We investigate the physics driving the cosmic star formation (SF) history using the more than fifty large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations (OWLS) project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift, then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion, and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photo-heating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift two, although more extreme feedback from SF, for example in the form of a top-heavy IMF at high gas pressures, can help.Comment: Accepted for publication in MNRAS, 27 pages and 18 figures. Revised version: minor change

Freak waves are very large, rare events in a random ocean wave train. Here we study the numerical generation of freak waves in a random sea state characterized by the JONSWAP power spectrum. We assume, to cubic order in nonlinearity, that the wave dynamics are governed by the nonlinear Schroedinger (NLS) equation. We identify two parameters in the power spectrum that control the nonlinear dynamics: the Phillips parameter α and the enhancement coefficient γ. We discuss how freak waves in a random sea state are more likely to occur for large values of α and γ. Our results are supported by extensive numerical simulations of the NLS equation with random initial conditions. Comparison with linear simulations are also reported.Freak waves are extraordinarily large water waves whose heights exceed by a factor of 2.2 the significant wave height of a measured wave train [1]. The mechanism of freak wave generation has become an issue of principal interest due to their potentially devastating effects on offshore structures and ships. In addition to the formation of such waves in the presence of strong currents [2] or as a result of a simple chance superposition of Fourier modes with coherent phases, it has recently been established that the nonlinear Schroedinger (NLS) equation can describe many of the features of the dynamics of freak waves which are found to arise as a result of the nonlinear self-focusing phenomena [3][4][5]. The self-focusing effect arises from the Benjamin-Feir instability [7]: a monochromatic wave of amplitude, a 0 , and wave number, k 0 , modulationally perturbed on a wavelength L = 2π/∆k, is unstable whenever ∆k/(k 0 ε) < 2 √ 2, where ε is the steepness of the carrier wave defined as ε=k 0 a 0 . The instability causes a local exponential growth in the amplitude of the wave train. This result is established from a linear stability analysis of the NLS equation [8] and has been confirmed, for small values of the steepness, by numerical simulations of the fully nonlinear water wave equations [5,6] (for high values of steepness wave breaking, which is clearly not included in the NLS model, can occur). Moreover, it is known that small-amplitude instabilities are but a particular case of the much more complicated and general analytical solutions of the NLS equation obtained by exploiting its integrability properties via Inverse Scattering theory in the θ-function representation [11,12].Even though the above results are well understood and robust from a physical and mathematical point of view, it is still unclear how freak waves are generated via the Benjamin-Feir instability in more realistic oceanic conditions, i.e. in those characterized not by a simple monochromatic wave perturbed by two small side-bands, but instead by a complex spectrum whose perturbation of the carrier wave cannot be viewed as being small. Furthermore, the focus herein is not to attempt to model ocean waves but instead to study leading order effects using the nonlinear Schroedinger equation, as suggested by [3][4][5]. Research at hi...

We present results of a new feedback scheme implemented in the Munich galaxy formation model. The new scheme includes a dynamical treatment of galactic winds powered by supernova explosions and stellar winds in a cosmological context. We find that such a scheme is a good alternative to empirically motivated recipes for feedback in galaxy formation. Model results are in good agreement with the observed luminosity functions and stellar mass function for galaxies in the local universe. In particular, the new scheme predicts a number density of dwarfs that is lower than in previous models. This is a consequence of a new feature of the model, which allows an estimate of the amount of mass and metals that haloes can permanently deposit into the intergalactic medium (IGM). This loss of material leads to the suppression of star formation in small haloes and therefore to the decrease in the number density of dwarf galaxies. The model is able to reproduce the observed mass–stellar metallicity and luminosity–gas metallicity relationships. This demonstrates that our scheme provides a significant improvement in the treatment of the feedback in dwarf galaxies. Despite these successes, our model does not reproduce the observed bimodality in galaxy colours and predicts a larger number of bright galaxies than observed. Finally, we investigate the efficiency of metal injection in winds and in the IGM. We find that galaxies that reside in haloes with Mvir < 1012 h−1 M⊙ may deposit most of their metal mass into the IGM, while groups and clusters at z= 0 have lost at most a few per cent of their metals before the bulk of the halo mass was accreted.

The origin of intergalactic magnetic fields is still a mystery and several scenarios have been proposed so far: among them, primordial phase transitions, structure‐formation shocks and galactic outflows. In this work, we investigate how efficiently galactic winds can provide an intense and widespread ‘seed’ magnetization. This may be used to explain the magnetic fields observed today in clusters of galaxies and in the intergalactic medium (IGM). We use semi‐analytic simulations of magnetized galactic winds coupled to high‐resolution N‐body simulations of structure formation to estimate lower and upper limits for the fraction of the IGM which can be magnetized up to a specified level. We find that galactic winds are able to seed a substantial fraction of the cosmic volume with magnetic fields. Most regions affected by winds have magnetic fields in the range 10−12 < B < 10−8 G, while higher seed fields can be obtained only rarely and in close proximity to wind‐blowing galaxies. These seed fields are sufficiently intense for a moderately efficient turbulent dynamo to amplify them to the observed values. The volume‐filling factor of the magnetized regions strongly depends on the efficiency of winds to load mass from the ambient medium. However, winds never completely fill the whole Universe and pristine gas can be found in cosmic voids and regions unaffected by feedback even at z= 0. This means that, in principle, there might be the possibility to probe the existence of primordial magnetic fields in such regions.