Cool Outflows in Galaxies and their Implications

Abstract

Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.

Properties of the Multiphase Outflows in Local (Ultra)luminous Infrared Galaxies

“…In recent years, galactic winds and outflows have been observed in several different gas phases, including in the hot highly-ionized (using X-ray spectroscopy, Tombesi et al (2013); Nardini et al (2015)), the warm ionized (Cano-Diaz et al 2012;Bellocchi et al 2013;Arribas et al 2014;Harrison et al 2014;Zakamska & Greene 2014;Carniani et al 2015Carniani et al , 2016Cicone et al 2016;Concas et al 2017;Rupke et al 2017;Gallagher et al 2019), the neutral atomic phase through absorption in the sodium doublet (Cazzoli et al 2016;Perna et al 2017;Concas et al 2019;Roberts-Borsani & Saintonge 2019), [CII] emission (Maiolino et al 2012;Cicone et al 2015;Janssen et al 2016;Bischetti et al 2019b) or in HI absorption (Morganti et al 2016) and the molecular gas phase (Feruglio et al 2010;Sturm et al 2011;Cicone et al 2014;González-Alfonso et al 2017;Fluetsch et al 2019;Jones et al 2019;Veilleux et al 2020). These measurements trace outflows at different spatial scales (from sub-pc in the X-ray to tens of kpc in the cold molecular and ionized phase) and in different physical conditions (e.g.…”

The observable properties of cool winds from galaxies, AGN, and star clusters -- II. 3D models for the multiphase wind of M82

“…This paper marked the discovery of galactic winds, and shows that galaxies are not "isolated islands" as had been thought in the eighteenth century (Wright 1750). Modern observations reveal that galactic winds are ubiquitous across nearly all starburst galaxies and active galactic nuclei (AGN), at both low and high redshift (Veilleux et al 2005(Veilleux et al , 2020Heckman & Thompson 2017), and these winds typically contain multiple phases within them. The hot phase, characterised by high temperature (∼ 10 8 K) and speed, is widely observed in X-ray emission, and carries most of the energy budget in galaxies with strong outflows.…”

MusE GAs FLOw and Wind V. The dust/metallicity-anisotropy of the circum-galactic medium

“…In the ΛCDM cosmological context, supernovae(SN)-driven outflows (Dekel & Silk 1986) are thought to play a major role in galaxy formation given that galactic winds are ubiquitous and necessary to account for the observed mass-metallicity relation. They could eject a significant fraction of the baryons and play a key role in enriching the inter-galactic medium (Veilleux et al 2005(Veilleux et al , 2020Heckman & Thompson 2017). SN-driven outflows and gas inflows usually have rather low particle densities and are hence difficult to observe.…”