We have obtained spatially resolved spectra of the z_em=3.911 triply imaged QSO APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We study the line of sight equivalent width (EW) differences and velocity shear of high and low ionization absorbers (including a damped Lyman alpha [DLA] system identified in a spatially unresolved ground based spectrum) in the three lines of sight. The combination of a particularly rich spectrum and three sight-lines allow us to study 27 intervening absorption systems over a redshift range 1.1 < z_abs < 3.8, probing proper transverse dimensions of 30 h₇₀^-1 pc up to 2.7 h₇₀^-1 kpc. We find that high ionization systems (primarily C IV absorbers) do not exhibit strong EW variations on scales <0.4 h₇₀^-1 kpc; their fractional EW differences are typically less than 30%. When combined with previous work on other QSO pairs, we find that the fractional variation increases steadily with separation out to at least ∼ 100 h₇₀^-1 kpc. Conversely, low ionization systems (primarily Mg II absorbers) show strong variations (often > 80%) over kpc scales. A minimum radius for strong (EW > 0.3 Å) Mg II systems of >1.4 h₇₀^-1 kpc is inferred from absorption coincidences in all lines of sight. For weak Mg II absorbers (EW < 0.3 Å), a maximum likelihood analysis indicates a most probable coherence scale of 2.0 h₇₀^-1 kpc for a uniform spherical geometry, with 95% confidence limits ranging between 1.5 and 4.4 h₇₀^-1 kpc. The weak Mg II absorbers may therefore represent a distinct population of smaller galaxies compared with the strong Mg II systems which we know to be associated with luminous galaxies whose halos extend over tens of kpc. Alternatively, the weak systems may reside in the outer parts of larger galaxies, where their filling factor may be lower. By cross-correlating spectra along different lines of sight, we infer shear velocities of typically less than 20 km s^-1 for both high and low ionization absorbers. Finally, for systems with weak absorption that can be confidently converted to column densities, we find constant N(C IV)/N(Si IV) across the three lines of sight. Similarly, the [Al/Fe] ratios in the z_abs = 2.974 DLA are consistent with solar relative abundances over a transverse distance of ∼ 0.35 h₇₀^-1 kpc.

Based on Hubble Space Telescope data from program 8626.