Gas Accretion via Condensation and Fountains

Abstract

For most of their lives, galaxies are surrounded by large and massive coronae of hot gas, which constitute vast reservoirs for gas accretion. This Chapter describes a mechanism that allows star-forming disc galaxies to extract gas from their coronae. Stellar feedback powers a continuous circulation (galactic fountain) of gas from the disc into the halo, producing mixing between metal-rich disc material and metal-poor coronal gas. This mixing causes a dramatic reduction of the cooling time of the corona making it condense and accrete onto the disc. This fountain- driven accretion model makes clear predictions for the kinematics of the extraplanar cold/warm gas in disc galaxies, which are in good agreement with a number of independent observations. The amount of gas accretion predicted by the model is of the order of what is needed to sustain star formation. Accretion is expected to occur preferentially in the outer parts of discs and its efficiency drops for higher coronal temperatures. Thus galaxies are able to gather new gas as long as they do not become too massive nor fall into large halos and maintain their star-forming gaseous discs.Comment: 30 pages, 13 figures; invited review to appear in Gas Accretion onto Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by Springe

How cold gas continuously entrains mass and momentum from a hot wind

“…At the same time, we see infall in the form of 'high-velocity' and 'intermediate-velocity' clouds (HVCs and IVCs;Putman et al 2012) with relatively low metallicities, as well as a galactic fountain with continuous circulation of material between the disk and corona (Shapiro & Field 1976;Fraternali & Binney 2008). Fountain-driven accretion could supply the disk with gas for star formation, and explain the observed kinematics of extra-planar gas (Armillotta et al 2016;Fraternali 2017). Our simulations give credence to the idea that star formation in the disk exerts a form of positive feedback: cold gas thrown up into the halo 'comes back with interest', by mixing with low metallicity halo gas which cools and increases the cold gas mass.…”

Gas accretion and galactic fountain flows in the Auriga cosmological simulations: angular momentum and metal redistribution

“…Fountain flows are expected to play an important role in increasing the vertical component of the specific angular momentum, l z , of discs (Fraternali 2017). To determine whether this holds true, we show in Fig.…”

“…Hani et al 2019). For example, Fraternali et al (2013) and Fraternali (2017) argue that the cold gas clouds around the Milky Way are part of a fountain flow that mix with the hot corona, and instigate the condensation of gas with high specific angular momentum acquired from large-scale tidal torques (e.g. Peebles 1969;Fall & Efstathiou 1980;Mo et al 1998) at early times.…”

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

“…There is observational evidence of a significant burst of star formation near the tangent of the Scutum arm, in the form of a large density of protoclusters around W43 (Motte et al 2003) and multiple red supergiant clusters (Figer et al 2006;Davies et al 2007;Alexander et al 2009). This star formation burst can be associated to an enhancement of SN feedback forming a "Galactic fountain" (Shapiro & Field 1976;Bregman 1980;Fraternali 2017;Kim & Ostriker 2018), whose remants shape the vertical HI filaments.…”