A Graphical Interpretation of Circumgalactic Precipitation

Abstract

Both observations and recent numerical simulations of the circumgalactic medium (CGM) support the hypothesis that a self-regulating feedback loop suspends the gas density of the ambient CGM close to the galaxy in a state with a ratio of cooling time to freefall time ≳10. This limiting ratio is thought to arise because circumgalactic gas becomes increasingly susceptible to multiphase condensation as the ratio declines. If the timescale ratio gets too small, then cold clouds precipitate out of the CGM, rain into the galaxy, and fuel energetic feedback that raises the ambient cooling time. The astrophysical origin of this so-called precipitation limit is not simple but is critical to understanding the CGM and its role in galaxy evolution. This paper therefore attempts to interpret its origin as simply as possible, relying mainly on conceptual reasoning and schematic diagrams. It illustrates how the precipitation limit can depend on both the global configuration of a galactic atmosphere and the degree to which dynamical disturbances drive CGM perturbations. It also frames some tests of the precipitation hypothesis that can be applied to both CGM observations and numerical simulations of galaxy evolution.

Feedback from Active Galactic Nuclei in Galaxy Groups

“…Indeed, unlike in classical linear TI, turbulence acts as an irreducible background of fluctuations over the whole IGrM (e.g., Lau et al [164]). In particular, AGN bubbles are a key recurrent mechanism for inducing such fluctuations in the IGrM (e.g., McNamara et al [252], Voit [253]). In this regard, it is not surprising that t cool /t ff profiles show a large non-trivial deviation above unity, as well as a large intrinsic scatter (e.g., Singh et al [254]; also see Figure 6).…”

The High-Velocity Clouds Above the Disk of the Outer Milky Way: Misty Precipitating Gas in a Region Roiled by Stellar Streams

“…McCourt et al (2018) argue that cooling will cause an optically thin plasma at T ≤ 10 6 K to naturally fragment into a mist-like arrangement of small and denser "cloudlets" that comprise an astrophysical "cloud"; because the fragmentation occurs rapidly, they refer to this as cloud "shattering" (see also Liang & Remming 2020;Gronke et al 2021). In a galactic atmosphere with gravity, it has been argued that circumgalactic "precipitation" McCourt et al 2012;Voit et al 2015;Voit 2021) regulates galaxy evolution. In brief, in this model the ratio of the cooling time,…”

“…This concept provides a neat solution to some galaxy-evolution problems: the stability of the CGM in the high-t cool /t ff regime explains the persistence of the CGM and the small fraction of the baryons that turn into stars, and the periodic rain of (lowt cool /t ff ) cool gas enables prolonged star formation. Voit (2021) argues that convection will sort a circumgalactic atmosphere so that it has an entropy gradient with a constant median t cool /t ff ≈ 10; in this situation, "buoyancy damping" (see Voit 2021) often saturates the growth of a thermal instability before condensation occurs and multiphase gas develops. In this case, Voit (2021) suggests that dynamical disturbances from satellites, bulk flows, and turbulence may be important because they boost the likelihood of precipitation and growth of multiphase gas.…”

Characterising the turbulent multiphase halos with periodic box simulations

“…But in the presence of stratification, this thermally heated gas can rise against the density gradient, expand and cool adiabatically. The ratio between the cooling time and the freefall time is known to play an important role in thermal instability in these environments (Sharma et al 2012;Choudhury & Sharma 2016;Choudhury et al 2019;Voit 2021). We plan to include stratification and study its effects in a future thermal instability study.…”