arXiv subjects

Ellen G. Zweibel

Publications and source records attributed to Ellen G. Zweibel.

At least 37 records · Page 2Linked to original sources

arXiv2019

Constraints on Cosmic Ray Transport in Galaxy Clusters from Radio and Gamma Ray Observations

The nature of cosmic rays (CRs) and cosmic ray transport in galaxy clusters is probed by a number of observations. Radio observations reveal the synchrotron radiation of cosmic ray electrons (CRe) spiraling around cluster magnetic fields. $γ$-ray observations reveal hadronic reactions of cosmic ray protons (CRp) with ambient gas nuclei which produce pions. To date, no such cluster-wide $γ$-ray signal has been measured, putting an upper limit on the density of CRp present in clusters. But the presence of CRe implies some source of CRp, and consequently there must be some CRp loss mechanism. In this paper we quantify the observational constraints on this loss mechanism assuming that losses are dominated by CR transport, ultimately deriving a minimum diffusion coefficient of $\sim10^{31}$ cm$^2$ s$^{-1}$ in the Coma cluster. This lower limit on transport may help illuminate some unknown properties of the cluster field topology. Conversely, measurements of cluster field tangling scales can constrain other model parameters, such as the relative acceleration efficiency of protons to electrons. To be consistent with the Coma observations, protons cannot be accelerated more than 15 times more efficiently than electrons of the same energy.

astro-ph.HE↗

arXiv2019

Cosmic Ray Acceleration of Cool Clouds in the Circumgalactic Medium

We investigate a mechanism for accelerating cool (10$^4$ K) clouds in the circumgalactic medium (CGM) with cosmic rays (CRs), possibly explaining some characteristics of observed high velocity clouds (HVCs). Enforcing CRs to stream down their pressure gradient into a region of slow streaming speed results in significant buildup of CR pressure which can accelerate the CGM. We present the results of the first two-dimensional magnetohydrodynamic (MHD) simulations of such `CR bottlenecks,' expanding on simpler simulations in 1D from \cite{wiener17a}. Although much more investigation is required, we find two main results. First, radiative cooling in the interfaces of these clouds is sufficient to keep the cloud intact to CR wave heating. Second, cloud acceleration depends almost linearly with the injected CR flux at low values (comparable to that expected from a Milky Way-like star formation rate), but scales sublinearly at higher CR fluxes in 1D simulations. 2D simulations show hints of sublinear dependence at high CR fluxes but are consistent with pure linear dependence up to the CR fluxes tested. It is therefore plausible to accelerate cool clouds in the CGM to speeds of hundreds of km s$^{-1}$.

astro-ph.GA↗

arXiv2018

SOFIA Far Infrared Imaging Polarimetry of M82 and NGC 253: Exploring the Super-Galactic Wind

We present Far-Infrared polarimetry observations of M82 at 53 and $154~μ\rm{m}$ and NGC 253 at $89~μ\rm{m}$, which were taken with HAWC+ in polarimetry mode on the Stratospheric Observatory for Infrared Astronomy (SOFIA). The polarization of M82 at $53~μ\rm{m}$ clearly shows a magnetic field geometry perpendicular to the disk in the hot dust emission. For M82 the polarization at $154~μ\rm{m}$ shows a combination of field geometry perpendicular to the disk in the nuclear region, but closer to parallel to the disk away from the nucleus. The fractional polarization at $53~μ\rm{m}$ $(154~μ\rm{m})$ ranges from 7% (3%) off nucleus to 0.5% (0.3%) near the nucleus. A simple interpretation of the observations of M82 invokes a massive polar outflow, dragging the field along, from a region $\sim 700$~pc in diameter that has entrained some of the gas and dust, creating a vertical field geometry seen mostly in the hotter $(53~μ\rm{m})$ dust emission. This outflow sits within a larger disk with a more typical planar geometry that more strongly contributes to the cooler $(154~μ\rm{m})$ dust emission. For NGC 253, the polarization at $89~μ\rm{m}$ is dominated by a planar geometry in the tilted disk, with weak indication of a vertical geometry above and below the plane from the nucleus. The polarization observations of NGC 253 at $53~μ\rm{m}$ were of insufficient S/N for detailed analysis.

astro-ph.GA↗

arXiv2018

Atomic and Ionized Microstructures in the Diffuse Interstellar Medium

It has been known for half a century that the interstellar medium (ISM) of our Galaxy is structured on scales as small as a few hundred km, more than 10 orders of magnitude smaller than typical ISM structures and energy input scales. In this review we focus on neutral and ionized structures on spatial scales of a few to ~10^4 Astronomical Units (AU) which appear to be highly overpressured, as these have the most important role in the dynamics and energy balance of interstellar gas: the Tiny Scale Atomic Structure (TSAS) and Extreme Scattering Events (ESEs) as the most over-pressured example of the Tiny Scale Ionized Structures (TSIS). We review observational results and highlight key physical processes at AU scales. We present evidence for and against microstructures as part of a universal turbulent cascade and as discrete structures, and review their association with supernova remnants, the Local Bubble, and bright stars. We suggest a number of observational and theoretical programs that could clarify the nature of AU structures. TSAS and TSIS probe spatial scales in the range of what is expected for turbulent dissipation scales, therefore are of key importance for constraining exotic and not-well understood physical processes which have implications for many areas of astrophysics. The emerging picture is one in which a magnetized, turbulent cascade, driven hard by a local energy source and acting jointly with phenomena such as thermal instability, is the source of these microstructures.

astro-ph.GA↗

arXiv2018

The Fate of Supernova-Heated Gas in Star-Forming Regions of the LMC: Lessons for Galaxy Formation?

Galactic winds and fountains driven by supernova-heated gas play an integral role in re-distributing gas in galaxies, depositing metals in the circumgalactic medium (CGM), and quenching star formation. The interplay between these outflows and ram pressure stripping due to the galaxy's motion through an ambient medium may enhance these effects by converting fountain flows into expelled gas. In this paper, we present controlled, 3D simulations of ram pressure stripping combined with thermally driven, local outflows from clustered supernovae in an isolated disk galaxy modeled on the Large Magellanic Cloud (LMC), a dwarf satellite of the Milky Way on its first infall. Observational evidence of local outflows emanating from supergiant shells in the LMC and a trailing filament of HI gas originating from these regions - with no obvious Leading Arm counterpart - may represent a perfect example of this process. Our simulations present a proof-of-concept that ram pressure can convert fountain flows into expelled gas. We find that fountains launched near the peak star formation time of the LMC can comprise part of the LMC filament in the Trailing Stream, but with lower column densities than observed. Larger, more numerous outflows from the LMC may be possible and may contribute more mass, but higher inertia gas will lengthen the timescale for this gas to be swept away by ram pressure. Given the high resolution observations, increased knowledge of star formation histories, and growing evidence of multiphase, ionized outflows, the LMC is an ideal test-bed for future wind models.

astro-ph.GA↗

arXiv2018

The Parker Instability with Cosmic Ray Streaming

Recent studies have found that cosmic ray transport plays an important role in feedback processes such as star formation and the launching of galactic winds. Although cosmic ray buoyancy is widely held to be a destabilizing force in galactic disks, the effect of cosmic ray transport on the stability of stratified systems has yet to be analyzed. We perform a stability analysis of a stratified layer for three different cosmic ray transport models: decoupled (Classic Parker), coupled with $γ_c=4/3$ but not streaming (Modified Parker), and finally coupled with streaming at the Alfvén speed. When the compressibility of the cosmic rays is decreased the system becomes much more stable, but the addition of cosmic ray streaming to the Parker Instability severely destabilizes it. Through comparison of these three cases and analysis of the work contributions for the perturbed quantities of each system, we demonstrate that cosmic ray heating of the gas is responsible for the destabilization of the system. We find that a 3D system is unstable over a larger range of wavelengths than the 2D system. Therefore, the Parker Instability with cosmic ray streaming may play an important role in cosmic ray feedback.

astro-ph.HE↗

arXiv2018

The Propagation of Cosmic Rays from the Galactic Wind Termination Shock: Back to the Galaxy?

Although several theories for the origin of cosmic rays in the region between the spectral `knee' and `ankle' exist, this problem is still unsolved. A variety of observations suggest that the transition from Galactic to extragalactic sources occurs in this energy range. In this work we examine whether a Galactic wind which eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al. (2017) estimated that particles can be accelerated up to energies above the `knee' up to $R_\mathrm{max} = 10^{16}$ eV for parameters drawn from a model of a Milky Way wind (Everett et al. 2017). A remaining question is whether the accelerated cosmic rays can propagate back into the Galaxy. To answer this crucial question, we simulate the propagation of the cosmic rays using the low energy extension of the CRPropa framework, based on the solution of the transport equation via stochastic differential equations. The setup includes all relevant processes, including three-dimensional anisotropic spatial diffusion, advection, and corresponding adiabatic cooling. We find that, assuming realistic parameters for the shock evolution, a possible Galactic termination shock can contribute significantly to the energy budget in the `knee' region and above. We estimate the resulting produced neutrino fluxes and find them to be below measurements from IceCube and limits by KM3NeT.

astro-ph.HE↗

arXiv2018

Acoustic Disturbances in Galaxy Clusters

Galaxy cluster cores are pervaded by hot gas which radiates at far too high a rate to maintain any semblance of a steady state; this is referred to as the cooling flow problem. Of the many heating mechanisms that have been proposed to balance radiative cooling, one of the most attractive is dissipation of acoustic waves generated by Active Galactic Nuclei (AGN). Fabian (2005) showed that if the waves are nearly adiabatic, wave damping due to heat conduction and viscosity must be well below standard Coulomb rates in order to allow the waves to propagate throughout the core. Because of the importance of this result, we have revisited wave dissipation under galaxy cluster conditions in a way that accounts for the self limiting nature of dissipation by electron thermal conduction, allows the electron and ion temperature perturbations in the waves to evolve separately, and estimates kinetic effects by comparing to a semi-collisionless theory. While these effects considerably enlarge the toolkit for analyzing observations of wavelike structures and developing a quantitative theory for wave heating, the drastic reduction of transport coefficients proposed in Fabian (2005) remains the most viable path to acoustic wave heating of galaxy cluster cores.

astro-ph.HE↗

arXiv2017

High $β$ Effects on Cosmic Ray Streaming in Galaxy Clusters

Diffuse, extended radio emission in galaxy clusters, commonly referred to as radio halos, indicate the presence of high energy cosmic ray (CR) electrons and cluster-wide magnetic fields. We can predict from theory the expected surface brightness of a radio halo, given magnetic field and CR density profiles. Previous studies have shown that the nature of CR transport can radically effect the expected radio halo emission from clusters (Wiener et al. 2013). Reasonable levels of magnetohydrodynamic (MHD) wave damping can lead to significant CR streaming speeds. But a careful treatment of MHD waves in a high $β$ plasma, as expected in cluster environments, reveals damping rates may be enhanced by a factor of $β^{1/2}$. This leads to faster CR streaming and lower surface brightnesses than without this effect. In this work we re-examine the simplified, 1D Coma cluster simulations (with radial magnetic fields) of Wiener et al. (2013) and discuss observable consequences of this high $β$ damping. Future work is required to study this effect in more realistic simulations.

astro-ph.HE↗

arXiv2017

Gamma-Ray Puzzles in Cygnus X: Implications for High-Energy Neutrinos

The Cygnus X region contains giant molecular cloud complexes and populous associates of massive young stars. The discovery of spatially extended, hard gamma-ray emission in Cygnus X by both Milagro and Fermi indicates that Cygnus X is also a potential source of high-energy Galactic neutrinos. Here, we adapt our single-zone model for cosmic ray interactions in the central molecular zones of starburst galaxies for use in Cygnus X. We calculate the potential neutrino flux corresponding to the hard gamma-ray emission from the "Cygnus Cocoon" and to the soft, diffuse interstellar gamma-ray emission. We check our results by comparing the corresponding gamma-ray emission against the Fermi interstellar emission model and Milagro, ARGO-YBJ, and HAWC observations. In comparing our results against a recent IceCube analysis and the current sensitivity limits, we find that neutrino emission from the Cocoon has a large enough flux that it could plausibly be detected, provided hadronic interactions are occurring at sufficiently high energies. High-energy neutrinos from Cygnus X would provide direct evidence for the presence of as yet unidentified PeV energy accelerators in the Galactic disk.

astro-ph.HE↗

arXiv2017

Detection of Extraplanar Diffuse Ionized Gas in M83

We present the first kinematic study of extraplanar diffuse ionized gas (eDIG) in the nearby, face-on disk galaxy M83 using optical emission-line spectroscopy from the Robert Stobie Spectrograph on the Southern African Large Telescope. We use a Markov Chain Monte Carlo method to decompose the [NII]$λλ$6548, 6583, H$α$, and [SII]$λλ$6717, 6731 emission lines into HII region and diffuse ionized gas emission. Extraplanar, diffuse gas is distinguished by its emission-line ratios ([NII]$λ$6583/H$α\gtrsim 1.0$) and its rotational velocity lag with respect to the disk ($Δv = -24$ km/s in projection). With interesting implications for isotropy, the velocity dispersion of the diffuse gas, $σ= 96$ km/s, is a factor of a few higher in M83 than in the Milky Way and nearby, edge-on disk galaxies. The turbulent pressure gradient is sufficient to support the eDIG layer in dynamical equilibrium at an electron scale height of $h_{z} = 1$ kpc. However, this dynamical equilibrium model must be finely tuned to reproduce the rotational velocity lag. There is evidence of local bulk flows near star-forming regions in the disk, suggesting that the dynamical state of the gas may be intermediate between a dynamical equilibrium and a galactic fountain flow. As one of the first efforts to study eDIG kinematics in a face-on galaxy, this study demonstrates the feasibility of characterizing the radial distribution, bulk velocities, and vertical velocity dispersions in low-inclination systems.

astro-ph.GA↗

arXiv2017

Interaction of Cosmic Rays with Cold Clouds in Galactic Halos

We investigate the effects of cosmic ray (CR) dynamics on cold, dense clouds embedded in a hot, tenuous galactic halo. If the magnetic field does not increase too much inside the cloud, the local reduction in Alfvén speed imposes a bottleneck on CRs streaming out from the star-forming galactic disk. The bottleneck flattens the upstream CR gradient in the hot gas, implying that multi-phase structure could have global effects on CR driven winds. A large CR pressure gradient can also develop on the outward-facing edge of the cloud. This pressure gradient has two independent effects. The CRs push the cloud upward, imparting it with momentum. On smaller scales, the CRs pressurize cold gas in the fronts, reducing its density, consistent with the low densities of cold gas inferred in recent COS observations of local $L_{*}$ galaxies. They also heat the material at the cloud edge, broadening the cloud-halo interface and causing an observable change in interface ionic abundances. Due to the much weaker temperature dependence of cosmic ray heating relative to thermal conductive heating, CR mediated fronts have a higher ratio of low to high ions compared to conduction fronts, in better agreement with observations. We investigate these effects separately using 1D simulations and analytic techniques.

astro-ph.HE↗

arXiv2016

Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks

There are two distinct breaks in the cosmic ray (CR) spectrum: the so-called "knee" around $3 \times 10^{15}$ eV and the so-called "ankle" around $10^{18}$ eV. Diffusive shock acceleration (DSA) at supernova remnant (SNR) shock fronts is thought to accelerate galactic CRs to energies below the knee, while an extragalactic origin is presumed for CRs with energies beyond the ankle. CRs with energies between $3 \times 10^{15}$ and $10^{18}$ eV, which we dub the "shin," have an unknown origin. It has been proposed that DSA at galactic wind termination shocks, rather than at SNR shocks, may accelerate CRs to these energies. This paper uses the galactic wind model of Bustard et al. (2016) to analyze whether galactic wind termination shocks may accelerate CRs to shin energies within a reasonable acceleration time and whether such CRs can subsequently diffuse back to the galaxy. We argue for acceleration times on the order of 100 Myrs rather than a few billion years, as assumed in some previous works, and we discuss prospects for magnetic field amplification at the shock front. Ultimately, we generously assume that the magnetic field is amplified to equipartition. This formalism allows us to obtain analytic formulae, applicable to any wind model, for CR acceleration. Even with generous assumptions, we find that very high wind velocities are required to set up the necessary conditions for acceleration beyond $10^{17}$ eV. We also estimate the luminosities of CRs accelerated by outflow termination shocks, including estimates for the Milky Way wind.

astro-ph.HE↗

arXiv2016

Testing a Dynamical Equilibrium Model of the Extraplanar Diffuse Ionized Gas in NGC 891

The observed scale heights of extraplanar diffuse ionized gas (eDIG) layers exceed their thermal scale heights by a factor of a few in the Milky Way and other nearby edge-on disk galaxies. Here, we test a dynamical equilibrium model of the extraplanar diffuse ionized gas layer in NGC 891, where we ask whether the thermal, turbulent, magnetic field, and cosmic ray pressure gradients are sufficient to support the layer. In optical emission line spectroscopy from the SparsePak integral field unit on the WIYN 3.5-meter telescope, the H-alpha emission in position-velocity space suggests that the eDIG is found in a ring between galactocentric radii of R_min <= R <= 8 kpc, where R_min >= 2 kpc. We find that the thermal (sigma_th = 11 km/s) and turbulent (sigma_turb = 25 km/s) velocity dispersions are insufficient to satisfy the hydrostatic equilibrium equation given an exponential electron scale height of h_z = 1.0 kpc. Using a literature analysis of radio continuum observations from the CHANG-ES survey, we demonstrate that the magnetic field and cosmic ray pressure gradients are sufficient to stably support the gas at R >= 8 kpc if the cosmic rays are sufficiently coupled to the system (gamma_cr = 1.45). Thus, a stable dynamical equilibrium model is viable only if the extraplanar diffuse ionized gas is found in a thin ring around R = 8 kpc, and non-equilibrium models such as a galactic fountain flow are of interest for further study.

astro-ph.GA↗

arXiv2016

Detection of a ~20 kpc coherent magnetic field in the outskirt of merging spirals: the Antennae galaxies

We present a study of the magnetic field properties of NGC 4038/9 (the `Antennae' galaxies), the closest example of a late stage merger of two spiral galaxies. Wideband polarimetric observations were performed using the Karl G. Jansky Very Large Array between 2 and 4 GHz. Rotation measure synthesis and Faraday depolarization analysis was performed to probe the magnetic field strength and structure at spatial resolution of $\sim1$ kpc. Highly polarized emission from the southern tidal tail is detected with intrinsic fractional polarization close to the theoretical maximum ($0.62\pm0.18$), estimated by fitting the Faraday depolarization with a volume that is both synchrotron emitting and Faraday rotating containing random magnetic fields. Magnetic fields are well aligned along the tidal tail and the Faraday depths shows large-scale smooth variations preserving its sign. This suggests the field in the plane of the sky to be regular up to $\sim20$ kpc, which is the largest detected regular field structure on galactic scales. The equipartition field strength of $\sim8.5~μ$G of the regular field in the tidal tail is reached within a few 100 Myr, likely generated by stretching of the galactic disc field by a factor of 4--9 during the tidal interaction. The regular field strength is greater than the turbulent fields in the tidal tail. Our study comprehensively demonstrates, although the magnetic fields within the merging bodies are dominated by strong turbulent magnetic fields of $\sim20~μ$G in strength, tidal interactions can produce large-scale regular field structure in the outskirts.

astro-ph.GA↗

arXiv2016

A Versatile Family of Galactic Wind Models

We present a versatile family of model galactic outflows including non-uniform mass and energy source distributions, a gravitational potential from an extended mass source, and radiative losses. The model easily produces steady-state wind solutions for a range of mass-loading factors, energy-loading factors, galaxy mass and galaxy radius. We find that, with radiative losses included, highly mass-loaded winds must be driven at high central temperatures, whereas low mass-loaded winds can be driven at low temperatures just above the peak of the cooling curve, meaning radiative losses can drastically affect the wind solution even for low mass-loading factors. By including radiative losses, we are able to show that subsonic flows can be ignored as a possible mechanism for expelling mass and energy from a galaxy compared to the more efficient transonic solutions. Specifically, the transonic solutions with low mass-loading and high energy-loading are the most efficient. Our model also produces low-temperature, high-velocity winds that could explain the prevalence of low-temperature material in observed outflows. Finally, we show that our model, unlike the well-known Chevalier & Clegg (1985) model, can reproduce the observed linear relationship between wind X-ray luminosity and star formation rate (SFR) over a large range of SFR from $1-1000$ M$_{\odot}$/yr assuming the wind mass-loading factor is higher for low-mass, and hence, low-SFR galaxies. We also constrain the allowed mass-loading factors that can fit the observed X-ray luminosity vs. SFR trend, further suggesting an inverse relationship between mass-loading and SFR as explored in advanced numerical simulations.

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arXiv2015

Effects of Line-tying on Resistive Tearing Instability in Slab Geometry

The effects of line-tying on resistive tearing instability in slab geometry is studied within the framework of reduced magnetohydrodynamics (RMHD).\citep{KadomtsevP1974,Strauss1976} It is found that line-tying has a stabilizing effect. The tearing mode is stabilized when the system length $L$ is shorter than a critical length $L_{c}$, which is independent of the resistivity $η$. When $L$ is not too much longer than $L_{c}$, the growthrate $γ$ is proportional to $η$ . When $L$ is sufficiently long, the tearing mode scaling $γ\simη^{3/5}$ is recovered. The transition from $γ\simη$ to $γ\simη^{3/5}$ occurs at a transition length $L_{t}\simη^{-2/5}$.

physics.plasm-ph↗

arXiv2015

Equipartition and Cosmic Ray Energy Densities in Central Molecular Zones of Starbursts

The energy densities in magnetic fields and cosmic rays (CRs) in galaxies are often assumed to be in equipartition, allowing for an indirect estimate of the magnetic field strength from the observed radio synchrotron spectrum. However, both primary and secondary CRs contribute to the synchrotron spectrum, and the CR electrons also loose energy via bremsstrahlung and inverse Compton. While classical equipartition formulae avoid these intricacies, there have been recent revisions that account for the extreme conditions in starbursts. Yet, the application of the equipartition formula to starburst environments also presupposes that timescales are long enough to reach equilibrium. Here, we test equipartition in the central molecular zones (CMZs) of nearby starburst galaxies by modeling the observed gamma-ray spectra, which provide a direct measure of the CR energy density, and the radio spectra, which provide a probe of the magnetic field strength. We find that in starbursts, the magnetic field energy density is significantly larger than the CR energy density, demonstrating that the equipartition argument is frequently invalid for CMZs.

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