Studies of the Galactic center have provided some important highlights of the first 15 years of VLA research. The Galactic center mini-spiral was discovered at the VLA in 1983. One of the most exciting and unexpected VLA discoveries of the early 1980's was the sensational discovery in 1984 of the linear arcs near Sgr A. The thermal arched filaments and the non-thermal arcs at 0.16 deg Galactic longitude are two of the most fascinating and enigmatic Galactic objects. At the center of all of this unusual activity is Sgr A*-the nearest AGN-which can be studied with linear resolutions of less than 0.01 pc.
There have been a number of recombination line studies of the thermal gas near the the Galactic center. A study of the H 92 line (3.6cm) was carried out in 1993 with an angular resolution of about . The velocity range of 450 km/s was barely sufficient to cover the full range of velocities. In addition there are some portions of the thermal sources that are optically thick at 3.6cm and thus cannot be imaged. With the enhanced VLA it will be possible to image recombination lines at 2cm, 1.3cm and 0.7cm with a resolution of several arc seconds over the full velocity range of 1000 km/s. The electron temperatures and kinematics of the thermal gas will be determined.
The current VLA correlator limits high frequency observations of the molecular material in the Galactic center region. Both radio recombination line and molecular lines require a velocity coverage of at least 1000 km/s, which is impossible with the current VLA correlator. The new correlator and improved high frequency receivers (especially 7mm) will allow mosaicing of the Galactic center region with many simultaneous lines over a large velocity range. It will be possible to determine the physical conditions such as temperature and density over a wide range of linear scales of the gas that appears to be feeding the black hole at the center of the Galaxy.
Scattering of radio waves by thermal electrons along the line of sight distorts the images of OH/IR stars in the Galactic center, as well as of Sgr A*. The scattering has been shown to be asymmetric and to be related to the location with respect to Sgr A. The A+ configuration will be optimal for studying this phenomenon as the OH sizes are often in the range 0.1 to .