Arrest stress of uniformly sheared wet granular matter

We conduct extensive independent numerical experiments considering frictionless disks without internal degrees of freedom (rotation etc.) in two dimensions. We report here that for a large range of the packing fractions below random-close packing, all components of the stress tensor of wet granular materials remain finite in the limit of zero shear rate. This is direct evidence for a fluid-to-solid arrest transition. The offset value of the shear stress characterizes plastic deformation of the arrested state {which corresponds to {\em dynamic yield stress} of the system}. {Based on an analytical line of argument, we propose that the mean number of capillary bridges per particle, $\nu$, follows a non-trivial dependence on the packing fraction, $\phi$, and the capillary energy, \vareps. Most noticeably, we show that $\nu$ is a generic and universal quantity which does not depend on the driving protocol.} Using this universal quantity, we calculate the arrest stress, $\sigma_a$, analytically based on a balance of the energy injection rate due to the external force driving the flow and the dissipation rate accounting for the rupture of capillary bridges. The resulting prediction of $\sigma_a$ is a non-linear function of the packing fraction $\phi$, and the capillary energy \vareps. This formula provides an excellent, parameter-free prediction of the numerical data. Corrections to the theory for small and large packing fractions are connected to the emergence of shear bands and of contributions to the stress from repulsive particle interactions, respectively.Comment: 7 pages, g figure

Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501

VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The HI disk is sharply truncated to the southwest, well within the stellar disk. A region of low surface-density gas, which is more extended than the main HI disk, is discovered northeast of the galaxy center. These data are compared to existing 6cm polarized radio continuum emission, Halpha, and optical broad band images. We observe a coincidence between the western HI and polarized emission edges, on the one hand, and a faint Halpha emission ridge, on the other. The polarized emission maxima are located within the gaps between the spiral arms and the faint Halpha ridge. Based on the comparison of these observations with a sample of dynamical simulations with different values for maximum ram pressure and different inclination angles between the disk and the orbital plane,we conclude that ram pressure stripping can account for the main observed characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The southwestern ridge of enhanced gas surface density and enhanced polarized radio-continuum emission is due to ram pressure compression. It is argued that the faint western Halpha emission ridge is induced by nearly edge-on ram pressure stripping. NGC 4501 represents an especially clear example of early stage ram pressure stripping of a large cluster-spiral galaxy.Comment: 22 pages, 25 figures, accepted for publication in A&

The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals

The influence of the environment on the polarized and total power radio continuum emission of cluster spiral galaxies is investigated. We present deep scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo spiral galaxies. These data are combined with existing optical, HI, and Halpha data. Ram pressure compression leads to sharp edges of the total power distribution at one side of the galactic disk. These edges coincide with HI edges. In edge-on galaxies the extraplanar radio emission can extend further than the HI emission. In the same galaxies asymmetric gradients in the degree of polarization give additional information on the ram pressure wind direction. The local total power emission is not sensitive to the effects of ram pressure. The radio continuum spectrum might flatten in the compressed region only for very strong ram pressure. This implies that neither the local star formation rate nor the turbulent small-scale magnetic field are significantly affected by ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc) and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&

Rapid DNA minipreps from Neurospora

Rapid DNA minipreps from Neurospor

VIVA, VLA Imaging of Virgo spirals in Atomic gas: I. The Atlas & The HI Properties

We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA (the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes high resolution HI data of 53 carefully selected late type galaxies (48 spirals and 5 irregular systems). The goal is to study environmental effects on HI gas properties of cluster galaxies to understand which physical mechanisms affect galaxy evolution in different density regions, and to establish how far out the impact of the cluster reaches. As a dynamically young cluster, Virgo contains examples of galaxies experiencing a variety of environmental effects. Its nearness allows us to study each galaxy in great detail. We have selected Virgo galaxies with a range of star formation properties in low to high density regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr evious studies, more than half of the galaxies in the sample (~60%) are fainter than 12 mag in B_T. Overall, the selected galaxies represent the late type Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological type, systemic velocity, subcluster membership, HI mass and deficiency. In this paper (VIVA I: the atlas and the HI properties), we present HI maps and properties, and describe the HI morphology and kinematics of individual galaxies in detail (abbreviated).Comment: K band magnitudes for 6 galaxies in Table 3 have been corrected. One of the labels in Figure 8 is corrected and an omission in the acknowledgments has been added. The latter two were correct in the previous astro-ph version but are wrong in the journal version. A full resolution with the complete HI atlas can be downloaded at http://www.astro.yale.edu/viva/pub.htm

A dynamical model for the heavily ram pressure stripped Virgo spiral galaxy NGC 4522

A dynamical model including ram pressure stripping is applied to the strongly HI deficient Virgo spiral galaxy NGC 4522. A carefully chosen model snapshot is compared with existing VLA HI observations. The model successfully reproduces the large-scale gas distribution and the velocity field. However it fails to reproduce the large observed HI linewidths in the extraplanar component, for which we give possible explanations. In a second step, we solve the induction equation on the velocity fields of the dynamical model and calculate the large scale magnetic field. Assuming a Gaussian distribution of relativistic electrons we obtain the distribution of polarized radio continuum emission which is also compared with our VLA observations at 6 cm. The observed maximum of the polarized radio continuum emission is successfully reproduced. Our model suggests that the ram pressure maximum occurred only ~50 Myr ago. Since NGC 4522 is located far away from the cluster center (~1 Mpc) where the intracluster medium density is too low to cause the observed stripping if the intracluster medium is static and smooth, two scenarios are envisaged: (i) the galaxy moves very rapidly within the intracluster medium and is not even bound to the cluster; in this case the galaxy has just passed the region of highest intracluster medium density; (ii) the intracluster medium is not static but moving due to the infall of the M49 group of galaxies. In this case the galaxy has just passed the region of highest intracluster medium velocity. This study shows the strength of combining high resolution HI and polarized radio continuum emission with detailed numerical modeling of the evolution of the gas and the large-scale magnetic field.Comment: 15 pages, 11 figures, accepted for publication in A&