Dust in the Wind: Composition and Kinematics of Galaxy Outflows at the Peak Epoch of Star Formation

TL;DR

This study uses high-resolution spectra of gravitationally lensed galaxies at z=2-3 to analyze the composition, kinematics, and impact of galactic outflows on galaxy evolution and chemical enrichment.

Contribution

It provides detailed measurements of outflow properties, including velocities, mass loss rates, and chemical abundances, revealing their significant role in galaxy evolution at peak star formation epoch.

Findings

Outflows have typical velocities around -150 km/s.

Mass loss rates are comparable to or exceed star formation rates.

Outflows eject about half of the heavy element yield from star formation.

Abstract

Galactic-scale outflows regulate the stellar mass growth and chemical enrichment of galaxies, yet key outflow properties such as the chemical composition and mass loss rate remain largely unknown. We address these properties with Keck/ESI echellete spectra of nine gravitationally lensed z=2-3 star forming galaxies, probing a range of absorption transitions. Interstellar absorption in our sample is dominated by outflowing material, with typical velocities -150 km/s. Approximately 80% of the total column density is associated with a net outflow. Mass loss rates in the low ionization phase are comparable to or in excess of the star formation rate, with total outflow rates likely higher when accounting for ionized gas. Of order half of the heavy element yield from star formation is ejected in the low ionization phase, confirming that outflows play a critical role in regulating galaxy chemical evolution. Covering fractions vary and are in general non-uniform, with most galaxies having incomplete covering by the low ions across all velocities. Low ion abundance patterns show remarkably little scatter, revealing a distinct "chemical fingerprint" of outflows. Gas phase Si/Fe abundances are significantly super-solar ([Si/Fe]$\gtrsim$0.4) indicating a combination of $\alpha$-enhancement and dust depletion. Derived properties are comparable to the most kinematically broad, metal-rich, and depleted intergalactic absorption systems at similar redshifts, suggesting that these extreme systems are associated with galactic outflows at impact parameters conservatively within a few tens of kpc. We discuss implications of the abundance patterns in z=2-3 galaxies and the role of outflows at this epoch.