Greenhouse gases in intensive agriculture: contributions of individual gases to the radiative forcing of the atmosphere

Includes bibliographical references (pages 1924-1925).Agriculture plays a major role in the global fluxes of the greenhouse gases carbon dioxide, nitrous oxide, and methane. From 1991 to 1999, we measured gas fluxes and other sources of global warming potential (GWP) in cropped and nearby unmanaged ecosystems. Net GWP (grams of carbon dioxide equivalents per square meter per year) ranged from 110 in our conventional tillage systems to -211 in early successional communities. None of the annual cropping systems provided net mitigation, although soil carbon accumulation in no-till systems came closest to mitigating all other sources of GWP. In all but one ecosystem, nitrous oxide production was the single greatest source of GWP. In the late successional system, GWP was neutral because of significant methane oxidation. These results suggest additional opportunities for lessening the GWP of agronomic systems.Publisher version: http://www.jstor.org/stable/3077685

Beam optics study for a potential VHEE beam delivery system

VHEE (Very High Energy Electron) therapy can be superior to conventional radiotherapy for the treatment of deep seated tumours, whilst not necessarily requiring the space and cost of proton or heavy ion facilities. Developments in high gradient RF technology have allowed electrons to be accelerated to VHEE energies in a compact space, meaning that treatment could be possible with a shorter linac. A crucial component of VHEE treatment is the transfer of the beam from accelerator to patient. This is required to magnify the beam to cover the transverse extent of the tumour, whilst ensuring a uniform beam distribution. Two principle methodologies for the design of a compact transfer line are presented. The first of these is based upon a quadrupole lattice and optical magnification of beam size. A minimisation algorithm is used to enforce certain criteria on the beam distribution at the patient, defining the lattice through an automated routine. Separately, a dual scattering-foil based system is also presented, which uses similar algorithms for the optimisation of the foil geometry in order to achieve the desired beam shape at the patient location

A Physical Model for the Origin of Quasar Lifetimes

We propose a model of quasar lifetimes in which observational quasar lifetimes and an intrinsic lifetime of rapid accretion are strongly distinguished by the physics of obscuration by surrounding gas and dust. Quasars are powered by gas funneled to galaxy centers, but for a large part of the accretion lifetime are heavily obscured by the large gas densities powering accretion. In this phase, starbursts and black hole growth are fueled but the quasar is buried. Eventually, feedback from accretion energy disperses surrounding gas, creating a window in which the black hole is observable optically as a quasar, until accretion rates drop below those required to maintain a quasar luminosity. We model this process and measure the unobscured and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy merger. The source luminosity is determined from the black hole accretion rate, calculated from local gas properties. We calculate the column density of hydrogen to the source along multiple lines of sight and use these column densities and gas metallicities to determine B-band attenuation of the source. Defining the observable quasar lifetime as the total time with an observed B-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr for L_B,min=10^11 L_sun (M_B=-23), in good agreement with observationally determined quasar lifetimes. This is significantly smaller than the intrinsic lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed to intrinsic lifetime is also strong function of both the limiting luminosity and the observed frequency.Comment: 5 pages, 4 figures, submitted to ApJ Letter

Parallel Analysis: a Method for Determining Significant Principal Components

Numerous ecological studies use Principal Components Analysis (PCA) for exploratory analysis and data reduction. Determination of the number of components to retain is the most crucial problem confronting the researcher when using PCA. An incorrect choice may lead to the underextraction of components, but commonly results in overextraction. Of several methods proposed to determine the significance of principal components, Parallel Analysis (PA) has proven consistently accurate in determining the threshold for significant components, variable loadings, and analytical statistics when decomposing a correlation matrix. In this procedure, eigenvalues from a data set prior to rotation are compared with those from a matrix of random values of the same dimensionality (p variables and n samples). PCA eigenvalues from the data greater than PA eigenvalues from the corresponding random data can be retained. All components with eigenvalues below this threshold value should be considered spurious. We illustrate Parallel Analysis on an environmental data set. We reviewed all articles utilizing PCA or Factor Analysis (FA) from 1987 to 1993 from Ecology, Ecological Monographs, Journal of Vegetation Science and Journal of Ecology. Analyses were first separated into those PCA which decomposed a correlation matrix and those PCA which decomposed a covariance matrix. Parallel Analysis (PA) was applied for each PCA/FA found in the literature. Of 39 analyses (in 22 articles), 29 (74.4%) considered no threshold rule, presumably retaining interpretable components. According to the PA results, 26 (66.7%) overextracted components. This overextraction may have resulted in potentially misleading interpretation of spurious components. It is suggested that the routine use of PA in multivariate ordination will increase confidence in the results and reduce the subjective interpretation of supposedly objective methods

A Plea for Risk

Mountaineering is a dangerous activity. For many mountaineers, part of its very attraction is the risk, the thrill of danger. Yet mountaineers are often regarded as reckless or even irresponsible for risking their lives. In this paper, we offer a defence of risk-taking in mountaineering. Our discussion is organised around the fact that mountaineers and non-mountaineers often disagree about how risky mountaineering really is. We hope to cast some light on the nature of this disagreement – and to argue that mountaineering may actually be worthwhile because of the risks it involves. Section 1 introduces the disagreement and, in doing so, separates out several different notions of risk. Sections 2–4 then consider some explanations of the disagreement, showing how a variety of phenomena can skew people’s risk judgements. Section 5 then surveys some recent statistics, to see whether these illuminate how risky mountaineering is. In light of these considerations, however, we suggest that the disagreement is best framed not simply in terms of how risky mountaineering is but whether the risks it does involve are justified. The remainder of the paper, sections 6–9, argues that risk-taking in mountaineering often is justified – and, moreover, that mountaineering can itself be justified (in part) by and because of the risks it involves

The Evolution of the M-sigma Relation

(Abridged) We examine the evolution of the black hole mass - stellar velocity dispersion (M-sigma) relation over cosmic time using simulations of galaxy mergers that include feedback from supermassive black hole growth. We consider mergers of galaxies varying the properties of the progenitors to match those expected at redshifts z=0-6. We find that the slope of the resulting M-sigma relation is the same at all redshifts considered. For the same feedback efficiency that reproduces the observed amplitude of the M-sigma relation at z=0, there is a weak redshift-dependence to the normalization that results from an increasing velocity dispersion for a given galactic stellar mass. We develop a formalism to connect redshift evolution in the M-sigma relation to the scatter in the local relation at z=0. We show that the scatter in the local relation places severe constraints on the redshift evolution of both the normalization and slope of the M-sigma relation. Furthermore, we demonstrate that cosmic downsizing introduces a black hole mass-dependent dispersion in the M-sigma relation and that the skewness of the distribution about the locally observed M-sigma relation is sensitive to redshift evolution in the normalization and slope. In principle, these various diagnostics provide a method for differentiating between theories for producing the M-sigma relation. In agreement with existing constraints, our simulations imply that hierarchical structure formation should produce the relation with small intrinsic scatter.Comment: 12 pages, 6 figures, version accepted by Ap

A Theoretical Interpretation of the Black Hole Fundamental Plane

We examine the origin and evolution of correlations between properties of supermassive black holes (BHs) and their host galaxies using simulations of major galaxy mergers, including the effects of gas dissipation, cooling, star formation, and BH accretion and feedback. We demonstrate that the simulations predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found observationally. The simulations indicate that the BHFP can be understood roughly as a tilted intrinsic correlation between BH mass and spheroid binding energy, or the condition for feedback coupling to power a pressure-driven outflow. While changes in halo circular velocity, merger orbital parameters, progenitor disk redshifts and gas fractions, ISM gas pressurization, and other parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given the empirical trend of decreasing R_e for a given M_bulge at high redshift, the BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement with recent observations. This evolution in the structural properties of merger remnants, to smaller R_e and larger sigma (and therefore larger M_BH, conserving the BHFP) at a given M_bulge, is driven by the fact that bulge progenitors have characteristically larger gas fractions at high redshifts. Adopting the observed evolution of disk gas fractions with redshift, our simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ. Companion paper to arXiv:0707.400

Black Holes in Galaxy Mergers: Evolution of Quasars

Based on numerical simulations of gas-rich galaxy mergers, we discuss a model in which quasar activity is tied to the self-regulated growth of supermassive black holes in galaxies. Nuclear inflow of gas attending a galaxy collision triggers a starburst and feeds black hole growth, but for most of the duration of the starburst, the black hole is heavily obscured by surrounding gas and dust which limits the visibility of the quasar, especially at optical and UV wavelengths. Eventually, feedback energy from accretion heats the gas and expels it in a powerful wind, leaving a 'dead quasar'. Between buried and dead phases there is a window during which the galaxy would be seen as a luminous quasar. Because the black hole mass, radiative output, and distribution of obscuring gas and dust all evolve strongly with time, the duration of this phase of observable quasar activity depends on both the waveband and imposed luminosity threshold. We determine the observed and intrinsic lifetimes as a function of luminosity and frequency, and calculate observable lifetimes ~10 Myr for bright quasars in the optical B-band, in good agreement with empirical estimates and much smaller than the black hole growth timescales ~100 Myr, naturally producing a substantial population of 'buried' quasars. However, observed and intrinsic energy outputs converge in the IR and hard X-ray bands as attenuation becomes weaker and chances of observation greatly increase. We obtain the distribution of column densities along sightlines in which the quasar is seen above a given luminosity, and find that our result agrees remarkably well with observed estimates of the column density distribution from the SDSS for appropriate luminosity thresholds. (Abridged)Comment: 12 pages, 7 figures. Accepted for publication in ApJ (September 2005). Replacement with minor revisions from referee repor

Polyanionic Ligand Platforms for Methyl- and Dimethylaluminum Arrays

Trimethylaluminum finds widespread applications in chemical and materials synthesis, most prominently in its partially hydrolyzed form of methylalumoxane (MAO), which is used as a cocatalyst in the polymerization of olefins. This work investigates the sequential reactions of trimethylaluminum with hexaprotic phosphazenes (RNH)6P3N3 (=XH6) equipped with substituents R of varied steric bulk including tert-butyl (1H6), cyclohexyl (2H6), isopropyl (3H6), isobutyl (4H6), ethyl (5H6), propyl (6H6), methyl (7H6), and benzyl (8H6). Similar to MAO, the resulting complexes of polyanionic phosphazenates [XHn]n−6 accommodate multinuclear arrays of [AlMe2]+ and [AlMe]2+. Reactions were monitored by 31P NMR spectroscopy, and structures were determined by single-crystal X-ray diffraction. They included 1H4(AlMe2)2, 1H3(AlMe2)3, 2H3(AlMe2)3, 3(AlMe2)4AlMe, 4H­(AlMe2)5, 4(AlMe2)6, {5H­(AlMe2)4}2AlMe, 5(AlMe2)6, 6(AlMe2)6, {7(AlMe2)4AlMe}2, and 8(AlMe2)6. The study shows that subtle variations of the steric properties of the R groups influence the reaction pathways, levels of aggregation, and fluxional behavior. While [AlMe2]+ is the primary product of the metalation, [AlMe]2+ is utilized to alleviate overcrowding or to aid aggregation. At the later stages of metalation, [AlMe2]+ groups start to scramble around congested sites. The ligands proved to be very robust and extremely flexible, offering a unique platform to study complex multinuclear metal arrangements

The Evolution in the Faint-End Slope of the Quasar Luminosity Function

(Abridged) Based on numerical simulations of galaxy mergers that incorporate black hole (BH) growth, we predict the faint end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our simulations have yielded a new model for quasar lifetimes where the lifetime depends on both the instantaneous and peak quasar luminosities. This motivates a new interpretation of the QLF in which the bright end consists of quasars radiating at nearly their peak luminosities, but the faint end is mostly made up of quasars in less luminous phases of evolution. The faint-end QLF slope is then determined by the faint-end slope of the quasar lifetime for quasars with peak luminosities near the observed break. We determine this slope from the quasar lifetime as a function of peak luminosity, based on a large set of simulations spanning a wide variety of host galaxy, merger, BH, and ISM gas properties. Brighter peak luminosity (higher BH mass) systems undergo more violent evolution, and expel and heat gas more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From the quasar lifetime as a function of peak luminosity and this interpretation of the QLF, we predict the faint-end QLF slope and its evolution with redshift in good agreement with observations. Although BHs grow anti-hierarchically (with lower-mass BHs formed primarily at lower redshifts), the observed change in slope and differential or luminosity dependent density evolution in the QLF is completely determined by the luminosity-dependent quasar lifetime and physics of quasar feedback.Comment: 13 pages, 4 figures, submitted to ApJ (Replacement with minor revisions and changed sign convention