Economic and Environmental Contributions of Wetlands in Agricultural Landscapes

The purpose of this project was to determine key environmental and economic relationships between agricultural practices and wetlands in the Prairie Pothole Region (PPR) of eastern South Dakota. Water quality and water quantity variables were the key environmental parameters examined. Economic cost and returns by farm management system and wetland proximity were the key economic parameters. The three farm management systems examined were conventional (CON) transitional no-till (TNT), and organic (ORG). The TNT and CON management systems used synthetic fertilizers and chemical pesticides. The ORG system used no synthetic fertilizers and generally no chemical pesticides. The ORG system had greater emphasis on alfalfa and lower emphasis on corn and soybean production. Water budgets were determined for upland and wetland sites. At the wetland site, run-on was the major input to the water budget (60%). Overflow accounted for 3 6% of the wetland output and surface storage /seepage accounted for 40%. Evapotranspiration at the wetland site was much lower than at the upland site. Nitrate concentrations were consistently higher in the semi-permanent wetland areas than the seasonal wetland areas. The data show a steady decrease in phosphate concentration as we move upland in the landscape. Higher concentrations in wetland than upland groundwater may indicate that some soluble P is moving through the system and/or the sorption capacity of wetland soils is exceeded. Economic returns and costs of the three farming systems were estimated for 1992 - 1994 at the whole-farm and crop field level. The relative ranking of net returns by management systems were: TNT \u3e CON \u3e ORG, unless organic premiums are a major source of gross income. Production costs per acre by management system from lowest to highest were ORG \u3c TNT \u3c CON. The organic (ORG) system had lower reported average yields and considerably lower production costs per acre than the other management systems. The TNT system had the least diversity of crop rotations, intermediate-level production costs, and similar yields or higher yields than reported in the CON system. The added costs of more tillage and machinery operations in the CON system exceeded any reduction in chemical costs compared to the TNT system. Biomass production and most corn/soybean yields were lowest adjacent to wetland sites and increased to peak production at 150 to 300 feet out. Several years of crop budget estimates for ORG, CON, and TNT fields adjacent to monitored wetland sites indicated substandard net returns in most years

Cosmic Reionisation by Stellar Sources: Population III Stars

We combine fast radiative transfer calculations with high resolution hydrodynamical simulations to study an epoch of early hydrogen reionisation by primordial stellar sources at redshifts 15<z<30. With relatively conservative assumptions, population III star formation proceeds in a self-regulated manner both locally and globally and, for a conventional LCDM cosmology, can significantly reionise the intergalactic medium between 15<z<20 as long as a large fraction of ionising photons can escape from these earliest galaxies. We then combine these results with our earlier work focusing on the role of population II stars in galaxies with virial temperatures >10^4K at redshifts 5<z< 20. Hence, we construct a complete reionisation history of the Universe which matches the Thomson optical depths as measured by the WMAP satellite as well as the evolution of the Gunn Peterson optical depth as seen in the asborption spectra of the higest redshift quasars. We find that even with conservative estimates for the impact of negative feedback mechanisms, primordial stellar sources contribute significantly to early reionisation. Future observations of a Thomson optical depth of tau_e>~0.13 would bolster the claim for the existence of population III stars similar to the ones studied here.Comment: 40 pages, 9 figure

Simulations of Pregalactic Structure Formation with Radiative Feedback

We present results from three-dimensional hydrodynamic simulations of the high redshift collapse of pregalactic clouds including feedback effects from a soft H2 photodissociating UV radiation field. The simulations use an Eulerian adaptive mesh refinement technique to follow the nonequilibrium chemistry of nine chemical species with cosmological initial conditions drawn from a popular Lambda-dominated cold dark matter model. The results confirm that the soft UV background can delay the cooling and collapse of small halos (~10^6 Msun). For reasonable values of the photo-dissociating flux, the H2 fraction is in equilibrium throughout most of the objects we simulate. We determine the mass threshold for collapse for a range of soft-UV fluxes and also derive a simple analytic expression. Continuing the simulations beyond the point of initial collapse demonstrates that the fraction of gas which can cool depends mostly on the virial mass of the halo and the amount of soft-UV flux, with remarkably little scatter. We parameterize this relation, for use in semi-analytic models.Comment: 18 pages, 7 figures, submitted to Ap

The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations

By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas-phase, leading to large characteristic masses. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10^{-6} - 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well-resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5 percent to nearly the cosmic fraction in halos with merger histories rich in halos above 10^7 solar masses. A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10^{-3} Z_\odot and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyman alpha (DLA) systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.Comment: 11 pages, 7 figures; replaced with accepted version to ApJ; additional movies and images can be found at http://www.astro.princeton.edu/~jwise/research/GalaxyBirth.htm

The Birth of a Galaxy. II. The Role of Radiation Pressure

Massive stars provide feedback that shapes the interstellar medium of galaxies at all redshifts and their resulting stellar populations. Here we present three adaptive mesh refinement radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionising radiation to the absorbing gas on star formation in high-redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three-dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of five to 1.8 x 10^{-2} Msun/yr in a dwarf galaxy with a dark matter and stellar mass of 2.0 x 10^8 and 4.5 x 10^5 solar masses, respectively, when radiation pressure is included. Its mean metallicity of 10^{-2.1} Z_sun is consistent with the observed dwarf galaxy luminosity-metallicity relation. However, what one may naively expect from the calculation without radiation pressure, the central region of the galaxy overcools and produces a compact, metal-rich stellar population with an average metallicity of 0.3 Z_sun, indicative of an incorrect physical recipe. In addition to photo-heating in HII regions, radiation pressure further drives dense gas from star forming regions, so supernovae feedback occurs in a warmer and more diffuse medium, launching metal-rich outflows. Capturing this aspect and a temporal separation between the start of radiative and supernova feedback are numerically important in the modeling of galaxies to avoid the "overcooling problem". We estimate that dust in early low-mass galaxies is unlikely to aid in momentum transfer from radiation to the gas.Comment: 18 pages, 11 figures, replaced with accepted version, MNRAS. Minor changes with the conclusions unaffecte

Supermassive Black Hole Growth and Merger Rates from Cosmological N-body Simulations

Understanding how seed black holes grow into intermediate and supermassive black holes (IMBHs and SMBHs, respectively) has important implications for the duty-cycle of active galactic nuclei (AGN), galaxy evolution, and gravitational wave astronomy. Most studies of the cosmological growth and merger history of black holes have used semianalytic models and have concentrated on SMBH growth in luminous galaxies. Using high resolution cosmological N-body simulations, we track the assembly of black holes over a large range of final masses -- from seed black holes to SMBHs -- over widely varying dynamical histories. We used the dynamics of dark matter halos to track the evolution of seed black holes in three different gas accretion scenarios. We have found that growth of Sagittarius A* - size SMBH reaches its maximum mass M_{SMBH}~10^6Msun at z~6 through early gaseous accretion episodes, after which it stays at near constant mass. At the same redshift, the duty-cycle of the host AGN ends, hence redshift z=6 marks the transition from an AGN to a starburst galaxy which eventually becomes the Milky Way. By tracking black hole growth as a function of time and mass, we estimate that the IMBH merger rate reaches a maximum of R_{max}=55 yr^-1 at z=11. From IMBH merger rates we calculate N_{ULX}=7 per Milky Way type galaxy per redshift in redshift range 2<z<6.Comment: 12 pages, 8 figures, submitted to MNRA

Economic and Environmental Contributions of Wetlands in Agricultural Landscapes

The purpose of this project was to determine key environmental and economic relationships between agricultural practices and wetlands in the Prairie Pothole Region (PPR) of eastern South Dakota. Water quality and water quantity variables were the key environmental parameters examined. Economic cost and returns by farm management system and wetland proximity were the key economic parameters. The three farm management systems examined were conventional {CON), transitional no-till (TNT), and organic (ORG) • 'lhe 'lNr an::i a:N management systems used synthetic fertilizers and chemical pesticides. The ORG system used no synthetic fertilizers and generally no chemical pesticides. The ORG system had greater emphasis on alfalfa and lower emphasis on corn and soybean production. Water budgets were determined for upland and wetland sites. At the wetland site, runon was the major input to the water budget (60%). overflow accounted for 36% of the wetland output and surface storage /seepage accounted for 40%. Evapotranspiration at the wetland site was much lower than at the upland site. Nitrate concentrations were consistently higher in the semipermanent wetland areas than the seasonal wetland areas. The data show a steady decrease in phosphate concentration as we move upland in the landscape. Higher concentrations in wetland than upland groundwater may indicate that some soluble P is moving through the system and/or the sorption capacity of wetland soils is exceeded. . Economic returns and costs of the three farming systems were estimated for 1992 - 1994 at the whole-farm and crop field level. The relative ranking of net returns by management systems were: TNT > CON > ORG, unless organic premiums are a major source of gross income. Production costs per acre by management system from lowest to highest were ORG < TNT < CON. The organic (ORG) system had lower reported average yields and considerably lower production costs per acre than the other management systems. The TNT system had the least diversity of crop rotations, intermediate-level production costs, and similar yields or higher yields than reported in the CON system. The added costs of more tillage and machinery operations in the CON system exceeded any reduction in chemical costs compared to the TNT system. Biomass production and most corn/soybean yields were lowest adjacent to wetland sites and increase to peak production at 150' to 300' feet out. Several years of crop budget estimates for ORG, CON, and TNT fields adjacent to monitored wetland sites indicated substandard net returns in most years