Non-ideal magnetohydrodynamics versus turbulence II : which is the dominant process in stellar core formation?

Funding: JW acknowledges support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007- 2013 grant agreement no. 339248), and from the University of St Andrews. BTL acknowledges the support of the National Aeronautics and Space Administration (NASA) through grant NN17AK90G and from the National Science Foundation (NSF) through grants no. 1517488 and PHY-1748958.Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M☉ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis; we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence; only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.Publisher PDFPeer reviewe

Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?

Funding: European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 grant agreement no. 339248); University of St Andrews (JW). National Aeronautics and Space Administration (NASA) through grant NN17AK90G and from the National Science Foundation (NSF) through grants no 1517488 and PHY-1748958 (BTL).Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M☉ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.Publisher PDFPeer reviewe

Feedback from active galactic nuclei : a study of its impact and numerical implementations

xii, 220 leaves : ill., chiefly col. ; 29 cm.Includes abstract and appendices.Includes bibliographical references (leaves 190-204).Feedback from active galactic nuclei is now widely regarded as playing a fundamental role in modern theories of galaxy formation. Recent research has highlighted that not only may AGN feedback suppress star formation by heating, it may also promote star formation by producing rapidly cooling shocks. From both a theoretical and numerical perspective, modelling the physical processes responsible is highly challenging, as 13 orders of magnitude in both spatial and temporal scales are involved. In the literature, there are several different numerical approaches to modelling AGN feedback in galactic and cosmological contexts. The various models make different physical assumptions and are simulated using different numerical codes starting from different initial conditions. Thus, a direct comparison of the results is not possible. We present a study of six distinct approaches to modelling AGN feedback within gravitohydrodynamic simulations of major mergers of Milky Way-sized galaxies. To constrain differences to only be between AGN feedback models, all simulations are run using the Hydra code, including its associated star formation algorithm, and start from the same initial conditions. We focus on five key aspects of the AGN feedback algorithms: the black hole accretion rate, energy feedback rate and method, particle accretion algorithm, black hole advection algorithm and black hole merger algorithm. Our results yield a wide variation in the accretion behaviours of the models, which reinforces the fact that there remains much to be learnt about the evolution of galactic nuclei. Using an augmented version of Zeus-3D, we model an AGN in a system loosely resembling the Local Group. When the AGN is active, shocks are formed in the gas, and stars are born in these shocks. We track the stellar distributions over half a Hubble time to determine if triggered star formation in shocks can account for the observed stellar growth in a galaxy's outer regions. At all times, the stellar distribution can be fit with a Sersic profile, and the stars continually migrate to larger radii

The impact of non-ideal magnetohydrodynamic processes on discs, outflows, counter-rotation and magnetic walls during the early stages of star formation

Funding: JW and MRB acknowledge support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007- 2013 grant agreement no. 339248). JW and IAB acknowledge support from the University of St Andrews.Non-ideal magnetohydrodynamic (MHD) processes – namely Ohmic resistivity, ambipolar diffusion and the Hall effect – modify the early stages of the star formation process and the surrounding environment. Collectively, they have been shown to promote disc formation and promote or hinder outflows. But which non-ideal process has the greatest impact? Using three-dimensional smoothed particle radiation non-ideal MHD simulations, we model the gravitational collapse of a rotating, magnetised cloud through the first hydrostatic core phase to shortly after the formation of the stellar core. We investigate the impact of each process individually and collectively. Including any non-ideal process decreases the maximum magnetic field strength by at least an order of magnitude during the first core phase compared to using ideal MHD, and promotes the formation of a magnetic wall. When the magnetic field and rotation vectors are anti-aligned and the Hall effect is included, rotationally supported discs of r ≳ 20 au form; when only the Hall effect is included and the vectors are aligned, a counter-rotating pseudo-disc forms that is not rotationally supported. Rotationally supported discs of r ≲ 4 au form if only Ohmic resistivity or ambipolar diffusion are included. The Hall effect suppresses first core outflows when the vectors are anti-aligned and suppresses stellar core outflows independent of alignment. Ohmic resistivity and ambipolar diffusion each promote first core outflows and delay the launching of stellar core outflows. Although each non-ideal process influences star formation, these results suggest that the Hall effect has the greatest influence.PostprintPeer reviewe

A comparative study of AGN feedback algorithms

Publisher's Version/PDFModelling active galactic nuclei (AGN) feedback in numerical simulations is both technically and theoretically challenging, with numerous approaches having been published in the literature. We present a study of five distinct approaches to modelling AGN feedback within gravitohydrodynamic simulations of major mergers of Milky Way-sized galaxies. To constrain differences to only be between AGN feedback models, all simulations start from the same initial conditions and use the same star formation algorithm. Most AGN feedback algorithms have five key aspects: the black hole accretion rate, energy feedback rate and method, particle accretion algorithm, black hole advection algorithm and black hole merger algorithm. All models follow different accretion histories, and in some cases, accretion rates differ by up to three orders of magnitude at any given time. We consider models with either thermal or kinetic feedback, with the associated energy deposited locally around the black hole. Each feedback algorithm modifies the region around the black hole to different extents, yielding gas densities and temperatures within r &sim; 200 pc that differ by up to six orders of magnitude at any given time. The particle accretion algorithms usually maintain good agreement between the total mass accreted by Ṁdt and the total mass of gas particles removed from the simulation, although not all algorithms guarantee this to be true. The black hole advection algorithms dampen inappropriate dragging of the black holes by two-body interactions. Advecting the black hole a limited distance based upon local mass distributions has many desirably properties, such as avoiding large artificial jumps and allowing the possibility of the black hole remaining in a gas void. Lastly, two black holes instantly merge when given criteria are met, and we find a range of merger times for different criteria. This is important since the AGN feedback rate changes across the merger in a way that is dependent on the specific accretion algorithm used. Using the MBH&ndash;&sigma; relation as a diagnostic of the remnants yields three models that lie within the one-sigma scatter of the observed relation and two that fall below the expected relation. The wide variation in accretion behaviours of the models reinforces the fact that there remains much to be learnt about the evolution of galactic nuclei.</p

The genetic architecture of the human cerebral cortex

The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder

Accretion disc particle accretion in major merger simulations

Publisher's Version/PDFA recent approach to simulating localized feedback from active galactic nuclei (AGN) by Power et al. uses an accretion disc particle to represent both the black hole and its accretion disc. We have extrapolated and adapted this approach to simulations of Milky Way sized galaxy mergers containing black holes and explored the impact of the various parameters in this model as well as its resolution dependence. The two key parameters in the model are an effective accretion radius, which determines the radius within which gas particles are added to the accretion disc, and a viscous time-scale which determines how long it takes for material in the accretion disc to accrete on to the black hole itself. We find that there is a limited range of permitted accretion radii and viscous time-scales, with unphysical results produced outside this range. For permitted model parameters, the nuclear regions of simulations with the same resolution follow similar evolutionary paths, producing final black hole masses that are consistent within a factor of 2. When comparing the resolution dependence of the model, there is a trend towards higher resolution producing slightly lower mass black holes, but values for the two resolutions studied again agree within a factor of 2. We also compare these results to two other AGN feedback algorithms found in the literature.While the evolution of the systems vary, most notably the intermediate total black hole mass, the final black hole masses differ by less than a factor of 5 among all of our models, and the remnants exhibit similar structural parameters. The implication of this accretion model is that, unlike most accretion algorithms, a decoupling of the accretion rate on to the black hole and the local gas properties is permitted and obtained; this allows for black hole growth even after feedback has prevented additional accretion events on to the disc