Imaging the Black Hole in the Center of the Galaxy: ALMA and submm-VLBI

Sheperd Doeleman
MIT Haystack Observatory, Off Route 40, Westford, MA. 01886 
sdoeleman@haystack.mit.edu

There is now abundant evidence that the compact radio source SgrA* marks the
position of a massive black hole at the center of the Galaxy.  Measured
stellar orbits, proper motion studies, detection of polarized emission,
identification of IR and X-ray flares, and sizes derived from Very Long
Baseline Interferometry (VLBI) effectively rule out all but the most exotic
and improbable black hole alternatives.  What remains missing is the detection
of an unambiguous 'event-horizon' signature and, once found, the means to
study strong GR and accretion/outflow effects in the *immediate* environment
of a massive black hole.
                                                                                                  
With its combination of large collecting area, mm/submm frequency range and
location in the Southern Hemisphere, ALMA will be the essential element of a
high frequency VLBI array capable of making detailed images of SgrA*.  Use of
new VLBI recording systems with bandwidths of 4 Giga-bits/second and higher
will further boost the sensitivity of this VLBI array.  Initial VLBI
observations at 230 GHz with resolutions of 35 micro arc seconds (5
Schwarzschild radii for SgrA*) could be carried out using only a small number
of ALMA dishes phased together.  A fully phased ALMA array with all dishes will
enable even higher frequency VLBI observations with resolutions at the 3
Schwarzschild radii level.  Strong GR effects, including the 'shadow' expected
to be observed as a result of light bending near the event horizon, will be
targets of this VLBI array.
                                                                                                  
Building VLBI requirements into ALMA design, including the ability to phase and
sum signals over the entire ALMA array, can make this Galactic Center project a
unique early science result for ALMA.  We discuss the scientific objectives,
technical requirements, and capabilities of a submm VLBI array anchored by
ALMA.