Shock-multicloud interactions in galactic outflows -- II. Radiative fractal clouds and cold gas thermodynamics

Abstract

Galactic winds are crucial to the cosmic cycle of matter, transporting material out of the dense regions of galaxies. Observations show the coexistence of different temperature phases in such winds, which is not easy to explain. We present a set of 3D shock-multicloud simulations that account for radiative heating and cooling at temperatures between 10 2 K and 10 7 K. The interplay between shock heating, dynamical instabilities, turbulence, and radiative heating and cooling creates a complex multi-phase flow with a rain-like morphology. Cloud gas fragments and is continuously eroded, becoming efficiently mixed and mass loaded. The resulting warm mixed gas then cools down and precipitates into new dense cloudlets, which repeat the process. Thus, radiative cooling is able to sustain fast-moving dense gas by aiding condensation of gas from warm clouds and the hot wind. In the ensuing outflow, hot gas with temperatures 10 6 K outruns the warm and cold phases, which reach thermal equilibrium near ≈ 10 4 K and ≈ 10 2 K, respectively. Although the volume filling factor of hot gas is higher in the outflow, most of the mass is concentrated in dense gas cloudlets and filaments with these temperatures. More porous multicloud layers result in more vertically extended outflows, and dense gas is more efficiently produced in more compact layers. The cold phase is not accelerated by ram-pressure, but, instead, precipitates from warm and mixed gas out of thermal equilibrium. This cycle can explain the presence of high-velocity H gas with 𝑁 H I = 10 19−21 cm −2 and Δ𝑣 FWHM 37 km s −1 in the Galactic centre outflow.

Dust in RCW 58: clues to common envelope channel formation?

“…Individual clouds are stretched and produce filamentary tails. Banda-Barragán et al (2020a) show that, in radiative models, strong cooling can promote the clumping and fragmentation of dense gas. In RCW 58 there is the added process of photoionization, which will prevent the flow from cooling below 6500 K. Turbulence and mixing occur in the low-density, heated flow behind the accelerating forward shock.…”

“…Not only do the clouds interact with the main shock but there are subsequent interactions within the shocked layer. Although the parameters (intercloud density and shock velocity) are not tuned for nebulae around massive stars, the general scenario portrayed in simulations such as those of Banda-Barragán et al (2020a) can be appreciated.…”

“…In addition, blueshifted absorption components with v LSR between −150 km s −1 and −100 km s −1 have been detected in UV and optical spectra of the central star, WR 40 (Smith et al 1984). In the radiative simulations of Banda-Barragán et al (2020a), the forward shock re-accelerates to at most 0.4 times the initial shock velocity when it exits the cloudy layer, i.e., v as ∼ 0.4v b , and the post-shock flow velocity is v psh = 0.75v as for a fast shock. If we take the range of observed velocities as the post-shock flow velocities, then the initial shock velocity would be 300 < v b < 500 km s −1 .…”

Tracing the Ionization Structure of the Shocked Filaments of NGC 6240

“…If molecular clouds are instead swept up and carried away by outflowing gas, we expect the molecular gas to be evenly distributed across the face of an outflow. Depending on the density and size of the clouds, they would eventually evaporate into the wind and become less frequent farther from the launch of the outflow (Banda- Barragán et al 2020). In this scenario, gas on the edges of each cloud may still be shocked even as the interior is shielded.…”

Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

“…Begelman & Fabian 1990;Klein et al 1994;Scannapieco & Brüggen 2015;Armillotta et al 2017;Gronke & Oh 2018;Mandelker et al 2020a). A key finding is that, in the presence of efficient radiative cooling, cold gas structures can survive for much longer than the classical gas destruction time and even gain mass by mixing and cooling the surrounding hot medium (Gronke & Oh 2018;Ji et al 2019;Gronke & Oh 2020a;Mandelker et al 2020a;Fielding et al 2020;Li et al 2020;Sparre et al 2020;Banda-Barragán et al 2020;Kanjilal et al 2021;Abruzzo et al 2021;Farber & Gronke 2021). While still a developing field, these insights have wide-ranging implications for the survival of cold clouds embedded in hot galactic winds, as well as the filaments and high-velocity cloud (HVC) complexes that comprise the Magellanic Stream.…”