Galactic Fountains and Gas Accretion

Abstract

Abstract. Star-forming disc galaxies such as the Milky Way need to accrete > ∼ 1 M ⊙ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.Keywords: turbulence -ISM: kinematics and dynamics -Galaxy: kinematics and dynamics -Galaxy: structure -galaxies: formation PACS: 98.35.Ac, 98.38.Am, 98.58.Ay, 98.62.Ai THE PROPOSED SCENARIOStar-forming disc galaxies like the Milky Way must accrete > ∼ 1 M ⊙ of fresh gas each year [see 1, and references therein] and have built their discs gradually over the last 10 Gyr [e.g. 2]. A central question is the origin of the accreted gas and how this gas reaches the thin disc whitin which the process of star formation takes place. The virial-temperature corona, in which disc galaxies are embedded, is the only reservoir of baryons capable of sustaining an accretion rate of ∼ 1 M ⊙ yr −1 for a Hubble time. We present the results of a set of grid-based hydrodynamical simulations supporting the idea that the gas needed by the disc to form stars is drawn from this corona.Coronae of disc galaxies are similar in many respects to the hot atmospheres of giant elliptical galaxies and galaxy clusters, but with lower gas temperature and density [e.g. 3, 4]. As the hot gas of these more massive systems, the coronal gas is unlikely to fragment into clouds via thermal instability [5], but it is expected to cool monolithically and feed the central black hole rather than produce an extended cold disc in which stars can form. However, if the gas needed to feed star formation has to be drawn from the corona, a mechanism that makes the hot gas accrete onto the disc must be at work.There is abundant evidence that star formation in galaxies like the Milky Way powers a galactic fountain: ejection of gas from the mid-plane by supernova explosions [6]. Through the fountain a significant fraction (from 10 to 25 %) of the whole HI content of the galaxy is carried into its halo [see 7, and references therein]. In this work, supported by several lines of argument, we hypothesise that the transfer of gas from the corona to the star-forming disc is effected by the HI clouds ejected by the galactic fountain [8].Our hydrodynamical simulations suggest that the gas accretion proceeds through the following steps: (i) stripping of gas from fountain clouds by the corona as a result of the Kelvin-Helmholtz instability, (ii) mixing of the (high metallicity) stripped gas with a comparable amount of coronal gas in the turbulent ...

Modelling the H i halo of the Milky Way

“…This process slows down the rotation of the Galactic fountain clouds and produces gas accretion towards the disk. Hydrodynamical simulations show that the Galactic corona could be the reservoir of this accreting gas (Marinacci et al 2010a). The typical orbits of this "accreting Galactic fountain" differ from those of a fountain without accretion for the presence of inward radial motions (Fig.…”

Cold gas in massive early-type galaxies: the case of NGC 1167

“…However, according to e.g. Marinacci et al (2010), this galactic fountain gas would be needed to create Kelvin-Helmholtz instabilities in the hot corona such that (hot) gas can condense to clouds that are able to reach the disk. With the presence of supernovae, star-forming galaxies can accumulate enough cold gas to sustain their observed star formation rates (see also Hopkins et al 2008) while in early-type galaxies, like NGC 1167, the creation of instabilities in the halo is prevented (due to the absence of galactic fountains).…”

An Introduction to Gas Accretion onto Galaxies

“…The dominant mode of accretion at the star forming component of a galaxy remains to be determined. The interplay between enriched outflowing gas with gas coming in may be key in this process (Fraternali & Binney, 2008;Putman et al, 2012;Marinacci et al, 2010;Voit et al, 2015).…”