LOW-IONIZATION LINE EMISSION FROM A STARBURST GALAXY: A NEW PROBE OF A GALACTIC-SCALE OUTFLOW

We study the kinematically narrow, low-ionization line emission from a bright, starburst galaxy at z = 0.69 using slit spectroscopy obtained with Keck/LRIS. The spectrum reveals strong absorption in Mg II and Fe II resonance transitions with Doppler shifts of -200 to -300 km s(-1), indicating a cool gas outflow. Emission in Mg II near and redward of systemic velocity, in concert with the observed absorption, yields a P-Cygni-like line profile similar to those observed in the Ly alpha transition in Lyman break galaxies. Further, the Mg II emission is spatially resolved and extends significantly beyond the emission from stars and H II regions within the galaxy. Assuming that the emission has a simple, symmetric surface brightness profile, we find that the gas extends to distances greater than or similar to 7 kpc. We also detect several narrow Fe II* fine-structure lines in emission near the systemic velocity, arising from energy levels that are radiatively excited directly from the ground state. We suggest that the Mg II and Fe II* emission is generated by photon scattering in the observed outflow and emphasize that this emission is a generic prediction of outflows. These observations provide the first direct constraints on the minimum spatial extent and morphology of the wind from a distant galaxy. Estimates of these parameters are crucial for understanding the impact of outflows in driving galaxy evolution.