Showing papers by "Eliot Quataert published in 2020"

Synthetic Gaia surveys from the FIRE cosmological simulations of Milky-Way-mass galaxies

Abstract: With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing observations to simulations of Milky-Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and we use ananke to generate a suite of synthetic surveys designed to resemble Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky-Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments (FIRE) project, which offer many advantages for generating synthetic stellar surveys: self-consistent clustering of star formation in dense molecular clouds, thin stellar and gaseous disks, cosmological accretion and enrichment histories, all in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We generate synthetic stars assuming that each simulation’s star particles (of mass 7070 M_⊙ ) represent a single stellar population, and we use a kernel density representation to distribute synthetic stars accurately in position and velocity. At each viewpoint, we compute a self-consistent dust extinction map, using the gas metallicity distribution in each simulation. Finally, we apply a simple error model to produce a synthetic Gaia-like survey at each solar viewpoint, though we also provide quantities without error convolution. This results in a catalog of synthetic stars, as if measured by Gaia, that includes both observational properties and a pointer to each generating star particle in the simulation. We also provide the complete snapshot–including star, gas, and dark matter particles–at z = 0 for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades to ananke. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data.

Ab Initio Horizon-scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds

Abstract: We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.

Ab Initio Horizon-Scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds

Abstract: We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.

The Effects of Tilt on the Images of Black Hole Accretion Flows

Abstract: We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent difference is that the central shadow in the image is noticeably eccentric in tilted models, with the ring of emission relatively unchanged. Applying this procedure to M87 with a viewing angle based on the large-scale jet, we find that adding tilt increases the angular size of the ring for fixed black hole mass and distance, while at the same time increasing the number of bright spots in the image. Our findings illustrate observable features that can distinguish tilted from aligned flows. They also show that tilted models can be viable for M87, and that not accounting for tilt can bias inferences of physical parameters. Future modeling of horizon-scale observations should account for potential angular momentum misalignment, which is likely generic at the low accretion rates appropriate for EHT targets.

Direct Detection of Black Hole-driven Turbulence in the Centers of Galaxy Clusters

Abstract: Supermassive black holes (SMBHs) are thought to provide energy that prevents catastrophic cooling in the centers of massive galaxies and galaxy clusters. However, it remains unclear how this "feedback" process operates. We use high-resolution optical data to study the kinematics of multi-phase filamentary structures by measuring the velocity structure function (VSF) of the filaments over a wide range of scales in the centers of three nearby galaxy clusters: Perseus, Abell 2597 and Virgo. We find that the motions of the filaments are turbulent in all three clusters studied. There is a clear correlation between features of the VSFs and the sizes of bubbles inflated by SMBH driven jets. Our study demonstrates that SMBHs are the main driver of turbulent gas motions in the centers of galaxy clusters and suggests that this turbulence is an important channel for coupling feedback to the environment. Our measured amplitude of turbulence is in good agreement with Hitomi Doppler line broadening measurement and X-ray surface brightness fluctuation analysis, suggesting that the motion of the cold filaments is well-coupled to that of the hot gas. The smallest scales we probe are comparable to the mean free path in the intracluster medium (ICM). Our direct detection of turbulence on these scales provides the clearest evidence to date that isotropic viscosity is suppressed in the weakly-collisional, magnetized intracluster plasma.

The Impact of Type Ia Supernovae in Quiescent Galaxies. II. Energetics and Turbulence

Abstract: Type Ia supernovae (SNe Ia) provide unique and important feedback in quiescent galaxies, but their impact has been underappreciated. In this paper, we analyze a series of high-resolution simulations to examine the energetics and turbulence of the medium under SNe Ia. We find that when SN remnants are resolved, their effects differ distinctly from a volumetric heating term, as is commonly assumed in unresolved simulations. First, the net heating is significantly higher than expected, by 30$\pm$10\% per cooling time. This is because a large fraction of the medium is pushed into lower densities which cool inefficiently. Second, the medium is turbulent; the root-mean-squared (RMS) velocity of the gas to 20-50 km s$^{-1}$ on a driving scale of tens of parsec. The velocity field of the medium is dominated by compressional modes, which are larger than the solenoidal components by a factor of 3-7. Third, the hot gas has a very broad density distribution. The ratio between the density fluctuations and the RMS Mach number, parameterized as $b$, is 2-20. This is in contrast to previous simulations of turbulent media, which have found $b\lesssim$ 1. The reason for the difference is mainly caused by the \textit{localized} heating of SNe Ia, which creates a large density contrast. Last, the typical length scale of a density fluctuation grows with time, forming increasingly larger bubbles and filamentary ridges. These underlying density fluctuations need to be included when X-ray observations are interpreted.

The Structure of Radiatively Inefficient Black Hole Accretion Flows

Abstract: We run three long-timescale general-relativistic magnetohydrodynamic simulations of radiatively inefficient accretion flows onto non-rotating black holes. Our aim is to achieve steady-state behavior out to large radii and understand the resulting flow structure. A simulation with adiabatic index Gamma = 4/3 and small initial alternating poloidal magnetic field loops is run to a time of 440,000 GM/c^3, reaching inflow equilibrium inside a radius of 370 GM/c^2. Variations with larger alternating field loops and with Gamma = 5/3 are run to 220,000 GM/c^3, attaining equilibrium out to 170 GM/c^2 and 440 GM/c^2. There is no universal self-similar behavior obtained at radii in inflow equilibrium: the Gamma = 5/3 simulation shows a radial density profile with power law index ranging from -1 in the inner regions to -1/2 in the outer regions, while the others have a power-law slope ranging from -1/2 to close to -2. Both simulations with small field loops reach a state with polar inflow of matter, while the more ordered initial field has polar outflows. However, unbound outflows remove only a factor of order unity of the inflowing material over a factor of ~300 in radius. Our results suggest that the dynamics of radiatively inefficient accretion flows are sensitive to how the flow is fed from larger radii, and may differ appreciably in different astrophysical systems. Millimeter images appropriate for Sgr A* are qualitatively (but not quantitatively) similar in all simulations, with a prominent asymmetric image due to Doppler boosting.

Sound-wave instabilities in dilute plasmas with cosmic rays: implications for cosmic ray confinement and the Perseus X-ray ripples

Abstract: Weakly collisional, magnetised plasmas characterised by anisotropic viscosity and conduction are ubiquitous in galaxies, halos and the intracluster medium (ICM). Cosmic rays (CRs) play an important role in these environments as well, by providing additional pressure and heating to the thermal plasma. We carry out a linear stability analysis of weakly collisional plasmas with cosmic rays using Braginskii MHD for the thermal gas. We assume that the CRs stream at the Alfven speed, which in a weakly collisional plasma depends on the pressure anisotropy ($\Delta p$) of the thermal plasma. We find that this $\Delta p$-dependence introduces a phase shift between the CR-pressure and gas-density fluctuations. This drives a fast-growing acoustic instability: CRs offset the damping of acoustic waves by anisotropic viscosity and give rise to wave growth when the ratio of CR pressure to gas pressure is $\gtrsim \alpha \beta^{-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure, and $\alpha$, typically $\lesssim 1$, depends on other dimensionless parameters. In high-$\beta$ environments like the ICM, this condition is satisfied for small CR pressures. We speculate that the instability studied here may contribute to the scattering of high-energy CRs and to the excitation of sound waves in galaxy-halo, group and cluster plasmas, including the long-wavelength X-ray fluctuations in \textit{Chandra} observations of the Perseus cluster. It may also be important in the vicinity of shocks in dilute plasmas (e.g., cluster virial shocks or galactic wind termination shocks), where the CR pressure is locally enhanced.

The Impact of Type Ia Supernovae in Quiescent Galaxies. I. Formation of the Multiphase Interstellar Medium

Abstract: A cool phase of the interstellar medium has been observed in many giant elliptical galaxies, but its origin remains unclear. We propose that uneven heating from Type Ia supernovae (SNe Ia), together with radiative cooling, can lead to the formation of the cool phase. The basic idea is that since SNe Ia explode randomly, gas parcels which are not directly heated by SN shocks will cool, forming multiphase gas. We run a series of idealized high-resolution numerical simulations, and find that cool gas develops even when the overall SNe heating rate $H$ exceeds the cooling rate $C$ by a factor as large as 1.4. We also find that the time for multiphase gas development depends on the gas temperature. When the medium has a temperature $T = 3\times 10^6$ K, the cool phase forms within one cooling time \tc; however, the cool phase formation is delayed to a few times \tc\ for higher temperatures. The main reason for the delay is turbulent mixing. Cool gas formed this way would naturally have a metallicity lower than that of the hot medium. For constant $H/C$, there is more turbulent mixing for higher temperature gas. We note that this mechanism of producing cool gas cannot be captured in cosmological simulations, which usually fail to resolve individual SN remnants.

The Effects of Tilt on the Images of Black Hole Accretion Flows

Abstract: We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent difference is that the central shadow in the image is noticeably eccentric in tilted models, with the ring of emission relatively unchanged. Applying this procedure to M87 with a viewing angle based on the large-scale jet, we find that adding tilt increases the angular size of the ring for fixed black hole mass and distance, while at the same time increasing the number of bright spots in the image. Our findings illustrate observable features that can distinguish tilted from aligned flows. They also show that tilted models can be viable for M87, and that not accounting for tilt can bias inferences of physical parameters. Future modeling of horizon-scale observations should account for potential angular momentum misalignment, which is likely generic at the low accretion rates appropriate for EHT targets.