Hubble Space Telescope STIS spectroscopy of the optical outflow from DG Tauri:: Structure and kinematics on subarcsecond scales

We have carried out a spatio-kinematic study of the outflow from the classical T Tauri star DG Tau using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. A series of seven spatially offset long-slit spectra spaced by 0 ".07 were obtained along the axis of the outflow to build up a three-dimensional intensity-velocity "cube" in various forbidden emission lines (FELs) and Her. Here we present high spatial resolution synthetic lille images close to the star in distinct radial velocity intervals (from similar to+50 to similar to-450 km s(-1) in four bins, each similar to 125 km s(-1) wide). The lowest velocity emission is also examined in finer detail (from +60 to -70 km s(-1) in five bins similar to 25 km s(-1) wide). We have found that the highest velocity and most highly collimated component, i.e., the jet, can be traced from DG Tau to a distance D similar to 0 ".7. The jet is on the axis of a gear-shaped limb-brightened bubble which extends between 0 ".4 and 1 ".5 from the source and which we interpret as a bow shock. Other condensations are seen close to the star, indicating ongoing temporal variations in the flow. The low-velocity component of the outflow is found to be spatially wide close to the source (similar to 0 ".2 at D = 0 ".2), in contrast to the high-velocity jet (width less than or similar to 0 ".1). We have also found evidence to suggest that not only does the density in the outflow increase longitudinally with proximity to the source, but it also increases laterally toward the flow axis. Thus, at least in the case of DG Tan, the flow becomes gradually denser as it increases in velocity and becomes more collimated. Our observations show a continuous bracketing of the higher speed central flow within the lower speed, less collimated, broader flow, down to the lowest velocity scales. This suggests that the low- and high-velocity FELs in the highly active T Tauri star DG Tau are intimately related. Implications of these observations fur FEL models will be considered in a future paper.