Differences Between Emission and Absorption Tracers of Spatially Resolved Outflows in Clumpy z∼0.1 Star-forming Galaxies

Abstract We present spatially resolved Keck/LRIS spectroscopy of three clumpy star-forming galaxies at z ∼ 0.1, comparing outflow properties traced by Hα and Mg ii emission with those probed by Mg ii and Na i D absorption. Outflow velocities measured using Mg ii absorption (〈vout〉 = −560 ± 30 km s−1) are consistently higher than those traced by Hα emission (〈vout〉 = −124 ± 3 km s−1) across ∼5 kpc2 regions. Despite this offset, the correlation between vout and galaxy properties, such as SFR and ΣSFR, show similar slopes for both tracers, with Mg ii absorption systematically offset by ∼0.4 dex. In two galaxies, Mg ii emission is also detected, yielding velocities consistent with Hα. In one galaxy we also detect outflows in Na i D absorption and find similar velocities as Mg ii in absorption, which leads to a ∼0.4 dex higher Na i D outflow velocities compared to those measured in emission. Our spatially resolved results are consistent with those found for galactic-scale measurements, implying the outflow relationships are similar from the sales of ∼1-2 kpc to global measurements. Combined with literature measurements, these results suggest that the offset in velocities is driven not by ionisation state, but rather by the systematics associated to how absorption and emission measures trace the gas density.