Dynamics and Morphology of Cold Gas in Fast, Radiatively Cooling Outflows: Constraining AGN Energetics with Horseshoes

Abstract

Warm ionized and cold neutral outflows with velocities exceeding 100 km s−1 are commonly observed in galaxies and clusters. However, theoretical studies indicate that ram pressure from a hot wind, driven either by the central active galactic nucleus (AGN) or a starburst, cannot accelerate existing cold gas to such high speeds without destroying it. In this work we explore a different scenario, where cold gas forms in a fast, radiatively cooling outflow with temperature T ≲ 107 K. Using 3D hydrodynamic simulations, we demonstrate that cold gas continuously fragments out of the cooling outflow, forming elongated filamentary structures extending tens of kiloparsecs. For a range of physically relevant temperature and velocity configurations, a ring of cold gas perpendicular to the direction of motion forms in the outflow. This naturally explains the formation of transverse cold gas filaments such as the blue loop and the horseshoe filament in the Perseus cluster. Based on our results, we estimate that the AGN outburst responsible for the formation of these two features drove bipolar outflows with velocity >2000 km s−1 and total kinetic energy >8 × 1057 erg about ∼10 Myr ago. We also examine the continuous cooling in the mixing layer between hot and cold gas, and find that radiative cooling only accounts for ∼10% of the total mass cooling rate, indicating that observations of soft X-ray and far-ultraviolet emission may significantly underestimate the growth of cold gas in the cooling flow of galaxy clusters.

Signature of Supersonic Turbulence in Galaxy Clusters Revealed by AGN-driven Hα Filaments

“…In all cases, the cold gas fragments continuously out of the radiatively cooling outflow after ∼10 Myr, forming a filamentary trail as the outflow rises in the cluster potential (Qiu et al 2019a(Qiu et al , 2020. As discussed in Qiu et al (2021a), the morphology of the cold gas may take both longitudinal and transverse shapes depending on the initial outflow properties, such as the outer horseshoe-shaped shell at t = 20 Myr for v out = 1500 km s −1 . Note that in the slowest outflow case, it takes much longer for the low-temperature plasma to stretch and form elongated cold gas filaments under the effect of cluster gravity, unless the initial thermally unstable region is already spatially extended.…”

“…As demonstrated in Qiu et al (2021a), the radiative cooling time of the 10 7 K outflow in initial pressure equilibrium with the ambient ICM is t cool  10 Myr. This is a few times larger than the sound-crossing time t cross ≡ Δl/c s ≈ 2 Myr(Δl/kpc), where Δl ∼ kpc is the typical width of each observed filament complex, and c s is the sound speed.…”

“…In order to study the turbulent velocity structure of the cold gas, we perform a set of simulations where cold gas cools and fragments out of an AGN-driven outflow. The simulation setup is similar to that presented in Qiu et al (2021a). In each simulation, a spherical gas clump is launched from the center of the cluster, with temperature T out , outflow velocity v out , and mass M out .…”

“…Note that in the slowest outflow case, it takes much longer for the low-temperature plasma to stretch and form elongated cold gas filaments under the effect of cluster gravity, unless the initial thermally unstable region is already spatially extended. We refer the readers to our previous work on the dynamical and morphological evolution of the cold gas, which can be described reasonably well by a 1D model comprising radiative cooling and ICM ram pressure (Qiu et al 2020(Qiu et al , 2021a. In this work, we instead focus on the small-scale turbulent structure that can be extracted from the cold gas velocity.…”

“…10 In a rich cluster environment, however, direct detection of such outflows is more difficult due to the X-ray emission from the massive ICM. On the other hand, recent simulations of galaxy clusters and AGN feedback indicate that cold gas may fragment out of a supersonic, radiatively cooling outflow (Qiu et al 2020(Qiu et al , 2021a, giving rise to extended filamentary Hα nebulae, such as observed in the Perseus cluster (Conselice et al 2001;Gendron-Marsolais et al 2018). Therefore, the velocity structure of the 10 4 K gas offers a unique opportunity for probing the properties of the original hot outflow in galaxy clusters.…”