Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies

TL;DR

This study uses cosmological simulations to show that UV metal line emission from high-redshift galaxy halos is highly time-variable, primarily driven by star formation bursts and energetic outflows, with implications for observational detection.

Contribution

It demonstrates the strong time variability of UV metal line emission in high-redshift galaxy halos and links emission peaks to starburst-driven outflows, using FIRE simulations.

Findings

UV metal line emission is strongly time-variable with up to 2 dex variation.

Emission peaks follow star formation bursts and energetic outflows.

Spatially-extended emission should be detectable with current instruments.

Abstract

We use cosmological simulations from the Feedback In Realistic Environments (FIRE) project, which implement a comprehensive set of stellar feedback processes, to study ultra-violet (UV) metal line emission from the circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of Lyman break galaxies. Of the transitions we analyze, the low-ionization C III (977 A) and Si III (1207 A) emission lines are the most luminous, with C IV (1548 A) and Si IV (1394 A) also showing interesting spatially-extended structures. The more massive halos are on average more UV-luminous. The UV metal line emission from galactic halos in our simulations arises primarily from collisionally ionized gas and is strongly time variable, with peak-to-trough variations of up to ~2 dex. The peaks of UV metal line luminosity correspond closely to massive and energetic mass outflow events, which follow bursts of star formation and inject sufficient energy into galactic halos to power the metal line emission. The strong time variability implies that even some relatively low-mass halos may be detectable. Conversely, flux-limited samples will be biased toward halos whose central galaxy has recently experienced a strong burst of star formation. Spatially-extended UV metal line emission around high-redshift galaxies should be detectable by current and upcoming integral field spectrographs such as the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web Imager (KCWI).