Galaxies in Present-day Clusters: Evolutionary Constraints from Their Distributions and Kinematics

We discuss evidence in local, present-day clusters of galaxies (from the ENACS survey) about the way in which those clusters have evolved and about the evolutionary relationships between the galaxies of different morphological types in them. This evidence is complementary to that obtained from the study of clusters at intermediate and high redshifts. We argue that the spatial distribution and the kinematics of the various types of galaxies in and outside substructures support the following picture. The ELLIPTICAL AND S0 GALAXIES have been around for a long time and have obtained an isotropic velocity distribution. The spatial distribution and kinematics of the EARLY SPIRALS are consistent with the idea that many of their kind have transformed into an S0, but that they have survived, most likely because of their velocities. The distribution and kinematics of the LATE SPIRALS are consistent with a picture in which they have been accreted fairly recently. They have mildly radial orbits and hardly populate the central regions, most likely because they suffer tidal disruption. Finally, the distribution and kinematics of the GALAXIES IN SUBSTRUCTURES, when taken at face value, imply tangential velocity anisotropy for these galaxies, but this result may be (partly) due to the procedure by which these galaxies are selected. A first attempt to take the effects of selection into account shows that isotropic (or even mildly radial) orbits of subcluster galaxies cannot be excluded.Comment: 7 pages, 3 figures. To appear in the proceedings of the IAU colloquium No. 195: "Outskirts of Galaxy Clusters: intense life in the suburbs", A. Diaferio ed. (invited contribution

The Jamming Perspective on Wet Foams

Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can {\em jam} into a rigid, disordered state where they withstand finite shear stresses before yielding. The jamming transition has been studied extensively, in particular in computer simulations of frictionless, soft, purely repulsive spheres. Foams and emulsions are the closest realizations of this model, and in foams, the (un)jamming point corresponds to the wet limit, where the bubbles become spherical and just form contacts. Here we sketch the relevance of the jamming perspective for the geometry and flow of foams --- and also discuss the impact that foams studies may have on theoretical studies on jamming. We first briefly review insights into the crucial role of disorder in these systems, culminating in the breakdown of the affine assumption that underlies the rich mechanics near jamming. Second, we discuss how crucial theoretical predictions, such as the square root scaling of contact number with packing fraction, and the nontrivial role of disorder and fluctuations for flow have been observed in experiments on 2D foams. Third, we discuss a scaling model for the rheology of disordered media that appears to capture the key features of the flow of foams, emulsions and soft colloidal suspensions. Finally, we discuss how best to confront predictions of this model with experimental data.Comment: 7 Figs., 21 pages, Review articl

Parsec-scale structure in the warm ISM from polarized galactic radio background observations

We present multi-frequency polarization observations of the diffuse radio synchrotron background modulated by Faraday rotation, in two directions of positive latitude. No extended total intensity I is observed, which implies that total intensity has no structure on scales smaller than approximately a degree. Polarized intensity and polarization angle, however, show abundant small-scale structure on scales from arcminutes to degrees. Rotation Measure (RM) maps show coherent structure over many synthesized beams, but also abrupt large changes over one beam. RM's from polarized extragalactic point sources are correlated over the field in each of the two fields, indicating a galactic component to the RM, but show no correlation with the RM map of the diffuse radiation. The upper limit in structure in I puts constraints on the random and regular components of the magnetic field in the galactic interstellar medium and halo. The emission is partly depolarized so that the observed polarization mostly originates from a nearby part of the medium. This explains the lack of correlation between RM from diffuse emission and from extragalactic point sources as the latter is built up over the entire path length through the medium.Comment: To appear in "Astrophysical Polarized Backgrounds", Conference Proceedings, eds S. Cecchini, S. Cortiglioni, R. Sault and C. Sbarra, in pres

Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations

We discuss observations of the linearly polarized component of the diffuse galactic radio background. These observations, with an angular resolution of 4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5 frequency bands in the range 341-375 MHz. The linearly polarized intensity P (with polarized brightness temperature going up to 10K) shows a `cloudy' structure, with characteristic scales of 15'-30', which contains relatively long, but very narrow `canals' (essentially unresolved) in which P is only a small fraction of that in the neighbouring beams. These `canals' are generally seen in more than one frequency band, although their appearance changes between bands. They are probably due to depolarization within the synthesized beam, because the change in polarization angle across the deepest `canals' is in general close to 90 degrees (or 270 etc.). These very abrupt changes in polarization angle, which are seen only across the `canals', seem to be accompanied by abrupt changes in the Rotation Measure (RM), which may have the right magnitude to create the difference of close to 90 degrees in polarization angle, and thereby the `canals'. The structure in the polarization maps is most likely due to Faraday rotation modulation of the probably smooth polarized radiation emitted in the halo of our Galaxy by the fairly local ISM (up to 500 pc). Therefore, the abrupt changes of RM across the `canals' provide evidence for very thin (about 1 pc), and relatively long transition regions in the ISM, across which the RM changes by as much as 100%. Such drastic RM changes may well be due primarily to abrupt changes in the magnetic field.Comment: 4 pages, 4 figures, to be published in A&A Letter