Probing Outflows in Z = 1~2 Galaxies Through Fe II/Fe II Multiplets

We report on a study of the 2300 − 2600°A Fe II/Fe II∗ multiplets in the rest–UV spectra of star forming galaxies at 1.0 \u3c z \u3c 2.6 as probes of galactic–scale outflows. We extracted a mass–limited sample of 97 galaxies at z _ 1.0 − 2.6 from ultra–deep spectra obtained during the GMASS spetroscopic survey in the GOODS South field with the VLT and FORS2. We obtain robust measures of the rest equivalent width of the Fe II absorption lines down to a limit of Wr \u3e 1.5°A and of the Fe II∗ emission lines to Wr \u3e 0.5°A. Whenever we can measure the systemic redshift of the galaxies from the [O II] emission line, we find that both the Fe II and Mg II absorption lines are blueshifted, indicative that both species trace gaseous outflows. We also find, however, that the Fe II gas has generally lower outflow velocity relative to that of Mg II. We investigate the variation of Fe II line profiles as a function of the radiative transfer properties of the lines, and find that transitions with higher oscillator strengths are more blueshifted in terms of both line centroids and line wings. We discuss the possibility that Fe II lines are suppressed by stellar absorptions. The lower velocities of the Fe II lines relative to the Mg II doublet, as well as the absence of spatially extended Fe II∗ emission in 2D stacked spectra, suggest that most clouds responsible for the Fe II absorption lie close (3 _ 4 kpc) to the disks of galaxies. We show that the Fe II/Fe II∗ multiplets offer unique probes of the kinematic structure of galactic outflows