Starburst Galaxies: Outflows of Metals and Energy into the IGM

TL;DR

Starburst galaxies produce powerful outflows that significantly contribute to the enrichment and energy input of the Intergalactic Medium, especially through their hot X-ray emitting gas phase, which is crucial for understanding galaxy evolution.

Contribution

This paper highlights the importance of the hot X-ray emitting phase of superwinds and advocates for high-resolution X-ray spectroscopy to measure gas velocities and compositions.

Findings

Superwinds are common in high star formation rate galaxies.

Hot X-ray gas contains most of the energy and metals in outflows.

Future X-ray missions can measure gas velocities with ~100 km/s accuracy.

Abstract

What is the contribution of mass, metals and energy from starburst galaxies to the Intergalactic Medium? Starburst galaxies drive galactic-scale outflows or "superwinds" that may be responsible for removing metals from galaxies and polluting the Intergalactic Medium (IGM). In the last decade tremendous progress was made in mapping cool entrained gas in superwinds through UV/optical imaging and absorption line spectroscopy. These studies demonstrated that superwinds are ubiquitous in galaxies forming stars at high surface densities and that the most powerful starbursts can drive outflows near escape velocity. Theoretical models of galaxy evolution have begun to incorporate superwinds, using various ad-hoc prescriptions based on our knowledge of the cool gas. However, these efforts are fundamentally impeded by our lack of information about the hot phase of these outflows. The hot X-ray emitting phase of a superwind contains the majority of its energy and newly-synthesized metals, and given its high specific energy and inefficient cooling it is also the component most likely escape from the galaxy's gravitational potential well. Knowledge of the chemical composition and velocity of the hot gas are crucial to assess the energy and chemical feedback from a starburst. A high priority for the next decade is to enable direct measurements to be made of the rates at which starburst galaxies of all masses eject gas, metals, and energy into the IGM. This will require a high sensitivity X-ray imaging spectrometer capable of measuring velocities in faint diffuse X-ray emission with a velocity accuracy of ~ 100 km/s. Such spectral resolution automatically allows detailed line-based plasma diagnostics, and thus composition, energetics and flow rates can be derived.