arXiv subjects

Eliot Quataert

Publications and source records attributed to Eliot Quataert.

At least 145 records · Page 8Linked to original sources

arXiv2021

A Validated Nonlinear Kelvin-Helmholtz Benchmark for Numerical Hydrodynamics

The nonlinear evolution of the Kelvin-Helmholtz instability is a popular test for code verification. To date, most Kelvin-Helmholtz problems discussed in the literature are ill-posed: they do not converge to any single solution with increasing resolution. This precludes comparisons among different codes and severely limits the utility of the Kelvin-Helmholtz instability as a test problem. The lack of a reference solution has led various authors to assert the accuracy of their simulations based on ad-hoc proxies, e.g., the existence of small-scale structures. This paper proposes well-posed Kelvin-Helmholtz problems with smooth initial conditions and explicit diffusion. We show that in many cases numerical errors/noise can seed spurious small-scale structure in Kelvin-Helmholtz problems. We demonstrate convergence to a reference solution using both Athena, a Godunov code, and Dedalus, a pseudo-spectral code. Problems with constant initial density throughout the domain are relatively straightforward for both codes. However, problems with an initial density jump (which are the norm in astrophysical systems) exhibit rich behavior and are more computationally challenging. In the latter case, Athena simulations are prone to an instability of the inner rolled-up vortex; this instability is seeded by grid-scale errors introduced by the algorithm, and disappears as resolution increases. Both Athena and Dedalus exhibit late-time chaos. Inviscid simulations are riddled with extremely vigorous secondary instabilities which induce more mixing than simulations with explicit diffusion. Our results highlight the importance of running well-posed test problems with demonstrated convergence to a reference solution. To facilitate future comparisons, we include the resolved, converged solutions to the Kelvin-Helmholtz problems in this paper in machine-readable form.

astro-ph.IM↗

arXiv2021

The Effects of Tilt on the Time Variability of Millimeter and Infrared Emission from Sagittarius A*

Using a combination of general-relativistic magnetohydrodynamics simulations and ray tracing of synchrotron emission, we study the effect of modest (24 degrees) misalignment between the black hole spin and plasma angular momentum, focusing on the variability of total flux, image centroids, and image sizes. We consider both millimeter and infrared (IR) observables motivated by Sagittarius A* (Sgr A*), though our results apply more generally to optically thin flows. For most quantities, tilted accretion is more variable, primarily due to a significantly hotter and denser "coronal" region well off the disk midplane. We find (1) a 150% increase in millimeter light curve variability when adding tilt to the flow; (2) the tilted image centroid in the millimeter shifts on a scale of 3.7 microarcseconds over 28 hours (5000 gravitational times) for some electron temperature models; (3) tilted disk image diameters in the millimeter can be 10% larger (52 versus 47 microarcseconds) than those of aligned disks at certain viewing angles; (4) the tilted models produce significant IR flux, similar to that seen in Sgr A*, with comparable or even greater variability than observed; (5) for some electron models, the tilted IR centroid moves by more than 50 microarcseconds over several hours, in a similar fashion to the centroid motion detected by the GRAVITY interferometer.

astro-ph.HE↗

arXiv2021

Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs

We present a systematic survey for mass-transferring and recently-detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low-mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia color-magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period $P_{\rm orb} < 6$ hr. This yields 51 candidates brighter than G=18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely -- or nearly -- Roche lobe filling close binaries. 13 show evidence of ongoing mass transfer, which has likely just ceased in the other 8. Most of the secondaries are hotter than any previously known CV donors, with temperatures $4700<T_{\rm eff}/{\rm K}<8000$. Remarkably, all secondaries with $T_{\rm eff} \gtrsim 7000\,\rm K$ appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donor's convective envelope. Most of the proto-WD secondaries have masses near $0.15\,M_{\odot}$; their companions have masses near $0.8\,M_{\odot}$. We infer a space density of $\sim 60\,\rm kpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, $\mathcal{R}\sim 60\,\rm Myr^{-1}$, is half that of AM CVn binaries. Most systems are well-described by MESA models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods $5\lesssim P_{\rm orb}/{\rm min}\lesssim30$ within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population.

astro-ph.SR↗

arXiv2021

Thermal Electrons in Mildly-relativistic Synchrotron Blast-waves

Numerical models of collisionless shocks robustly predict an electron distribution comprised of both thermal and non-thermal electrons. Here, we explore in detail the effect of thermal electrons on the emergent synchrotron emission from sub-relativistic shocks. We present a complete `thermal + non-thermal' synchrotron model and derive properties of the resulting spectrum and light-curves. Using these results we delineate the relative importance of thermal and non-thermal electrons for sub-relativistic shock-powered synchrotron transients. We find that thermal electrons are naturally expected to contribute significantly to the peak emission if the shock velocity is $\gtrsim 0.2c$, but would be mostly undetectable in non-relativistic shocks. This helps explain the dichotomy between typical radio supernovae and the emerging class of `AT2018cow-like' events. The signpost of thermal electron synchrotron emission is a steep optically-thin spectral index and a $ν^2$ optically-thick spectrum. These spectral features are also predicted to correlate with a steep post-peak light-curve decline rate, broadly consistent with observed AT2018cow-like events. We expect that thermal electrons may be observable in other contexts where mildly-relativistic shocks are present, and briefly estimate this effect for gamma-ray burst afterglows and binary neutron star mergers. Our model can be used to fit spectra and light-curves of events and accounts for both thermal and non-thermal electron populations with no additional physical degrees of freedom.

astro-ph.HE↗

arXiv2021

Characterizing mass, momentum, energy and metal outflow rates of multi-phase galactic winds in the FIRE-2 cosmological simulations

We characterize mass, momentum, energy and metal outflow rates of multi-phase galactic winds in a suite of FIRE-2 cosmological "zoom-in" simulations from the Feedback in Realistic Environments (FIRE) project. We analyze simulations of low-mass dwarfs, intermediate-mass dwarfs, Milky Way-mass halos, and high-redshift massive halos. Consistent with previous work, we find that dwarfs eject about 100 times more gas from their interstellar medium (ISM) than they form in stars, while this mass "loading factor" drops below one in massive galaxies. Most of the mass is carried by the hot phase ($>10^5$ K) in massive halos and the warm phase ($10^3-10^5$ K) in dwarfs; cold outflows ($<10^3$ K) are negligible except in high-redshift dwarfs. Energy, momentum and metal loading factors from the ISM are of order unity in dwarfs and significantly lower in more massive halos. Hot outflows have $2-5\times$ higher specific energy than needed to escape from the gravitational potential of dwarf halos; indeed, in dwarfs, the mass, momentum, and metal outflow rates increase with radius whereas energy is roughly conserved, indicating swept up halo gas. Burst-averaged mass loading factors tend to be larger during more powerful star formation episodes and when the inner halo is not virialized, but we see effectively no trend with the dense ISM gas fraction. We discuss how our results can guide future controlled numerical experiments that aim to elucidate the key parameters governing galactic winds and the resulting associated preventative feedback.

astro-ph.GA↗

arXiv2021

Novae heat their food: mass transfer by irradiation

A nova eruption irradiates and heats the donor star in a cataclysmic variable to high temperatures $T_{\rm irr}$, causing its outer layers to expand and overflow the Roche lobe. We calculate the donor's heating and expansion both analytically and numerically, under the assumption of spherical symmetry, and find that irradiation drives enhanced mass transfer from the donor at a rate $\dot{m}\propto T_{\rm irr}^{5/3}$, which reaches $\dot{m}\sim 10^{-6}\textrm{ M}_\odot\textrm{ yr}^{-1}$ at the peak of the eruption - about a thousand times faster than during quiescence. As the nova subsides and the white dwarf cools down, $\dot{m}$ drops to lower values. We find that under certain circumstances, the decline halts and the mass transfer persists at a self-sustaining rate of $\dot{m}\sim 10^{-7}\textrm{ M}_\odot\textrm{ yr}^{-1}$ for up to $\sim 10^3$ yr after the eruption. At this rate, irradiation by the white dwarf's accretion luminosity is sufficient to drive the mass transfer on its own. The self-sustaining rate is close to the white dwarf's stable burning limit, such that this bootstrapping mechanism can simultaneously explain two classes of puzzling binary systems: recurrent novae with orbital periods $\approx 2$ h (T Pyxidis and IM Normae) and long-lived supersoft X-ray sources with periods $\approx 4$ h (RX J0537.7-7034 and 1E 0035.4-7230). Whether or not a system reaches the self-sustaining state is sensitive to the donor's chromosphere structure, as well as to the orbital period change during nova eruptions.

astro-ph.SR↗

arXiv2021

The impact of r-process heating on the dynamics of neutron star merger accretion disc winds and their electromagnetic radiation

Neutron star merger accretion discs can launch neutron-rich winds of $>10^{-2}\,\mathrm{M}_\odot$. This ejecta is a prime site for r-process nucleosynthesis, which will produce a range of radioactive heavy nuclei. The decay of these nuclei releases enough energy to accelerate portions of the wind by ~0.1c. Here, we investigate the effect of r-process heating on the dynamical evolution of disc winds. We extract the wind from a 3D general relativistic magnetohydrodynamic simulation of a disc from a post-merger system. This is used to create inner boundary conditions for 2D hydrodynamic simulations that continue the original 3D simulation. We perform two such simulations: one that includes the r-process heating, and another one that does not. We follow the hydrodynamic simulations until the winds reach homology (60 seconds). Using time-dependent multi-frequency multi-dimensional Monte Carlo radiation transport simulations, we then calculate the kilonova light curves from the winds with and without dynamical r-process heating. We find that the r-process heating can substantially alter the velocity distribution of the wind, shifting the mass-weighted median velocity from 0.06c to 0.12c. The inclusion of the dynamical r-process heating makes the light curve brighter and bluer at ~1 d post-merger. However, the high-velocity tail of the ejecta distribution and the early light curves are largely unaffected.

astro-ph.HE↗

arXiv2021

Thermal Instability in the CGM of $L_{\star}$ Galaxies: Testing "Precipitation" Models with the FIRE Simulations

We examine the thermodynamic state and cooling of the low-$z$ Circum-Galactic Medium (CGM) in five FIRE-2 galaxy formation simulations of Milky Way-mass galaxies. We find that the CGM in these simulations is generally multiphase and dynamic, with a wide spectrum of largely nonlinear density perturbations sourced by the accretion of gas from the Inter-Galactic Medium (IGM) and outflows from both the central and satellite galaxies. We investigate the origin of the multiphase structure of the CGM with a particle tracking analysis and find that most of the low entropy gas has cooled from the hot halo as a result of thermal instability triggered by these perturbations. The ratio of cooling to free-fall timescales $t_{\rm cool}/t_{\rm ff}$ in the hot component of the CGM spans a wide range $\sim 1-100$ at a given radius, but exhibits approximately constant median values $\sim 5-20$ at all radii $0.1 R_{\rm vir} < r < R_{\rm vir}$. These are similar to the $\approx 10-20$ value typically adopted as the thermal instability threshold in ``precipitation'' models of the ICM. Consequently, a one-dimensional model based on the assumption of a constant $t_{\rm cool}/t_{\rm ff}$ and hydrostatic equilibrium approximately reproduces the number density and entropy profiles of each simulation, but only if it assumes the metallicity profile and temperature boundary condition taken directly from the simulation. We explicitly show that the $t_{\rm cool}/t_{\rm ff}$ value of a gas parcel in the hot component of the CGM does not predict its probability of subsequently accreting onto the central galaxy. This suggests that the value of $t_{\rm cool}/t_{\rm ff}$ is a poor predictor of thermal stability in gaseous halos in which large-amplitude density perturbations are prevalent.

astro-ph.GA↗

arXiv2021

Neutral CGM as damped Lyα absorbers at high redshift

Recent searches for the hosts of high-redshift ($z \sim 4$) damped Ly$α$ absorbers (DLAs) have detected bright galaxies at distances of tens of kpc from the DLA. Using the FIRE-2 cosmological zoom simulations, we argue that these relatively large distances are due to a predominantly cool and neutral inner circumgalactic medium (CGM) surrounding high-redshift galaxies. The inner CGM is cool because of the short cooling time of hot gas in $\lesssim10^{12}$ Msun halos, which implies that accretion and feedback energy are radiated quickly, while it is neutral due to the high volume densities and column densities at high redshift which shield cool gas from photoionization. Our analysis predicts large DLA covering factors ($\gtrsim50\%$) out to impact parameters $\sim0.3((1 + z)/5)^{3/2}\ R_{\rm vir}$ from the central galaxies at $z > 1$, equivalent to a physical distance of $\sim 21 M_{12}^{1/3} ((1 + z)/5)^{1/2}$ kpc ($R_{\rm vir}$ and $M_{12}$ are the halo virial radius and mass in units of $10^{12}$ Msun, respectively). This implies that DLA covering factors at $z \sim 4$ may be comparable to unity out to a distance $\sim 10$ times larger than stellar half-mass radii. A predominantly neutral inner CGM in the early universe suggests that its mass and metallicity can be directly constrained by CGM absorption surveys, without resorting to large ionization corrections as required for ionized CGM.

astro-ph.GA↗

arXiv2021

The Physics of Galactic Winds Driven by Cosmic Rays II: Isothermal Streaming Solutions

We use analytic calculations and time-dependent spherically-symmetric simulations to study the properties of isothermal galactic winds driven by cosmic-rays (CRs) streaming at the Alfvén velocity. The simulations produce time-dependent flows permeated by strong shocks; we identify a new linear instability of sound waves that sources these shocks. The shocks substantially modify the wind dynamics, invalidating previous steady state models: the CR pressure $p_c$ has a staircase-like structure with $dp_c/dr \simeq 0$ in most of the volume, and the time-averaged CR energetics are in many cases better approximated by $p_c \propto ρ^{1/2}$, rather than the canonical $p_c \propto ρ^{2/3}$. Accounting for this change in CR energetics, we analytically derive new expressions for the mass-loss rate, momentum flux, wind speed, and wind kinetic power in galactic winds driven by CR streaming. We show that streaming CRs are ineffective at directly driving cold gas out of galaxies, though CR-driven winds in hotter ISM phases may entrain cool gas. For the same physical conditions, diffusive CR transport (Paper I) yields mass-loss rates that are a few-100 times larger than streaming transport, and asymptotic wind powers that are a factor of $\simeq 4$ larger. We discuss the implications of our results for galactic wind theory and observations; strong shocks driven by CR-streaming-induced instabilities produce gas with a wide range of densities and temperatures, consistent with the multiphase nature of observed winds. We also quantify the applicability of the isothermal gas approximation for modeling streaming CRs and highlight the need for calculations with more realistic thermodynamics.

astro-ph.GA↗

arXiv2021

Cosmological simulations of quasar fueling to sub-parsec scales using Lagrangian hyper-refinement

We present cosmological hydrodynamic simulations of a quasar-mass halo ($M_{\rm halo} \approx 10^{12.5}\,{\rm M}_{\odot}$ at z=2) that for the first time resolve gas transport down to the inner 0.1 pc surrounding the central massive black hole. We model a multi-phase interstellar medium including stellar feedback by supernovae, stellar winds, and radiation, and a hyper-Lagrangian refinement technique increasing the resolution dynamically approaching the black hole. We do not include black hole feedback. We show that the sub-pc inflow rate (1) can reach ~6 M$_{\odot}$yr$^{-1}$ roughly in steady state during the epoch of peak nuclear gas density (z~2), sufficient to power a luminous quasar, (2) is highly time variable in the pre-quasar phase, spanning 0.001-10 M$_{\odot}$yr$^{-1}$ on Myr timescales, and (3) is limited to short (~2 Myr) active phases (0.01-0.1 M$_{\odot}$yr$^{-1}$) followed by longer periods of inactivity at lower nuclear gas density and late times (z~1), owing to the formation of a hot central cavity. Inflowing gas is primarily cool, rotational support dominates over turbulence and thermal pressure, and star formation can consume as much gas as provided by inflows across 1 pc - 10 kpc. Gravitational torques from multi-scale stellar non-axisymmetries dominate angular momentum transport over gas self-torquing and pressure gradients, with accretion weakly dependent on black hole mass. Sub-pc inflow rates correlate with nuclear (but decouple from global) star formation and can exceed the Eddington rate by x10. The black hole can move ~10 pc from the galaxy center on ~0.1 Myr. Accreting gas forms pc-scale, rotationally supported, obscuring structures often misaligned with the galaxy-scale disk. These simulations open a new avenue to investigate black hole-galaxy co-evolution.

astro-ph.GA↗

arXiv2021

Surface Manifestation of Stochastically Excited Internal Gravity Waves

Recent photometric observations of massive stars show ubiquitous low-frequency "red-noise" variability, which has been interpreted as internal gravity waves (IGWs). Simulations of IGWs generated by convection show smooth surface wave spectra, qualitatively matching the observed red-noise. On the other hand, theoretical calculations by Shiode et al (2013) and Lecoanet et al (2019) predict IGWs should manifest at the surface as regularly-spaced peaks associated with standing g-modes. In this work, we compare these theoretical approaches to simplified 2D numerical simulations. The simulations show g-mode peaks at their surface, and are in good agreement with Lecoanet et al (2019). The amplitude estimates of Shiode et al (2013) did not take into account the finite width of the g-mode peaks; after correcting for this finite width, we find good agreement with simulations. However, simulations need to be run for hundreds of convection turnover times for the peaks to become visible; this is a long time to run a simulation, but a short time in the life of a star. The final spectrum can be predicted by calculating the wave energy flux spectrum in much shorter simulations, and then either applying the theory of Shiode et al (2013) or Lecoanet et al (2019).

astro-ph.SR↗

arXiv2021

LAMOST J0140355+392651: An evolved cataclysmic variable donor transitioning to become an extremely low mass white dwarf

We present LAMOST J0140355+392651 (hereafter J0140), a close ($P_{\rm orb} = 3.81$ hours) binary containing a bloated, low-mass ($M \approx 0.15 M_{\odot}$) proto-white dwarf (WD) and a massive ($M\approx 0.95\,M_{\odot}$) WD companion. The system's optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler ($T_{\rm eff} = 6800\pm 100$ K) and more puffy ($\log\left[g/\left({\rm cm\,s^{-2}}\right)\right]=4.74\pm0.07$) than any known extremely low mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe ($R/R_{\rm Roche\,lobe}=0.99\pm0.01$), suggesting ongoing or recently terminated mass transfer. No dwarf nova-like outbursts have been observed. The spectrum is dominated by the proto-WD but shows tentative hints of H$α$ emission, perhaps due to accretion onto the massive WD. The properties of the system are well-matched by MESA binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving toward higher temperatures at near-constant luminosity to become an ELM WD. If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient. Evolutionary models predict that the binary will shrink to $P_{\rm orb}\lesssim 10$ minutes within a few Gyr, when it will either merge or become an AM CVn binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems.

astro-ph.SR↗

arXiv2021

The contribution of globular clusters to cosmic reionization

We study the escape fraction of ionizing photons (f_esc) in two cosmological zoom-in simulations of galaxies in the reionization era with halo mass M_halo~10^10 and 10^11 M_sun (stellar mass M*~10^7 and 10^9 M_sun) at z=5 from the Feedback in Realistic Environments project. These simulations explicitly resolve the formation of proto-globular clusters (GCs) self-consistently, where 17-39% of stars form in bound clusters during starbursts. Using post-processing Monte Carlo radiative transfer calculations of ionizing radiation, we compute f_esc from cluster stars and non-cluster stars formed during a starburst over ~100 Myr in each galaxy. We find that the averaged f_esc over the lifetime of a star particle follows a similar distribution for cluster stars and non-cluster stars. Clusters tend to have low f_esc in the first few Myrs, presumably because they form preferentially in more extreme environments with high optical depths; the f_esc increases later as feedback starts to disrupt the natal cloud. On the other hand, non-cluster stars formed between cluster complexes or in the compressed shell at the front of a superbubble can also have high f_esc. We find that cluster stars on average have comparable f_esc to non-cluster stars. This result is robust across several star formation models in our simulations. Our results suggest that the fraction of ionizing photons from proto-GCs to cosmic reionization is comparable to the cluster formation efficiency in high-redshift galaxies and hence proto-GCs likely contribute an appreciable fraction of photons but are not the dominant sources for reionization.

astro-ph.GA↗

arXiv2021

Suppressed heat conductivity in the intracluster medium: implications for the magneto-thermal instability

In the outskirts of the intracluster medium (ICM) in galaxy clusters, the temperature decreases with radius. Due to the weakly collisional nature of the plasma, these regions are susceptible to the magneto-thermal instability (MTI), which can sustain turbulence and provide turbulent pressure support in the ICM. This instability arises due to heat conduction directed along the magnetic field, with a heat conductivity which is normally assumed to be given by the Spitzer value. Recent numerical studies of the ion mirror and the electron whistler instability using particle-in-cell codes have shown that microscale instabilities can lead to a reduced value for the heat conductivity in the ICM. This could in turn influence the efficiency with which the MTI drives turbulence. In this paper we investigate the influence of reduced heat transport on the nonlinear evolution of the MTI. We study plane-parallel, initially static atmospheres and employ a subgrid model that mimics the influence of the mirror instability on the heat conductivity. We use this subgrid model to assess the effect of microscales on the large scale dynamics of the ICM. We find that the nonlinear saturation of the MTI is surprisingly robust in our simulations. Over a factor of $\sim 10^3$ in the thermal-to-magnetic pressure ratio and collisionality we find at most modest changes to the saturation of the MTI with respect to reference simulations where heat transport is unsuppressed.

astro-ph.CO↗

arXiv2021

The bursty origin of the Milky Way thick disc

We investigate thin and thick stellar disc formation in Milky-Way-mass galaxies using twelve FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at $z=0$, whilst stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at $z=0$ correlates strongly with this transition time. We also find that galaxies with an earlier transition from bursty to steady star formation have a higher thin-disc fractions at $z=0$. Three of our systems have minor mergers with LMC-size satellites during the thin-disc phase. These mergers trigger short starbursts but do not destroy the thin disc nor alter broad trends between the star formation transition time and thin/thick disc properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way (or M31) provide a window into past star-formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that the Milky Way transitioned from bursty to steady phase $\sim$6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognisable thin disc.

astro-ph.GA↗

arXiv2021

The Physics of Galactic Winds Driven by Cosmic Rays I: Diffusion

The physics of Cosmic ray (CR) transport remains a key uncertainty in assessing whether CRs can produce galaxy-scale outflows consistent with observations. In this paper, we elucidate the physics of CR-driven galactic winds for CR transport dominated by diffusion. A companion paper considers CR streaming. We use analytic estimates validated by time-dependent spherically-symmetric simulations to derive expressions for the mass-loss rate, momentum flux, and speed of CR-driven galactic winds, suitable for cosmological-scale or semi-analytic models of galaxy formation. For CR diffusion coefficients $κ\gtrsim r_0 c_i$ where $r_0$ is the base radius of the wind and $c_i$ is the isothermal gas sound speed, the asymptotic wind energy flux is comparable to that supplied to CRs, and the outflow rapidly accelerates to supersonic speeds. By contrast, for $κ\lesssim r_0 c_i$, CR-driven winds accelerate more slowly and lose most of their energy to gravity, a CR analogue of photon-tired stellar winds. Given CR diffusion coefficients estimated using Fermi gamma-ray observations of pion decay, we predict mass-loss rates in CR-driven galactic winds of order the star formation rate for dwarf and disc galaxies. The dwarf galaxy mass-loss rates are small compared to the mass-loadings needed to reconcile the stellar and dark matter halo mass functions. For nuclear starbursts (e.g., M82, Arp 220), CR diffusion and pion losses suppress the CR pressure in the galaxy and the strength of CR-driven winds. We discuss the implications of our results for interpreting observations of galactic winds and for the role of CRs in galaxy formation.

astro-ph.GA↗

arXiv2021

Magnetically Modified Spherical Accretion in GRMHD: Reconnection-Driven Convection and Jet Propagation

We present 3D general relativistic magnetohydrodynamic(GRMHD) simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider serveral angles between the magnetic field direction and the black hole spin. In the resulting flows, the midplane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ~10-200% occupy the polar regions, reaching several hundred gravitational radii before they dissipate due to the kink instability. The jet directions fluctuate in time and can be tilted by as much as ~30 degrees with respect to black hole spin, but this tilt does not depend strongly on the tilt of the initial magnetic field. A jet forms even when there is no initial net vertical magnetic flux since turbulent, horizon-scale fluctuations can generate a net vertical field locally. Peak jet power is obtained for an initial magnetic field tilted by 40-80 degrees with respect to the black hole spin because this maximizes the amount of magnetic flux that can reach the black hole. These simulations may be a reasonable model for low luminosity black hole accretion flows such as Sgr A* or M87.

astro-ph.HE↗