147 research outputs found
Clustered Supernovae Drive Powerful Galactic Winds After Super-Bubble Breakout
We use three-dimensional hydrodynamic simulations of vertically stratified
patches of galactic discs to study how the spatio-temporal clustering of
supernovae (SNe) enhances the power of galactic winds. SNe that are randomly
distributed throughout a galactic disc drive inefficient galactic winds because
most supernova remnants lose their energy radiatively before breaking out of
the disc. Accounting for the fact that most star formation is clustered
alleviates this problem. Super-bubbles driven by the combined effects of
clustered SNe propagate rapidly enough to break out of galactic discs well
before the clusters' SNe stop going off. The radiative losses post-breakout are
reduced dramatically and a large fraction () of the energy
released by SNe vents into the halo powering a strong galactic wind. These
energetic winds are capable of providing strong preventative feedback and eject
substantial mass from the galaxy with outflow rates on the order of the star
formation rate. The momentum flux in the wind is only of order that injected by
the SNe, because the hot gas vents before doing significant work on the
surroundings. We show that our conclusions hold for a range of galaxy
properties, both in the local Universe (e.g., M82) and at high redshift (e.g.,
star forming galaxies). We further show that if the efficiency of
forming star clusters increases with increasing gas surface density, as
suggested by theoretical arguments, the condition for star cluster-driven
super-bubbles to break out of galactic discs corresponds to a threshold star
formation rate surface density for the onset of galactic winds
M yr kpc, of order that observed.Comment: 19 pages, 12 figures, and 3 page appendix with 6 figures. Movies
available at http://w.astro.berkeley.edu/~dfielding/#SNeDrivenWinds
Simulations of Jet Heating in Galaxy Clusters: Successes and Challenges
We study how jets driven by active galactic nuclei influence the cooling flow
in Perseus-like galaxy cluster cores with idealised, non-relativistic,
hydrodynamical simulations performed with the Eulerian code ATHENA using
high-resolution Godunov methods with low numerical diffusion. We use novel
analysis methods to measure the cooling rate, the heating rate associated to
multiple mechanisms, and the power associated with adiabatic
compression/expansion. A significant reduction of the cooling rate and cooling
flow within 20 kpc from the centre can be achieved with kinetic jets. However,
at larger scales and away from the jet axis, the system relaxes to a cooling
flow configuration. Jet feedback is anisotropic and is mostly distributed along
the jet axis, where the cooling rate is reduced and a significant fraction of
the jet power is converted into kinetic power of heated outflowing gas. Away
from the jet axis weak shock heating represents the dominant heating source.
Turbulent heating is significant only near the cluster centre, but it becomes
inefficient at 50 kpc scales where it only represents a few percent of the
total heating rate. Several details of the simulations depend on the choice
made for the hydro solver, a consequence of the difficulty of achieving proper
numerical convergence for this problem: current physics implementations and
resolutions do not properly capture multi-phase gas that develops as a
consequence of thermal instability. These processes happen at the grid scale
and leave numerical solutions sensitive to the properties of the chosen hydro
solver.Comment: Accepted for publication on MNRA
Cloud Atlas: Navigating the Multiphase Landscape of Tempestuous Galactic Winds
Galaxies comprise intricate networks of interdependent processes which
together govern their evolution. Central among these are the multiplicity of
feedback channels, which remain incompletely understood. One outstanding
problem is the understanding and modeling of the multiphase nature of galactic
winds, which play a crucial role in galaxy formation and evolution. We present
the results of three dimensional magnetohydrodynamical tall box interstellar
medium patch simulations with clustered supernova driven outflows.
Fragmentation of the interstellar medium during superbubble breakout seeds the
resulting hot outflow with a population of cool clouds. We focus on analyzing
and modeling the origin and properties of these clouds. Their presence induces
large scale turbulence, which in turn leads to complex cloud morphologies.
Cloud sizes are well described by a power law distribution and mass growth
rates can be modelled using turbulent radiative mixing layer theory. Turbulence
provides significant pressure support in the clouds, while magnetic fields only
play a minor role. We conclude that many of the physical insights and analytic
scalings derived from idealized small scale simulations translate well to
larger scale, more realistic turbulent magnetized winds, thus paving a path
towards their necessary yet challenging inclusion in global-scale galaxy
models.Comment: 34 pages, 37 figures; Accepted for publication in MNRA
Plasmoid Instability in the Multiphase Interstellar Medium
The processes controlling the complex clump structure, phase distribution,
and magnetic field geometry that develops across a broad range of scales in the
turbulent interstellar medium remains unclear. Using unprecedentedly high
resolution three-dimensional magnetohydrodynamic simulations of thermally
unstable turbulent systems, we show that large current sheets unstable to
plasmoid-mediated reconnection form regularly throughout the volume. The
plasmoids form in three distinct environments: (i) within cold clumps, (ii) at
the asymmetric interface of the cold and warm phases, and (iii) within the
warm, volume-filling phase. We then show that the complex magneto-thermal phase
structure is characterized by a predominantly highly magnetized cold phase, but
that regions of high magnetic curvature, which are the sites of reconnection,
span a broad range in temperature. Furthermore, we show that thermal
instabilities change the scale dependent anisotropy of the turbulent magnetic
field, reducing the increase in eddy elongation at smaller scales. Finally, we
show that most of the mass is contained in one contiguous cold structure
surrounded by smaller clumps that follow a scale free mass distribution. These
clumps tend to be highly elongated and exhibit a size versus internal velocity
relation consistent with supersonic turbulence, and a relative clump
distance-velocity scaling consistent with subsonic motion. We discuss the
striking similarity of cold plasmoids to observed tiny scale atomic and ionized
structures and HI fibers, and consider how the prevalence of plasmoids will
modify the motion of charged particles thereby impacting cosmic ray transport
and thermal conduction in the ISM and other similar systems.Comment: 19 pages, 10 figures. For associated movies, see
https://dfielding14.github.io/movies
The Anatomy of a Turbulent Radiative Mixing Layer: Insights from an Analytic Model with Turbulent Conduction and Viscosity
Turbulent Radiative Mixing Layers (TRMLs) form at the interface of cold,
dense gas and hot, diffuse gas in motion with each other. TRMLs are ubiquitous
in and around galaxies on a variety of scales, including galactic winds and the
circumgalactic medium. They host the intermediate temperature gases that are
efficient in radiative cooling, thus play a crucial role in controlling the
cold gas supply, phase structure, and spectral features of galaxies. In this
work, we introduce a simple parameterization of the effective turbulent
conductivity and viscosity that enables us to develop a simple and intuitive
analytic 1.5 dimensional model for TRMLs. Our analytic model reproduces the
mass flux, total cooling, and phase structure of 3D simulations of TRMLs at a
fraction of the computational cost. It also reveals essential insights into the
physics of TRMLs, particularly the importance of the viscous dissipation of
relative kinetic energy in balancing radiative cooling. This dissipation takes
place both in the intermediate temperature phase, which offsets the enthalpy
flux from the hot phase, and in the cold phase, which enhances radiative
cooling. Additionally, our model provides a fast and easy way of computing the
column density and surface brightness of TRMLs, which can be directly linked to
observations.Comment: 32 pages, 22 figures. Submitted to Ap
TuRMoiL of Survival: A Unified Survival Criterion for Cloud-Wind Interactions
Cloud-wind interactions play an important role in long-lived multiphase flows
in many astrophysical contexts. When this interaction is primarily mediated by
hydrodynamics and radiative cooling, the survival of clouds can be phrased in
terms of the comparison between a timescale that dictates the evolution of the
cloud-wind interaction, (the dynamical time-scale ) and the
relevant cooling timescale . Previously proposed survival
criteria, which can disagree by large factors about the size of the smallest
surviving clouds, differ in both their choice of and (to a
lesser extent) . Here we present a new criterion which agrees
with a previously proposed empirical formulae but is based on simple physical
principles. The key insight is that clouds can grow if they are able to mix and
cool gas from the hot wind faster than it advects by the cloud. Whereas prior
criteria associate with the cloud crushing timescale, our new
criterion links it to the characteristic cloud-crossing timescale of a
hot-phase fluid element, making it more physically consistent with shear-layer
studies. We develop this insight into a predictive expression and validate it
with hydrodynamic ENZO-E simulations of ,
pressure-confined clouds in hot supersonic winds, exploring, in particular,
high wind/cloud density contrasts, where disagreements are most pronounced.
Finally, we illustrate how discrepancies among previous criteria primarily
emerged due to different choices of simulation conditions and cooling
properties, and discuss how they can be reconciled.Comment: 6.5 pages, 4 figures, submitted to ApJ
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