Growing galaxies via superbubble-driven accretion flows

We use a suite of cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas on to star-forming galaxies. Correlated supernova (SN) events create converging ‘superbubbles' in the halo gas. Where these collide, the density increases, driving cooling filaments of low-metallicity gas that feed the disc. At our current numerical resolution (∼20pc; mgas = 4 × 104 M⊙) we are only able to resolve the most dramatic events; however, as we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SN-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate of ∼3 M⊙ yr−1, consistent with estimates for the Milky Way (MW). The filaments we resolve extend to ∼50 kpc, reaching column densities of N ∼ 1018cm−2. We show that such structures can plausibly explain the broad dispersion in Mgii absorption seen along sightlines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas disc

Voluntary simplicity strongly backed by all three main normative-ethical traditions

We examine the growing ‘Voluntary Simplicity’ (VS) movement from the perspectives of Utilitarianism, Kantian deontology, and Virtue Ethics. We argue that, from each of these three diverse perspectives, there is a compelling argument to the conclusion that citizens of the ‘developed’ world ought to embrace such simplicity in their own lives, and to facilitate its greater adoption societally and globally. We conclude by asking why it is that this compelling conclusion has not already been more widely found and acted upon. In reflecting on this question we outline some arguments for why a culture of voluntary simplicity may be needed to drive political and macroeconomic change

Entropy: The Markov Ordering Approach

The focus of this article is on entropy and Markov processes. We study the properties of functionals which are invariant with respect to monotonic transformations and analyze two invariant "additivity" properties: (i) existence of a monotonic transformation which makes the functional additive with respect to the joining of independent systems and (ii) existence of a monotonic transformation which makes the functional additive with respect to the partitioning of the space of states. All Lyapunov functionals for Markov chains which have properties (i) and (ii) are derived. We describe the most general ordering of the distribution space, with respect to which all continuous-time Markov processes are monotonic (the {\em Markov order}). The solution differs significantly from the ordering given by the inequality of entropy growth. For inference, this approach results in a convex compact set of conditionally "most random" distributions.Comment: 50 pages, 4 figures, Postprint version. More detailed discussion of the various entropy additivity properties and separation of variables for independent subsystems in MaxEnt problem is added in Section 4.2. Bibliography is extende

Phase field model of premelting of grain boundaries

We present a phase field model of solidification which includes the effects of the crystalline orientation in the solid phase. This model describes grain boundaries as well as solid-liquid boundaries within a unified framework. With an appropriate choice of coupling of the phase field variable to the gradient of the crystalline orientation variable in the free energy, we find that high angle boundaries undergo a premelting transition. As the melting temperature is approached from below, low angle grain boundaries remain narrow. The width of the liquid layer at high angle grain boundaries diverges logarithmically. In addition, for some choices of model coupling, there may be a discontinuous jump in the width of the fluid layer as function of temperature.Comment: 6 pages, 9 figures, RevTeX

A mass-dependent density profile for dark matter haloes including the influence of galaxy formation

We introduce a mass-dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the Making Galaxies In a Cosmological Context project, which have been shown to match a wide range of disc scaling relationships. We find that the best-fitting parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M⋆/Mhalo constrains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transition between the slopes (α), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way-mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs. This mass-dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi-analytic galaxy formation models as a significant improvement over the commonly used NFW profile

Spectral bounds for Schrödinger operators in dimensions one and two

This thesis investigates Lieb-Thirring and Cwikel-Lieb-Rozenblum (CLR) type inequalities for Schrödinger operators in dimensions one and two, with a focus on overcoming the weak coupling problem associated with the failure of the CLR inequality in these dimensions. To this end we start by investigating three two-dimensional Schrödinger operators with extra repulsive factors in the form of: a repulsive Hardy potential, a restriction to antisymmetric functions, and an Aharonov-Bohm magnetic field. Under the assumption that the electric potential is radially non-increasing we establish semi-classical bounds on the number of negative eigenvalues for each of these operators. Our proof lies in generalising a one-dimensional bound of Calogero and Cohn to operator-valued potentials. For the same operators we then derive a family of weighted CLR type inequalities by applying the Birman-Schwinger principle. An interpolation argument leads us to ascertain weak forms, which we show to be saturated in the strong coupling limit by a class of long-range potentials. In the Aharonov-Bohm case this enables us to deduce the optimal dependence of the constants on the flux of the field. Finally in one dimension, we examine Schrödinger operators with an additional fixed attractive potential. Following the method of factorisation we derive Lieb-Thirring type bounds which measure the distance between the eigenvalues of the original and the perturbed operator. Applying this to the Pölsch-Teller potential leads to an improvement of the Lieb-Thirring inequality for partial sums of eigenvalues.Open Acces

Theories of Low-Energy Quasi-Particle States in Disordered d-Wave Superconductors

The physics of low-energy quasi-particle excitations in disordered d-wave superconductors is a subject of ongoing intensive research. Over the last decade, a variety of conceptually and methodologically different approaches to the problem have been developed. Unfortunately, many of these theories contradict each other, and the current literature displays a lack of consensus on even the most basic physical observables. Adopting a symmetry-oriented approach, the present paper attempts to identify the origin of the disagreement between various previous approaches, and to develop a coherent theoretical description of the different low-energy regimes realized in weakly disordered d-wave superconductors. We show that, depending on the presence or absence of time-reversal invariance and the microscopic nature of the impurities, the system falls into one of four different symmetry classes. By employing a field-theoretical formalism, we derive effective descriptions of these universal regimes as descendants of a common parent field theory of Wess-Zumino-Novikov-Witten type. As well as describing the properties of each universal regime, we analyse a number of physically relevant crossover scenarios, and discuss reasons for the disagreement between previous results. We also touch upon other aspects of the phenomenology of the d-wave superconductor such as quasi-particle localization properties, the spin quantum Hall effect, and the quasi-particle physics of the disordered vortex lattice.Comment: 42 Pages, 8 postscript figures, published version with updated reference

Feeding SMBHs through supersonic turbulence and ballistic accretion

It has long been recognised that the main obstacle to accretion of gas onto supermassive black holes (SMBHs) is large specific angular momentum. It is feared that the gas settles in a large scale disc, and that accretion would then proceed too inefficiently to explain the masses of the observed SMBHs. Here we point out that, while the mean angular momentum in the bulge is very likely to be large, the deviations from the mean can also be significant. Indeed, cosmological simulations show that velocity and angular momentum fields of gas flows onto galaxies are very complex. Furthermore, inside bulges the gas velocity distribution can be further randomised by the velocity kicks due to feedback from star formation. We perform hydrodynamical simulations of gaseous rotating shells infalling onto an SMBH, attempting to quantify the importance of velocity dispersion in the gas at relatively large distances from the black hole. We implement this dispersion by means of a supersonic turbulent velocity spectrum. We find that, while in the purely rotating case the circularisation process leads to efficient mixing of gas with different angular momentum, resulting in a low accretion rate, the inclusion of turbulence increases this accretion rate by up to several orders of magnitude. We show that this can be understood based on the notion of "ballistic" accretion, whereby dense filaments, created by convergent turbulent flows, travel through the ambient gas largely unaffected by hydrodynamical drag. We derive a simple analytical formula that captures the numerical results to within a factor of a few. Rescaling our results to astrophysical bulges, we argue that this "ballistic" mode of accretion could provide the SMBHs with a sufficient supply of fuel without the need to channel the gas via large-scale discs or bars, and therefore that star formation in bulges can be a strong catalyst for SMBH accretion.Comment: 21 pages, 21 figures. Accepted and published by MNRA

Vertical stiffness asymmetries during drop jumping are related to ankle stiffness asymmetries

Asymmetry in vertical stiffness has been associated with increased injury incidence and impaired performance. The determinants of vertical stiffness asymmetry have not been previously investigated. Eighteen healthy males performed three unilateral drop jumps during which vertical stiffness and joint stiffness of the ankle and knee were calculated. Reactive strength index was also determined during the jumps using the ratio of flight time to ground contact time. ‘Moderate’ differences in vertical stiffness (t17 = 5.49; P < 0.001), ‘small’ differences in centre of mass displacement (t17 = -2.19; P = 0.043) and ‘trivial’ differences in ankle stiffness (t17 = 2.68; P = 0.016) were observed between stiff and compliant limbs. A model including ankle stiffness and reactive strength index symmetry angles explained 79% of the variance in vertical stiffness asymmetry (R2 = 0.79; P < 0.001). None of the symmetry angles were correlated to jump height or reactive strength index. Results suggest that asymmetries in ankle stiffness may play an important role in modulating vertical stiffness asymmetry in recreationally trained males