Differences between emission and absorption tracers of spatially resolved outflows in clumpy z ~ 0.1 star-forming galaxies

TL;DR

This study compares outflow velocities in star-forming galaxies using emission and absorption lines, revealing systematic differences driven by how these tracers relate to gas density.

Contribution

It provides spatially resolved evidence that velocity offsets between emission and absorption tracers are due to systematic effects, not ionisation state, across galaxy scales.

Findings

Mg II absorption shows higher outflow velocities than Hα emission.

Velocity correlations with galaxy properties are similar for both tracers.

Na I D absorption velocities are comparable to Mg II absorption velocities.

Abstract

We present spatially resolved Keck/LRIS spectroscopy of three clumpy star-forming galaxies at $z\sim0.1$, comparing outflow properties traced by H$\alpha$ and Mg II emission with those probed by Mg II and Na I D absorption. Outflow velocities measured using Mg II absorption ($\langle v_{\rm out} \rangle = -560 \pm 30$~\kms) are consistently higher than those traced by H$\alpha$ emission ($\langle v_{\rm out} \rangle = -124 \pm 3$~\kms) across $\sim$5 kpc$^{2}$ regions. Despite this offset, the correlation between $v_{\rm out}$ and galaxy properties, such as SFR and $\Sigma_{\rm SFR}$, show similar slopes for both tracers, with Mg II absorption systematically offset by $\sim 0.4$ dex. In two galaxies, Mg II emission is also detected, yielding velocities consistent with H$\alpha$. 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 $\sim$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 $\sim$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.