been identified, which may be caused by transient emission regions in the inner accretion envelope near the last stable orbit, or in a precessing jet.

The SMA and JCMT have been linked to form a VLBI station at 230 GHz in a network that includes telescopes on Mauna Kea, Mt Hopkins, Arizona, and Cedar Flat, CA. Observations in 2007 have definitively resolved the intrinsic emission region of Sgr A*. Its diameter is 37 μas (3.7 R_s, where R_s is the Schwarzschild radius of 1 x 10¹² cm) based on a Gaussian source model. The data are also consistent with a thick annulus model with inner and outer diameters of 35 and 80 μas, respectively. If this model is correct, it provides the first indication of the existence of the predicted black hole "shadow." Future observations with higher sensitivity and additional stations, e.g. Mexico (LMT) or Chile (ASTE), will improve the VLBI imaging capabliity dramatically.

CfA:

Jim Moran, Diego Munoz, Jun-Hui Zhao, Jonathan Weintroub, Hua Bai Li

ASIAA:

Ram Rao

On-going collaborators

Dan Marrone, Shepard Doeleman, Alan Rogers

Ray Blundell