Abelian quotients of subgroups of the mapping class group and higher Prym representations

A well-known conjecture asserts that the mapping class group of a surface (possibly with punctures/boundary) does not virtually surject onto $\Z$ if the genus of the surface is large. We prove that if this conjecture holds for some genus, then it also holds for all larger genera. We also prove that if there is a counterexample to this conjecture, then there must be a counterexample of a particularly simple form. We prove these results by relating the conjecture to a family of linear representations of the mapping class group that we call the higher Prym representations. They generalize the classical symplectic representation.Comment: 20 pages, 3 figures; appendix added containing a new counterexample in genus 1; to appear in J. London Math. So

Stability of Localized Wave Fronts in Bistable Systems

Localized wave fronts are a fundamental feature of biological systems from cell biology to ecology. Here, we study a broad class of bistable models subject to self-activation, degradation, and spatially inhomogeneous activating agents. We determine the conditions under which wave-front localization is possible and analyze the stability thereof with respect to extrinsic perturbations and internal noise. It is found that stability is enhanced upon regulating a positional signal and, surprisingly, also for a low degree of binding cooperativity. We further show a contrasting impact of self-activation to the stability of these two sources of destabilization. DOI: 10.1103/PhysRevLett.110.03810

A Life Cycle Analysis of Land Use in US Pork Production

The goal of this study was to analyze land use in the production of US pork using Life Cycle Assessment (LCA). LCA is a comprehensive methodology for quantitatively analyzing potential environmental impacts associated with complex systems. Identification of processes contributing to high environmental impacts often highlights opportunities for gains in efficiency, which can increase the profitability and sustainability of US pork. The environmental impact category analyzed in this assessment was land use. After reviewing existing information regarding land use in agriculture and livestock production, analysis for US pork production was performed at two scales: cradle-to-grave and cradle-to-farm gate. The cradle-to-grave analysis provided a scan-level overview of land use associated with the production and consumption of lean pork at an aggregated national level. The cradle-to-farm gate analysis provided a more granular assessment of the land use required for live swine production, and evaluated the use of alternate ration formulation as a tool for reducing environmental impacts. [Excerpt from report]

The scattered debris of the Magellanic Stream

Searching the HI Parkes All-Sky Survey (HIPASS) and its northern extension, we detected a population of very compact high-velocity clouds (HVCs) with similar velocities in the Galactic standard-of-rest frame which appear to be arranged in several filaments aligned with the nearby Magellanic Stream. A comparison with published OVI/CaII absorption and HI emission line measurements suggests that the HVCs are condensations within an extended and mainly ionised component of the Magellanic Stream. They coincide in position with a faint gas stream predicted in numerical simulations of the Magellanic Clouds by Gardiner & Noguchi (1996). Consequently, the Magellanic Stream could be much more extended than generally believed.Comment: 5 pages, 4 figures, accepted for publication in MNRA

Sustainability Assessment of U.S. Beef Production Systems

With increasing public concern and awareness of agricultural sustainability issues, comprehensive methodologies such as life cycle assessment are required to benchmark the beef industry and identify areas of opportunity for continuous improvement. To that end, the Beef Checkoff completed a retrospective sustainability assessment benchmark in 2013 by using Eco-efficiency analysis to compare the years 2005 and 2011. At the time of the analysis, the methodology used was the most up-to-date and comprehensive – indeed the analysis remains one of the only complete cradle-to-grave assessments of the U.S. beef industry. In 2015, a further refined version of the Eco-efficiency analysis was completed to incorporate new primary data sources from the beef value chain for the years 2011-2013. As the young and dynamic field of sustainability science continues to evolve, there is a need to adapt and update the methodologies used in life cycle and broader sustainability assessments of the beef industry. Consequently, this project updated and expanded the original Eco-efficiency analysis to the SimaPro™ computational platform. The move to the SimaPro™ platform will allow for direct linkages with the Integrated Farm Systems Model (USDA-ARS), which is the simulation model that has been used to generate life cycle inventories from the feed production, cow-calf, and backgrounding/feedlot segments of the beef industry. Additionally, the SimaPro™ platform will allow for even more transparent reporting of our inventories and results to the broader life cycle assessment, sustainability science, and beef communities, which is key to advancing the field and benchmarking beef’s sustainability. Finally, this project further expanded the economic sustainability evaluation of U.S. beef industry to include the direct, indirect, and induced economic activity and value that is generated from beef production. [Excerpt from report]

A Retrospective Assessment of US Pork Productions: 1960 to 2015

The primary goal of this study is to assess the carbon, energy, water and land footprints per kg (2.2 pounds) of live weight (LW) pork produced at five-year increments between 1960 and 2015. This assessment utilizes the Life Cycle Assessment (LCA) methodology, which is a technique to assess the potential environmental impacts associated with a product system by compiling an inventory of relevant energy and material flows, evaluating the associated burdens, and interpreting the results to assist in making more informed decisions and to provide an understanding of the drivers of change over the past 55 years. This LCA is “cradle-to-farm gate” e.g. covering the material and energy flows associated with the full supply chain beginning with extraction of raw materials through the production of live, market-weight swine, inclusive of culled sows, at the farm gate. On average, production-weighted metrics declined across all four categories over the assessment period. The largest decrease was seen in land use (75.9 percent), followed by water use (25.1 percent), then global warming potential (7.7 percent), and finally energy use (7.0 percent). [Excerpt from report]

Head-Tail Clouds: Drops to Probe the Diffuse Galactic Halo

A head-tail high-velocity cloud (HVC) is a neutral hydrogen halo cloud that appears to be interacting with the diffuse halo medium as evident by its compressed head trailed by a relatively diffuse tail. This paper presents a sample of 116 head-tail HVCs across the southern sky (d < 2 deg) from the HI Parkes All Sky Survey (HIPASS) HVC catalog, which has a spatial resolution of 15.5 arcmin (45 pc at 10 kpc) and a sensitivity of N_HI=2 x 10^(18) cm^(-2) (5 sigma). 35% of the HIPASS compact and semi-compact HVCs (CHVCs and :HVCs) can be classified as head-tail clouds from their morphology. The clouds have typical masses of 730 M_sun at 10 kpc (26,000 M_sun at 60 kpc) and the majority can be associated with larger HVC complexes given their spatial and kinematic proximity. This proximity, together with their similar properties to CHVCs and :HVCs without head-tail structure, indicate the head-tail clouds have short lifetimes, consistent with simulation predictions. Approximately half of the head-tail clouds can be associated with the Magellanic System, with the majority in the region of the Leading Arm with position angles pointing in the general direction of the movement of the Magellanic System. The abundance in the Leading Arm region is consistent with this feature being closer to the Galactic disk than the Magellanic Stream and moving through a denser halo medium. The head-tail clouds will feed the multi-phase halo medium rather than the Galactic disk directly and provide additional evidence for a diffuse Galactic halo medium extending to at least the distance of the Magellanic Clouds.Comment: MNRAS Accepted, 10 figures, 7 in colo

An absorption-selected survey of neutral gas in the Milky Way halo

We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set. Multi-wavelength studies were performed combining optical absorption line data of CaII and NaI with follow-up HI 21-cm emission line observations along 408 sight lines towards low- and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. HI data were extracted from the Effelsberg-Bonn HI survey and the Galactic All-Sky survey. For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope. CaII (NaI) halo absorbers at intermediate and high radial velocities are present in 40-55% (20-35%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65% of the cases, absorption is associated with HI 21-cm emission. The CaII (NaI) column density distribution function follows a power-law with a slope of -2.2 (-1.4). Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common HI high- and intermediate-velocity clouds seen in 21-cm observations. We find that CaII/NaI column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman-alpha systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.Comment: 17 pages, 20 figures, accepted by A&

Do High-Velocity Clouds trace the Dark Matter subhalo population?

Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos form the building blocks which merge hierarchically to more massive galaxies. Since intergalactic gas is accreted by massive galaxies, observable e.g. as high- velocity clouds (HVCs) around the Milky Way, with extremely low metallicities, these can be suggested to represent the baryonic content of primordial Dark Matter (DM) subhalos. Another possibility of their origin is that they stem from disrupted satellite galaxies, but in this case, these gas clouds move unaccompanied by a bound DM structure. Since HVCs are observed with long gas tails and with irregular substructures, numerical models are performed aiming at exploring their structure and compare them with observations. If HVCs are engulfed by DM subhalos, their gas must leave the DM gravitational potential and reflect this in their dynamics. On the other hand, the evolution and survival of pure gas models must be tested to distinguish between DM-dominated and DM-free clouds and to allow conclusions on their origin. The models demonstrate that purely baryonic HVCs with low masses are disrupted by ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive ones survive, losing their initially spherical shape and develop significant substructures including cometary elongations in the column density distribution ("head-tail structure"). On the contrary, HVCs with DM subhalos survive with more than 90% of their gas mass still bound and spherically shaped, approaching the Galactic disk like bullets. In addition, we find that velocity gradients along the cometary head-tail structures does not necessarily offer a possibility to distinguish between DM-dominated and purely gaseous HVCs. Comparison of models with observations let us conclude that HVCs are not embedded in a DM substructure and do not trace the cosmological subhalo population.Comment: Accepted for publication in A&

Effects of ram pressure on the gas distribution and star formation in the Large Magellanic Cloud

We use high resolution N-body/SPH simulations to study the hydrodynamical interaction between the Large Magellanic Cloud (LMC) and the hot halo of the Milky Way. We investigate whether ram-pressure acting on the satellite's ISM can explain the peculiarities observed in the HI distribution and the location of the recent star formation activity. Due to the present nearly edge-on orientation of the disk with respect to the orbital motion, compression at the leading edge can explain the high density region observed in HI at the south-east border. In the case of a face-on disk (according to Mastropietro et al. 2008 the LMC was moving almost face-on before the last perigalactic passage), ram-pressure directed perpendicularly to the disk produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes HI survey. As a consequence of the very recent edge-on motion, the H-alpha emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some H-alpha complexes form a patchy distribution on the entire disk. In this scenario only the youngest stellar complexes show a progression in age along the leading border of the disk.Comment: 18 pages, 18 figures, submitted to MNRA