Consequences of a New Hot Component of the Interstellar Medium

“However, ejected gas is able to rise to substantial heights in the halo and to displace infalling material, before it eventually falls back to the disk, forming a “galactic fountain” (Shapiro & Field, 1976).”

“In many formulations, the cooler low-ion traced gas precipitates out of the warmer O vitraced phase, owing to thermal instabilities (Shapiro & Field 1976; McCourt et al. 2012; Voit et al. 2015b; Thompson et al. 2016; see also Wang 1995), while the O vi-traced gas may be continually replenished by a…”

“The mechanism is that of a “galactic fountain” in which hot gas rises into the halo, condenses into cold clouds and returns to the disk (first suggested by Shapiro & Field 1976 [118]; see also [22]).”

“…is the case, some mechanism is required to move the gas as much as 140 pc out of the galactic plane; driving by stellar/supernovae energy input can naturally produce such structures (see, e.g., Shapiro & Field 1976, or more recent numerical simulations by Avillez 2001 and Avillez & Mac Low 2001).”

“…& Silk, 2010; Di Matteo et al., 2005, 2008), as well as momentum-driven winds due to radiation pressure from central sources on dust grains (Murray et al., 2005) 19 and a galactic fountain as enriched hot gas rises up from the disk (e.g., Shapiro & Field, 1976; Joung & Mac Low, 2006).”

“…argument in favor of the idea that some of the positive velocity wings trace gas expelled from the Galactic disk is that hot gas expelled from the Galactic disk, perhaps as part of a ‘‘Galactic fountain ’’ (Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989), would likely contain O vi.”

“M odels for HVCs range from condensed \G alactic Fountain" gas at D ’ 5kpc (Shapiro & Field 1976; Bregm an 1980),to large,extra-galactic objects associated with the Local G roup D ’ 1M pc (Verschuur 1969; Arp 1985; Blitz etal.”

“…include expansion of the hot regions to much larger volumes above the SN source region, with cooling occurring in the flow (a Galactic Fountain, Shapiro & Field 1976); the filling of the halo of the Galaxy with such material until it is dense enough to radiate the SN power input (an extension…”

“T he Galactic Fountain The most frequently discussed origin for the HVC phenomenon is the ““ galactic fountain ÏÏ model proposed by Shapiro & Field (1976) and elaborated by Bregman (1980) and others.”

“…of the ISM in normal spiral galaxies also predict hot gas in the halo, due to the interchange of material between the disk and halo through ‘‘galactic fountains’’ (Shapiro & Field 1976; Bregman 1980; de Avillez 2000), or in a more localized form through ‘‘chimneys’’ (Norman & Ikeuchi 1989).”

“The galactic fountain mechanism (Shapiro & Field 1976) has received the most attention to date.”

“Qualitatively, these outflows are similar to galactic fountain models in which outflows are driven by supernova-powered bubbles (see, e.g., Shapiro & Field 1976; Oosterloo et al. 2007; Marinacci et al. 2011).”

“More realistically, gas with vw vesc will circulate in the halo along a roughly ballistic trajectory and will cool forming a galactic fountain (Shapiro and Field 1976, Bregman 1980).”

“Above and below this layer, a galactic fountain (Shapiro & Field 1976) is set up, as observed in the Milky Way, enabling disk-halo interactions.”

“…of the ISM in normal spiral galaxies also predict hot gas in the halo, as a result of the interchange of material between the disk and halo through ‘‘galactic fountains’’ (Shapiro & Field 1976; Bregman 1980; de Avillez 2000), or in a more localized form through ‘‘chimneys’’ (Norman & Ikeuchi 1989).”

“…of cosmic gas which accretes from the intergalactic medium and shocks to the (¦ 106 K in local ∼ L∗ galaxies) virial temperature of the halo (Birnboim & Dekel, 2003), and partly of gas recycled from the galaxy disk in a galactic ‘fountain’ (e. g. Shapiro & Field, 1976; Oppenheimer et al., 2010).”

“Such processes are generally described as galactic fountains (Shapiro & Field 1976; Bregman 1980).”

“Even in the case where material is driven out of a galaxy disk but does not escape the galaxy (a “Galactic fountain” scenario, Shapiro & Field 1976), the redistribution of baryons may shape the future evolution of the galaxy.”

“…paper we use the term ‘galactic wind’ to describe such outflows without regard for their eventual fate — the gas may escape the halo potential well, or be recycled back into the disk in a process sometimes referred to as ‘galactic fountain’ activity (Shapiro & Field 1976; Bregman 1980; Kahn 1981).”

“This model is referred to as a turbulent mixing layer (TML), and is commonly invoked to explain the ionization state of Milky Way HVCs (Sembach et al. 2003; Fox et al. 2004; Wakker et al. 2012; Savage et al. 2014), and intergalactic intervening absorption (Tripp et al. 2008).”

“Such transition temperature gas might occur within the cooling gas of a ‘‘Galactic fountain ’’ (Shapiro & Field 1976; Edgar & Chevalier 1986; Shapiro & Benjamin 1991), in the conductively heated interface region between the hot and cool interstellar gas (Ballet, Arnaud, & Rothenflug 1986), in…”

“In the galactic fountain scheme, gas is pushed up by stellar activity, travels through the halo and eventually falls back to the disc (Shapiro & Field 1976; Bregman 1980).”

“Material ejected in a wind may then recycle back on to the galaxy as ‘wind recycling’ or a ‘galactic fountain’ (e.g. Shapiro & Field 1976; Bregman 1980; Oppenheimer et al. 2010; Ford et al. 2014; Anglés-Alcázar et al. 2017a).”

“The primary source of such coronal gas may actually be the action of a large-scale “galactic fountain” (Shapiro & Field 1976; Bregman 1980; Savage et al. 2003) in the recent or distant past.”

“One of the mechanisms proposed is the so called Galactic Fountain (Shapiro & Field 1976; Bregman 1980) according to which gas, heated by stars and supernovae, leaves the disk and moves via chimneys in the vertical direction to fall back, eventually, onto the disk.”

“These comprise two major models: the classical fountain (CF) model (Shapiro & Field, 1976; Bregman, 1980; Kahn, 1981) and the chimney model (Tomisaka & Ikeuchi, 1986; Norman & Ikeuchi, 1989).”

“…Leroy et al. (2015; see also Chisholm &Matsushita 2016) have shown a clear signature of decreasing outward mass fluxes in molecular and neutral gas as a function of the distance from the disk midplane, implying a fountain (Shapiro & Field 1976; Bregman 1980) rather than a wind for the cooler gas.”

“The expelled gas eventually falls back onto the disc, a mechanism known as a galactic fountain (Shapiro & Field 1976; Bregman 1980).”

“This absorber (and the similar O viwing toward Mrk 421; Savage et al. 2005) may be tracing a hot Galactic fountain (Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989; Houck & Bregman 1990): a circulation of interstellar material in which supernovae-heated gas rises buoyantly into the halo,…”

“Supernovae drive circulation of gas from the disk to heights of several kiloparsecs in a so-called Galactic fountain (Shapiro & Field 1976; Bregman 1980).”

“Conversely, high-mass galaxies retain their outflows, and the gas reaccretes onto the galaxy as a galactic fountain (Shapiro & Field 1976), providing a secondary source of star-forming material.”

“Previously-expelled material may be recycled to provide additional fuel for star formation (i.e., in a galactic fountain; Shapiro & Field 1976; Bregman 1980; Marasco et al. 2011), and likely gives rise to intermediate velocity clouds (IVCs) at distances < 5 kpc (Wakker 1 Max-Planck-Institut für…”

“Depending on its density, coronal gas may cool and condense into clouds that fall back to the disk, or remain hot and accelerate as a galactic wind to join the CGM (Shapiro & Field 1976; Bregman 1978; Chevalier & Clegg 1985).”

“This range probably reflects the variety in the origin of these systems, from metal-poor intergalactic medium accretion and small mergers to metal-rich gas participating in the galactic fountain (Shapiro & Field 1976).”

“…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).”

“The disk-halo interaction is described by theorists by means of galactic fountains (Shapiro & Field 1976; de Avillez & Breitschwerdt 2005), 2 R. Tüllmann et al.: The multi-phase gaseous halos of star forming late-type galaxies chimneys (Norman & Ikeuchi 1989), and galactic winds (Breitschwerdt et…”

“This gas will be recycled as a galactic fountain (Shapiro & Field 1976), and will eventually be converted into stars, accreted onto a supermassive black hole, or suspended in the circumgalactic medium.”

“The HVCs may have a galactic origin such as the Galactic fountain (Shapiro & Field 1976; Bregman 1980), or they may be extragalactic (e.g., Oort 1970; Blitz et al. 1999; Braun & Burton 1999).”

“Its three columns give, from left to right, the mass fraction of ISM removed by ‘outflows’,8 star formation, and ‘fountains’ (Shapiro & Field 1976), by which we refer to gas which remains bound to the galaxy halo, but is no longer sufficiently dense to form stars.”

“The ejected gas may re-accrete onto the galaxy in a large scale version of the “galactic fountain” (Shapiro & Field 1976; Bregman 1980; Breitschwerdt & Komossa 2000; Marinacci et al. 2011).”

“The gas flows to the halo where it cools and contracts at a height of several kiloparsecs and then falls downt the disc in the form of cold, relatively small clouds, forming what is known as the galactic fountain (Shapiro & Field 1976, Bregman 1980).”

“This last hypothesis has long been used to model the HVCs in the context of a ‘galactic fountain’ scenario in a multi-phase medium (e.g. Shapiro & Field 1976; Wolfire et al. 1995).”

“They may never actually reach the IGM but circle within a so called ‘halo fountain’, the name chosen as reference to the ‘galactic fountain’, first introduced by Shapiro & Field (1976), which describes a similar process on smaller galactic scales.”

“Through the years, theoretical investigations kept pace with the observations, starting with initial assessments by Shapiro & Field (1976) on the nature of a possible Galactic fountain, and an early proposal by Cox & Smith (1974) on the origin of a pervasive network (“tunnels”) of hot gas that is…”

“Shapiro & Field (1976) introduced the idea of the Galactic fountain (see also Bregman 1996) in which hot gas ejected from the disk cools and condenses into neutral clouds that fall back onto the Galaxy in a continuing cycle.”

“It has been suggested that HVCs may represent material taking part in a Galactic fountain (Shapiro & Field 1976; Bregman 1980).”

“5 kpc suggest that the IV Arch has its origin in the Milky Way disk, for instance as part of a Galactic fountain (Shapiro & Field 1976 ; Houck & Bregman 1990).”

“Promising mechanisms include hydrodynamic or magnetohydrodynamic flows, such as fountain (Shapiro & Field 1976; Bregman 1980; Houck & Bregman 1990; de Avillez, Berry, & Kahn 1998) and chimney (Norman & Ikeuchi 1989; see also Heiles 1990) flows; expulsion via radiation pressure on dust grains (e.g.,…”

“Aside from direct observations of such hot halos in the Milky Way and other nearby galaxies (e.g., Lynds & Sandage 1963; Shapiro & Field 1976; Lehnert & Heckman 1995; Armus et al. 1995; Strickland et al. 2004), the higher starformation-rate surface densities of, and the ubiquitous presence of…”

“…the typical velocity dispersion in the disk of a star-forming galaxy, coronal gas created in the disk can—in the absence of rapid radiative cooling—flow upward to large vertical scale heights and occupy the lower halo of the galaxy (Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989).”

“…perspective several scenarios have been suggested for the gas transport from the galactic disk into the halo, among them the galactic fountains (Shapiro & Field 1976; de Avillez 2000), chimneys (Norman & Ikeuchi 1989), superbubble outbreak (Mac Low & Ferrara 1999), or superwinds (Heckman et…”

“In the classic model of galactic fountain (Shapiro & Field 1976; Bregman 1980), a continuous gas circulation is produced by supernova explosions and stellar winds that cause the mostly ionized gas to leave the plane of the disk.”

“Undoubtedly much of the H i that is observed more than a kiloparsec from galactic planes has been pushed off the star-forming plane by the mechanical feedback from star formation (e.g. Fraternali & Binney 2008) – the galactic fountain effect (Shapiro & Field 1976).”

“This gas is expected to travel through the halo and eventually to fall back to the disc in a time-scale of ∼80–100 Myr, a mechanism which is known as galactic fountain (Shapiro & Field 1976; Bregman 1980; Houck & Bregman 1990).”

“The result is a disk-halo interface and Galactic fountain (Shapiro & Field 1976; Bregman 1980; Houck & Bregman 1990; Shull & Slavin 1994).”

“The first is supernova (SN) driven outflows from the Galactic disk (e.g., Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989; Joung & Mac Low 2006).”

“Such models have included galactic chimneys (Norman & Ikeuchi 1989), fountains (Shapiro & Field 1976), superbubble blowout (Mac Low & Ferrara 1999), and CR-driven Parker instabilities (e.g., Breitschwerdt et al. 1991; Hanasz et al. 2002).”

“Stellar winds and supernovae expel part of the interstellar gas out of the Galactic disk, but gravity generally forces the gas to fall back through the process known as “Galactic fountain” (Shapiro & Field 1976).”

“While models for the gas transport include magneto–hydrodynamic (MHD) flows such as the galactic fountains (Shapiro & Field 1976), chimneys (Norman & Ikeuchi 1989), favored expulsion models for the dust may be considered via radiation pressure on dust grains, termed photolevitation (e.g., Franco et…”

“The velocity structures show that they are falling back toward the galaxy, in a fashion perhaps similar to galactic fountains (Shapiro & Field 1976), on somewhat extended scales.”

“There are also HVCs with solar-like metallicity, suggestive of galactic fountain flows (e.g. Shapiro & Field 1976; Bregman 1980; Fraternali & Binney 2006).”

“Another suggests that they are the cooled condensate of material vented into the halo as a result of super-bubble breakout (the galactic fountain model, see Shapiro & Field 1976, Bregman 1980).”

“These phenomena set up a circulation (galactic fountain Shapiro & Field, 1976; Fraternali & Binney, 2006) that continuously lifts disc gas into the halo causing it to mix with the hot (T >∼ 106 K) gas of the corona.”

“At low outflow speeds, the material will fall back to the plane as a fountain (Shapiro & Field 1976; Bregman 1980; Houck & Bregman 1990).”

“…of hot gas into the Galactic halo wherein the gas cools and returns to the disk in a global circulation pattern (i.e., a “Galactic fountain” - Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989; Houck & Bregman 1990), ram pressure or tidal stripping of material from the Magellanic…”

“Above the midplane, outflows produced by SN explosions and inflows arising from cooling and disk gravity drive gas circulation in a fountain-like manner (Shapiro & Field 1976; Bregman 1980).”

“Several theoretical studies have addressed the issue of cloud formation in the Galactic halo, usually in the context of high-velocity clouds, not gas in normal Galactic rotation (Shapiro & Field 1976; Bregman 1980; Ferrara & Field 1994; Wolfire et al. 1995).”

“One of the most prominent hypotheses puts forward the Galactic fountain (Shapiro & Field 1976) as a means of forming HVCs.”

“Halo gas motions are likely to be dominated by complex physics, such as a Galactic fountain (Shapiro & Field 1976; Bregman 1980) where gas is pushed off of the disk, moves through the halo, and then returns to the disk.”

“There is a component of Galactic gas with an interesting history, circulating in a “Galactic fountain” (Shapiro & Field 1976; Bregman 1980).”

“Most of this gas is thought to have been driven out of the disc by supernova-powered bubbles (Shapiro & Field 1976; Houck & Bregman 1990) and must consist of clouds that are moving on essentially ballistic trajectories because if it were in hydrostatic equilibrium, its vertical density profile…”

“The outflow of hot (T ∼ 106 K) gas is expected in regions where there are multiple supernova explosions; the “chimneys” serve as conduits for the hot gas that may eventually return to the Galactic disk as cool high velocity clouds (e.g., Shapiro & Field 1976; Bregman 1980; Norman & Ikeuchi 1989).”

“Within this picture, it is not clear whether the source of HVCs is predominantly the accretion of low-metallicity gas onto the Galaxy (Wakker et al. 1999) or whether the material traces condensed outflows of hot enriched gas in a ‘‘Galactic fountain ’’ (Shapiro & Field 1976; Bregman 1980).”

“The halo gas may eventually cool down and fall back to the disc to feed subsequent star formation (i.e. the “galactic fountain” picture; Shapiro & Field 1976; Bregman 1980; de Avillez 2000), or it may be lost to the system through interactions with other galaxies.”