Thermal instability in gravitationally stratified plasmas: implications for multiphase structure in clusters and galaxy haloes

Abstract

We study the interplay among cooling, heating, conduction and magnetic fields in gravitationally stratified plasmas using simplified, plane‐parallel numerical simulations. Since the physical heating mechanism remains uncertain in massive haloes such as groups or clusters, we adopt a simple, phenomenological prescription which enforces global thermal equilibrium and prevents a cooling flow. The plasma remains susceptible to local thermal instability, however, and cooling drives an inward flow of material. For physically plausible heating mechanisms in clusters, the thermal stability of the plasma is independent of its convective stability. We find that the ratio of the cooling time‐scale to the dynamical time‐scale tcool/tff controls the non‐linear evolution and saturation of the thermal instability: when tcool/tff≲ 1, the plasma develops extended multiphase structure, whereas when tcool/tff≳ 1 it does not. (In a companion paper, we show that the criterion for thermal instability in a more realistic, spherical potential is somewhat less stringent, tcool/tff≲ 10.) When thermal conduction is anisotropic with respect to the magnetic field, the criterion for multiphase gas is essentially independent of the thermal conductivity of the plasma. Our criterion for local thermal instability to produce multiphase structure is an extension of the cold versus hot accretion modes in galaxy formation that applies at all radii in hot haloes, not just to the virial shock. We show that this criterion is consistent with data on multiphase gas in galaxy groups and clusters; in addition, when tcool/tff≳ 1, the net cooling rate to low temperatures and the mass flux to small radii are suppressed enough relative to models without heating to be qualitatively consistent with star formation rates and X‐ray line emission in groups and clusters.

Characterizing the chemically enriched circumgalactic medium of ∼38 000 luminous red galaxies in SDSS DR12

“…These earlier analytic models have relied on a simple hypothesis that thermal instabilities would develop if the cooling time (t cool ) is comparable to or smaller than the dynamical time (t ff ) of the gas. However, recent numerical simulations have provided more detailed insights into the process of forming a multi-phase medium (e.g., McCourt et al 2012;Sharma et al 2012). It has been shown that, in fact, a multi-phase medium starts to develop when the cooling time is 3 − 10 times the free-fall time ).…”

The jet feedback mechanism (JFM) in stars, galaxies and clusters

“…Overall, many observational and theoretical studies in the last decade have put the cold feedback mechanism on a very solid ground (e.g., Revaz et al 2008;Pope 2009;Wilman et al 2009;Edge et al 2010;Wilman et al 2011;Nesvadba et al 2011;Cavagnolo et al 2011;Gaspari et al 2012a,b;McCourt et al 2012;Sharma et al 2012a,b;Farage et al 2012;Wagh et al 2013;Li & Bryan 2014a,b;McNamara et al 2014;Voit et al 2015b;Li et al 2015;Prasad et al 2015;Singh & Sharma 2015;Tremblay et al 2015;Valentini & Brighenti 2015;Choudhury & Sharma 2016;Hamer et al 2016;Loubser et al 2016;Meece et al 2016;Russell et al 2016;McNamara et al 2016;Yang & Reynolds 2016b;Barai et al 2016;Tremblay et al 2016). As an example, Gaspari (2015) shows that the AGN-JFM self-regulated by accretion of cold clumps can explain the strong deficit of soft X-ray emission in clusters and groups of galaxies.…”

Regulation of star formation in giant galaxies by precipitation, feedback and conduction

“…6,7 Theoretical models in which cold clouds precipitate out of the hot gas via thermal instability and accrete onto the black hole exhibit the necessary tuning. [8][9][10] We have recently presented observational evidence showing that the abundance of cold gas in the central galaxy increases rapidly near the predicted threshold for instability. 11 Here we present observations showing that this threshold extends over a large range in cluster radius, cluster mass, and cosmic time, and incorporate the precipitation threshold into a comprehensive framework of theoretical models for the thermodynamic state of hot gas in galaxy clusters.…”