2012ApJ...759...26E

We study large-scale outflows in a sample of 96 star-forming galaxies at 1 ≲ z ≲ 2, using near-UV spectroscopy of Fe II and Mg II absorption and emission. The average blueshift of the Fe II interstellar absorption lines with respect to the systemic velocity is -85±10 km/s at z ∼ 1.5, with standard deviation 87 km/s; this is a decrease of a factor of two from the average blueshift measured for far-UV interstellar absorption lines in similarly selected galaxies at z ∼ 2. The profiles of the Mg II λλ2796, 2803 lines show much more variety than the Fe II profiles, which are always seen in absorption; Mg II ranges from strong emission to pure absorption, with emission more common in galaxies with blue UV slopes and at lower stellar masses. Outflow velocities, as traced by the centroids and maximum extent of the absorption lines, increase with increasing stellar mass with 2σ-3σ significance, in agreement with previous results. We study fine structure emission from Fe II*, finding several lines of evidence in support of the model in which this emission is generated by the re-emission of continuum photons absorbed in the Fe II resonance transitions in outflowing gas. In contrast, photoionization models indicate that Mg II emission arises from the resonant scattering of photons produced in H II regions, accounting for the differing profiles of the Mg II and Fe II lines. A comparison of the strengths of the Fe II absorption and Fe II* emission lines indicates that massive galaxies have more extended outflows and/or greater extinction, while two-dimensional composite spectra indicate that emission from the outflow is stronger at a radius of ∼10 kpc in high-mass galaxies than in low-mass galaxies.