Abstract
Abstract Multiphase media have very complex structure and evolution. Accurate numerical simulations are necessary to make advances in our understanding of this rich physics. Because simulations can capture both the linear and nonlinear evolution of perturbations with a relatively wide range of sizes, it is important to thoroughly understand the stability of condensation and acoustic modes between the two extreme wavelength limits of isobaric and isochoric instability as identified by Field. Partially motivated by a recent suggestion that large non-isobaric clouds can “shatter” into tiny cloudlets, we revisit the linear theory to survey all possible regimes of thermal instability. We uncover seven regimes in total, one of which allows three unstable condensation modes. Using the code Athena++, we determine the numerical requirements to properly evolve small amplitude perturbations of the entropy mode into the nonlinear regime. Our 1D numerical simulations demonstrate that for a typical AGN cooling function, the nonlinear evolution of a single eigenmode in an isobarically unstable plasma involves increasingly larger amplitude oscillations in cloud size, temperature, and density as the wavelength increases. Such oscillations are the hallmark behavior of non-isobaric multiphase gas dynamics and may be observable as correlations between changes in brightness and the associated periodic redshifts and blueshifts in systems that can be spatially resolved. Intriguingly, we discuss regimes and derive characteristic cloud sizes for which the saturation process giving rise to these oscillations can be so energetic that the cloud may indeed break apart. However, we dub this process “splattering” instead of “shattering,” as it is a different fragmentation mechanism that is triggered when the cloud suddenly “lands” on the stable cold branch of the equilibrium curve.
Highlights
The concept of thermal instability (TI) originated through an analysis of the heat equation by Parker (1953), who envisioned a runaway scenario that would uniformly cool the plasma
ALMA observations have provided strong evidence that TI operates in the central regions of cool-core clusters and within brightest cluster galaxies and brightest group galaxies (e.g., David et al 2014; McNamara et al 2014; Russell et al 2014, 2016; Voit et al 2015; Tremblay et al 2016; Vantyghem et al 2016; Pulido et al 2018; Temi et al 2018), as molecular gas must co-exist with the hot virialized plasma temperatures of the intracluster medium (ICM) or intergroup medium
While it is known that isochoric instability is difficult to trigger for realistic astrophysical cooling functions (e.g., Balbus 1995), there are circumstances in which it can occur, for example, when there is a deficit of soft X-rays in photoionized plasmas
Summary
The concept of thermal instability (TI) originated through an analysis of the heat equation by Parker (1953), who envisioned a runaway scenario that would uniformly cool the plasma. While it is known that isochoric instability is difficult to trigger for realistic astrophysical cooling functions (e.g., Balbus 1995), there are circumstances in which it can occur, for example, when there is a deficit of soft X-rays in photoionized plasmas (see, e.g., Figure 24 of Kallman & McCray 1982) We do find that thermally unstable plasmas can be prone to a fragmentation process, albeit one quite different from shattering This new mechanism, which we call “splattering,” arises because the saturation of TI is a sudden process in which the velocity of the condensating gas must reverse directions upon “landing” on the cold phase of the S-curve. In a companion paper (Waters & Proga 2019), we present an application of non-isobaric gas dynamics, addressing how clouds of different sizes interact
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