New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization

We present new measurements of the intensity of the ionizing ultraviolet background and the global emissivity of ionizing photons over 2 < z < 5. Our results are based on a suite of updated measurements of physical properties of the high-redshift intergalactic medium (IGM), including gas temperatures and the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with previous works, we find a relatively flat hydrogen photoionization rate over 2 < z < 5, although our measurements are roughly a factor of two higher than the 2008 values of Faucher-Giguere et al., due primarily to our lower gas temperatures. The ionizing emissivity we derive is also generally higher than other recent estimates due to a combination of lower gas temperatures, higher ionizing opacity, and an accounting of cosmological radiative transfer effects. We find evidence that the emissivity increases from z~3 to 5, reaching ~5 ionizing photons per atom per gigayear at z=4.75 for realistic galaxy spectra. We further find that galaxies must dominate the emissivity near 1 Ryd at z > 4, and possibly at all redshifts z > 2.4. Our results suggest that the globally-averaged ionizing "efficiency" of star-forming galaxies increases substantially with redshift over 3.2 < z < 4.75. This trend is consistent with the conclusion often drawn from reionization models that the ionizing efficiency of galaxies must be higher during reionization in order for galaxies to reionize the IGM by z=6. Our emissivity values at z~5 suggest that ionizing photons may have been a factor of two more abundant during the final stages of reionization than previously indicated. The evolution of the ionizing emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the photoionization rate from z~5 to 6 may indicate a rapid evolution in the mean free path at z > 5.Comment: 19 pages, 14 figures, MNRAS, in pres

Elections, Ideology, and Turnover in the U.S. Federal Government

A defining feature of public sector employment is the regular change in elected leadership. Yet, we know little about how elections influence public sector careers. We describe how elections alter policy outputs and disrupt the influence of civil servants over agency decisions. These changes shape the career choices of employees motivated by policy, influence, and wages. Using new Office of Personnel Management data on the careers of millions of federal employees between 1988 and 2011, we evaluate how elections influence employee turnover decisions. We find that presidential elections increase departure rates of career senior employees, particularly in agencies with divergent views relative to the new president and at the start of presidential terms. We also find suggestive evidence that vacancies in high-level positions after elections may induce lower-level executives to stay longer in hopes of advancing. We conclude with implications of our findings for public policy, presidential politics, and public management

Bored pile design in stiff clay II:Mechanisms and uncertainty

The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stress–strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays. </jats:p

The plastic limit of clays

The plastic limit of soils was first described by Atterberg in 1911. The thread-rolling test was standardised at the US Public Roads Bureau in the 1920s and 1930s, and has subsequently become one of the standard tests of soil mechanics. This paper reviews the original definitions of plastic limit as proposed by Atterberg, and proposes that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay. Critical state soil mechanics is used to show that the observed range of undrained shear strengths of soils at plastic limit is consistent with this hypothesis. The fallacy that strength at plastic limit is a constant is highlighted, and the implications for geotechnical practice are discussed. </jats:p

Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state

(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the SEI method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. The Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity $v$ increases with radius $r$ according to $v=v_\infty(1-r_\star/r)^\beta$, with$\beta\approx0.75$ and $v_\infty\approx1420$ km s$^{-1}$.The microturbulent velocity was $\approx160$ km s$^{-1}$. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate of L$_{X,38}/\dot M_{-6} \approx 0.33$, measured at $\dot M_{-6}$ = 4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L$_x$ matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap

Thermal constraints on the reionisation of hydrogen by population-II stellar sources

Measurements of the intergalactic medium (IGM) temperature provide a potentially powerful constraint on the reionisation history due to the thermal imprint left by the photo-ionisation of neutral hydrogen. However, until recently IGM temperature measurements were limited to redshifts 2 < z < 4.8, restricting the ability of these data to probe the reionisation history at z > 6. In this work, we use recent measurements of the IGM temperature in the near-zones of seven quasars at z ~ 5.8 - 6.4, combined with a semi-numerical model for inhomogeneous reionisation, to establish new constraints on the redshift at which hydrogen reionisation completed. We calibrate the model to reproduce observational constraints on the electron scattering optical depth and the HI photo-ionisation rate, and compute the resulting spatially inhomogeneous temperature distribution at z ~ 6 for a variety of reionisation scenarios. Under standard assumptions for the ionising spectra of population-II sources, the near-zone temperature measurements constrain the redshift by which hydrogen reionisation was complete to be z > 7.9 (6.5) at 68 (95) per cent confidence. We conclude that future temperature measurements around other high redshift quasars will significantly increase the power of this technique, enabling these results to be tightened and generalised.Comment: 15 pages, 8 figures, accepted for publication in MNRA

County of San Mateo Adolescent Report 2014-2015 Youth and Adults Working Together for a Healthy Future

The purpose of this paper is to highlight my fieldwork experience with the County of San Mateo Health System’s Health Policy and Planning Unit. Under the guidance of a Community Health Planner, my scope of work was within the area of school wellness, ranging from an adolescent report publication to researching breakfast in the classroom models to attending district wellness policy meetings. The San Mateo County Adolescent Report took precedence over most of the school wellness projects and initiatives because it was seeking to identify the needs of the County’s youth using evidence from various data sources and to develop policy recommendations. Although the Adolescent Report highlighted a multitude of youth health concerns, a large focus was on the mental health of youth. Throughout the report, I will provide an in-depth overview of the Health Policy and Planning Unit’s role in the development of the Adolescent Report, the implementation process, the findings and application of the results. I will briefly highlight other individual and group projects that I had the opportunity to be involved in and help execute. I will discuss my initial learning objectives, how they altered over time, and my achievements. I will identify which Master of Public Health Program competencies were addressed during the implementation of the Adolescent Report and other projects. Lastly, I will provide a short reflection on my overall field work experience from a personal and professional viewpoint