Spatially Resolved Spectroscopy of APM08279+5255

We propose to obtain spatially resolved, high resolution spectra of the z=3.911 BAL quasar, APM 08279+5255. This ultra-luminous QSO is a triply imaged gravitationally lensed system with image separations of 0.38 -- 0.15 arcsec. Ground- based observations of this bright source have revealed a rich absorption spectrum caused by both intervening material and the complex QSO environment and BAL flows. The proposed STIS spectrum, which will spatially resolve the individual quasar images, is a unique opportunity to probe the numerous intervening systems on sub-kpc scales, providing: 1} a sensitive probe of the structure of intervening galaxy halos and metal line systems on scales of ~ 0.2 -- 1.6 kpc h^-1, their kinematics and spatial extents, 2} multiple sightlines through the complex BAL flow on parsec scales, yielding information on ionization, kinematics and metal enrichment, We also request 5 orbits with WFPC2 in order to detect and measure the centroid of the lensing galaxy; this will provide the necessary information to remove the degeneracy in the present gravitational lensing models.