Positive feedback at the disc-halo interface

Abstract

The flat star formation (SF) history of the Milky Way (MW) requires gas in the Galactic disc to be replenished, most likely from a reservoir outside the Galaxy. Such a replenishment may be achieved by a form of 'positive' feedback, whereby SF feedback creates a Galactic fountain cycle that collects and cools additional gas from the hot halo surrounding the Galaxy. In this paper we present a model of this process for the MW. A section of the Galactic disc is allowed to form stars which subsequently explode as supernovae (SNe) and send gas out into the hot halo. The gas that is sent out is colder than the hot halo gas and, as it mixes, the halo gas is cooled, providing fuel for further SF as the mixture falls back onto the Galactic disc. We find that this process can be sufficient to maintain a roughly-constant cold gas mass in the MW over at least 3 Gyr. Our results further suggest that there is a positive feedback trend whereby increasing SF leads to an increase in the cold gas budget at average SF rates below 0.5 M yr −1 but above which becomes negative, where further increasing the SFR causes the cold gas budget to decrease. We have constructed an analytical model for this that reproduces the data well and could have profound implications for galaxy evolution in feedback-dominated regimes.

Black hole -- galaxy scaling relations in FIRE: the importance of black hole location and mergers

“…However, the gas ejected by SN feedback can cool down at the outskirts of the galaxy and return to the galaxy, a process known as "Galactic fountain" (Shapiro & Field 1976;Bregman 1980;Melioli et al 2008). It is also possible to accomplish such a replenishment in the form of positive SN feedback through which the feedback steals part of the hot gas from the intergalactic medium (IGM) and cools it down to constitute a galactic fountain (Hobbs & Feldmann 2020). These findings suggest that the refurbished cold gas budget may be crucial to sustain SF and black hole feeding, especially in the local Universe.…”