45 research outputs found
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
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
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
CloudFlex: A Flexible Parametric Model for the Small-Scale Structure of the Circumgalactic Medium
We present CloudFlex, a new open-source tool for predicting the
absorption-line signatures of cool gas in galaxy halos with complex small-scale
structure. Motivated by analyses of cool material in hydrodynamical simulations
of turbulent, multiphase media, we model individual cool gas structures as
assemblies of cloudlets with a power-law distribution of cloudlet mass and relative velocities drawn from a turbulent velocity
field. The user may specify , the lower limit of the cloudlet mass
distribution (), and several other parameters that set the
total mass, size, and velocity distribution of the complex. We then calculate
the MgII 2796 absorption profiles induced by the cloudlets along pencil-beam
lines of sight. We demonstrate that at fixed metallicity, the covering fraction
of sightlines with equivalent widths Ang increases
significantly with decreasing , cool cloudlet number density
(), and cloudlet complex size. We then present a first application,
using this framework to predict the projected distribution around
galaxies. We show that the observed incidences of
Ang sightlines within 10 kpc < < 50 kpc are consistent with our
model over much of parameter space. However, they are underpredicted by models
with and , in
keeping with a picture in which the inner cool circumgalactic medium (CGM) is
dominated by numerous low-mass cloudlets ()
with a volume filling factor . When used to simultaneously model
absorption-line datasets built from multi-sightline and/or spatially-extended
background probes, CloudFlex will enable detailed constraints on the size and
velocity distributions of structures comprising the photoionized CGM.Comment: 22 pages, 7 figures. Submitted to AAS Journals, with minor
modifications. Comments welcome. (1) Co-first authors who made equal
contributions to this wor
Multiphase Gas and the Fractal Nature of Radiative Turbulent Mixing Layers
A common situation in galactic and intergalactic gas involves cold dense gas
in motion relative to hot diffuse gas. Kelvin-Helmholtz instability creates a
turbulent mixing layer and populates the intermediate-temperature phase, which
often cools rapidly. The energy lost to cooling is balanced by the advection of
hot high enthalpy gas into the mixing layer, resulting in growth and
acceleration of the cold phase. This process may play a major role in
determining the interstellar medium and circumgalactic medium phase structure,
and accelerating cold gas in galactic winds and cosmic filaments. Cooling in
these mixing layers occurs in a thin corrugated sheet, which we argue has an
area with fractal dimension and a thickness that adjusts to match the
hot phase mixing time to the cooling time. These cooling sheet properties form
the basis of a new model for how the cooling rate and hot gas inflow velocity
depend on the size , cooling time , relative velocity , and density contrast of the system.
Entrainment is expected to be enhanced in environments with short , large , and large . Using
a large suite of three dimensional hydrodynamic simulations, we demonstrate
that this fractal cooling layer model accurately captures the energetics and
evolution of turbulent interfaces and can therefore be used as a foundation for
understanding multiphase mixing with strong radiative cooling.Comment: 11 pages, 5 figures, submitted to ApJL. Movies can be found here
https://dfielding14.github.io/movies
Regulation of Star Formation by a Hot Circumgalactic Medium
Galactic outflows driven by supernovae (SNe) are thought to be a powerful
regulator of a galaxy's star-forming efficiency. Mass, energy, and metal
outflows (, , and , here normalized by the star
formation rate, the SNe energy and metal production rates, respectively) shape
galaxy properties by both ejecting gas and metals out of the galaxy and by
heating the circumgalactic medium (CGM), preventing future accretion.
Traditionally, models have assumed that galaxies self-regulate by ejecting a
large fraction of the gas which enters the interstellar medium (ISM), even
though such high mass-loadings are in growing tension with observations. To
better understand how the relative importance of ejective (i.e. high
mass-loading) vs preventative (i.e. high energy-loading) feedback affects the
present-day properties of galaxies, we develop a simple gas-regulator model of
galaxy evolution, where the stellar mass, ISM, and CGM are modeled as distinct
reservoirs which exchange mass, metals, and energy at different rates within a
growing halo. Focusing on the halo mass range from to , we demonstrate that, with reasonable parameter choices, we can
reproduce the stellar-to-halo mass relation and the ISM-to-stellar mass
relation with low mass-loaded () but high energy-loaded
() winds, with self-regulation occurring primarily through
heating and cooling of the CGM. We show that the model predictions are robust
against changes to the mass-loading of outflows but are quite sensitive to our
choice of the energy-loading, preferring for the lowest mass
halos and for Milky Way-like halos.Comment: 19 pages, 9 Figures, submitted to Ap
Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium
Large reservoirs of cold (~10⁴ K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmic-ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas