Micro-arcsecond Astrometry with the VLBA

Recent breakthroughs in astrometric techniques with the VLBA have enabled parallax and proper motion accuracies of ~10 micro-arcsec and ~3 micro-arcsec/yr. I will present results of three astrometric programs:

1) the proper motion of Sgr A*, (the super-massive
black hole candidate at the center of the Milky
Way), which strongly constrains its mass and
the possibility of binary black holes in the
region;

2) the parallax and proper motion of the massive
star-forming region, W3OH, which resolves
a long-standing problem of the location of
the Perseus spiral arm, but raises questions
about spiral density waves; and

3) a direct measurement of the angular rotation
and proper motion of the galaxy M33, which
constrains models of our Local Group of galaxies.

Mark J. Reid