Most massive halos with Gumbel Statistics

We present an analytical calculation of the extreme value statistics for dark matter halos - that is, the probability distribution of the most massive halo within some region of the universe of specified shape and size. Our calculation makes use of the counts-in-cells formalism for the correlation functions, and the halo bias derived from the Sheth-Tormen mass function. We demonstrate the power of the method on spherical regions, comparing the results to measurements in a large cosmological dark matter simulation and achieving good agreement. Particularly good fits are obtained for the most likely value of the maximum mass and for the high-mass tail of the distribution, relevant in constraining cosmologies by observations of most massive clusters.Comment: Accepted to MNRA

Psychophysiological Measures of Cognitive Absorption

Cognitive absorption (CA) corresponds to a state of deep involvement with a software program. CA has widely been studied over the last decade in the IT literature using psychometric instruments. Measuring ongoing CA with psychometric tools requires interrupting a subject’s ongoing usage behavior to self-evaluate their level of absorption. Such interruptions may alter or contaminate the very CA state the researcher us attempting to measure. To circumvent this problem, we are investigating the effectiveness of psychophysiological measures of cognitive absorption. This paper reports preliminary results from an ongoing research project by looking at the correlation between electrodermal activity (EDA) and several dimensions of the CA construct

Stability Analysis of Superconducting Electroweak Vortices

We carry out a detailed stability analysis of the superconducting vortex solutions in the Weinberg-Salam theory described in Nucl.Phys. B826 (2010) 174. These vortices are characterized by constant electric current $I$ and electric charge density $I_0$, for ${I}\to 0$ they reduce to Z strings. We consider the generic field fluctuations around the vortex and apply the functional Jacobi criterion to detect the negative modes in the fluctuation operator spectrum. We find such modes and determine their dispersion relation, they turn out to be of two different types, according to their spatial behavior. There are non-periodic in space negative modes, which can contribute to the instability of infinitely long vortices, but they can be eliminated by imposing the periodic boundary conditions along the vortex. There are also periodic negative modes, but their wavelength is always larger than a certain minimal value, so that they cannot be accommodated by the short vortex segments. However, even for the latter there remains one negative mode responsible for the homogeneous expansion instability. This mode may probably be eliminated when the vortex segment is bent into a loop. This suggests that small vortex loops balanced against contraction by the centrifugal force could perhaps be stable.Comment: 42 pages, 11 figure

Extreme value statistics of smooth random Gaussian fields

We consider the Gumbel or extreme value statistics describing the distribution function p_G(x_max) of the maximum values of a random field x within patches of fixed size. We present, for smooth Gaussian random fields in two and three dimensions, an analytical estimate of p_G which is expected to hold in a regime where local maxima of the field are moderately high and weakly clustered. When the patch size becomes sufficiently large, the negative of the logarithm of the cumulative extreme value distribution is simply equal to the average of the Euler Characteristic of the field in the excursion x > x_max inside the patches. The Gumbel statistics therefore represents an interesting alternative probe of the genus as a test of non Gaussianity, e.g. in cosmic microwave background temperature maps or in three-dimensional galaxy catalogs. It can be approximated, except in the remote positive tail, by a negative Weibull type form, converging slowly to the expected Gumbel type form for infinitely large patch size. Convergence is facilitated when large scale correlations are weaker. We compare the analytic predictions to numerical experiments for the case of a scale-free Gaussian field in two dimensions, achieving impressive agreement between approximate theory and measurements. We also discuss the generalization of our formalism to non-Gaussian fields.Comment: 10 pages, 2 figures, accepted for publication in MNRA

Influence of grain size, shape and compaction on georadar waves: example of an Aeolian dune

Many Ground Penetrating Radar (GPR) profiles acquired in dry aeolian environment have shown good reflectivity inside present-day dunes. We show that the origin of this reflectivity is related to changes in grain size distribution, packing and/or grain shape in a sandy material. We integrate these three parameters into analytical models for bulk permittivity in order to predict the reflections and the velocity of GPR waves. We consider two GPR cross-sections acquired over Aeolian dunes in the Chadian desert. The 2D migration of GPR data suggests that dunes contain different kinds of bounding surfaces. We discuss and model three kinds of reflections using reasonable geological hypothesis about Aeolian sedimentation processes. The propagation and the reflection of radar waves are calculated using the 1D wavelet modelling method in spectral domain. The results of the forward modelling are in good accordance with real observed data

Merger Histories in Warm Dark Matter Structure Formation Scenario

Observations on galactic scales seem to be in contradiction with recent high resolution N-body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy halos. In this paper, we explore a somewhat different approach which consists of filtering the dark matter power spectrum on small scales, thereby altering the formation history of low mass objects. The physical motivation for damping these fluctuations lies in the possibility that the dark matter particles have a different nature i.e. are warm (WDM) rather than cold. We show that this leads to some interesting new results in terms of the merger history and large-scale distribution of low mass halos, as compared to the standard CDM scenario. However, WDM does not appear to be the ultimate solution, in the sense that it is not able to fully solve the CDM crisis, even though one of the main drawbacks, namely the abundance of satellites, can be remedied. Indeed, the cuspiness of the halo profiles still persists, at all redshifts, and for all halos and sub-halos that we investigated. Despite the persistence of the cuspiness problem of DM halos, WDM seems to be still worth taking seriously, as it alleviates the problems of overabundant sub-structures in galactic halos and possibly the lack of angular momentum of simulated disk galaxies. WDM also lessens the need to invoke strong feedback to solve these problems, and may provide a natural explanation of the clustering properties and ages of dwarfs.Comment: 11 pages, 17 figures, MNRAS submitted, high-res figures can be found at http://www-thphys.physics.ox.ac.uk/users/AlexanderKnebe/publications.html, replaced with accepted version (warmon masses corrected!

CMB anisotropy predictions for a model of double inflation

We consider a double-inflationary model with two massive scalar fields interacting only gravitationally in the context of a flat cold dark matter (CDM) Universe. The cosmic microwave background (CMB) temperature anisotropies produced in this theory are investigated in great details for a window of parameters where the density fluctuation power spectrum P(k) is in good agreement with observations. The first Doppler (``acoustic'') peak is a crucial test for this model as well as for other models. For the ``standard'' values of the cosmological parameters of CDM, our model is excluded if the height of the Doppler peak is sensibly higher than about three times the Sachs-Wolfe plateau.Comment: 12 pages LaTeX using revtex, to be published in Phys. Rev.

The hierarchical build-up of the Tully-Fisher relation

We use the semi-analytic model GalICS to predict the Tully-Fisher relation in the B, I and for the first time, in the K band, and its evolution with redshift, up to z~1. We refined the determination of the disk galaxies rotation velocity, with a dynamical recipe for the rotation curve, rather than a simple conversion from the total mass to maximum velocity. The new recipe takes into account the disk shape factor, and the angular momentum transfer occurring during secular evolution leading to the formation of bulges. This produces model rotation velocities that are lower by ~20-25% for the majority of the spirals. We implemented stellar population models with a complete treatment of the TP-AGB, which leads to a revision of the mass-to-light ratio in the near-IR. I/K band luminosities increase by ~0.3/0.5 mags at redshift z=0 and by ~0.5/1 mags at z=3. With these two new recipes in place, the comparison between the predicted Tully-Fisher relation with a series of datasets in the optical and near-IR, at redshifts between 0 and 1, is used as a diagnostics of the assembly and evolution of spiral galaxies in the model. At 0.4<z<1.2 the match between the new model and data is remarkably good, especially for later-type spirals (Sb/Sc). At z=0 the new model shows a net improvement in comparison with its original version of 2003, and in accord with recent observations in the K band, the model Tully-Fisher also shows a morphological differentiation. However, in all bands the z=0 model Tully-Fisher is too bright. We argue that this behaviour is caused by inadequate star formation histories in the model galaxies at low redshifts. The star-formation rate declines too slowly, due to continuous gas infall that is not efficiently suppressed. An analysis of the model disk scale lengths, at odds with observations, hints to some missing physics in the modeling of disk formation inside dark matter halos.Comment: Accepted for publication on MNRAS. 2 new plots, 1 new section, and extended discussion. 21 pages, 11 figures in tota