The Keck+Magellan Survey for Lyman Limit Absorption II: A Case Study on Metallicity Variations

We present an absorption line analysis of the Lyman limit system (LLS) at z=3.55 in our Magellan/MIKE spectrum of PKS2000-330. Our analysis of the Lyman limit and full HI Lyman series constrains the total HI column density of the LLS (N_HI = 10^[18.0 +/- 0.25] cm^{-2} for b_HI >= 20 km/s) and also the N_HI values of the velocity subsystems comprising the absorber. We measure ionic column densities for metal-line transitions associated with the subsystems and use these values to constrain the ionization state (>90% ionized) and relative abundances of the gas. We find an order of magnitude dispersion in the metallicities of the subsystems, marking the first detailed analysis of metallicity variations in an optically thick absorber. The results indicate that metals are not well mixed within the gas surrounding high $z$ galaxies. Assuming a single-phase photoionization model, we also derive an N_H-weighted metallicity, = -1.66 +/- 0.25, which matches the mean metallicity in the neutral ISM in high z damped Lya systems (DLAs). Because the line density of LLSs is ~10 times higher than the DLAs, we propose that the former dominate the metal mass-density at z~3 and that these metals reside in the galaxy/IGM interface. Considerations of a multi-phase model do not qualitatively change these conclusions. Finally, we comment on an anomalously large O^0/Si^+ ratio in the LLS that suggests an ionizing radiation field dominated by soft UV sources (e.g. a starburst galaxy). Additional abundance analysis is performed on the super-LLS systems at z=3.19.Comment: 20 pages, 7 figures (most in color). Accepted to Ap

Predicting QSO Continua in the Ly Alpha Forest

We present a method to make predictions with sets of correlated data values, in this case QSO flux spectra. We predict the continuum in the Lyman-Alpha forest of a QSO, from 1020 -- 1216 A, using the spectrum of that QSO from 1216 -- 1600 A . We find correlations between the unabsorbed flux in these two wavelengths regions in the HST spectra of 50 QSOs. We use principal component analysis (PCA) to summarize the variety of these spectra and we relate the weights of the principal components for 1020 -- 1600 A to the weights for 1216 -- 1600 A, and we apply this relation to make predictions. We test the method on the HST spectra, and we find an average absolute flux error of 9%, with a range 3 -- 30%, where individual predictions are systematically too low or too high. We mention several ways in which the predictions might be improved.Comment: 17 pages, 4 figures, submitted to Ap

Archeops: an instrument for present and future cosmology

Archeops is a balloon-borne instrument dedicated to measure the cosmic microwave background (CMB) temperature anisotropies. It has, in the millimetre domain (from 143 to 545 GHz), a high angular resolution (about 10 arcminutes) in order to constrain high l multipoles, as well as a large sky coverage fraction (30%) in order to minimize the cosmic variance. It has linked, before WMAP, Cobe large angular scales to the first acoustic peak region. From its results, inflation motivated cosmologies are reinforced with a flat Universe (Omega_tot=1 within 3%). The dark energy density and the baryonic density are in very good agreement with other independent estimations based on supernovae measurements and big bang nucleosynthesis. Important results on galactic dust emission polarization and their implications for Planck are also addressed.Comment: 4 pages, 2 figures, to appear in Proceedings of the Multiwavelength Cosmology Conference, June 2003, Mykonos Island, Greec

QSO 0130-4021: A third QSO showing a low Deuterium to Hydrogen Abundance Ratio

We have discovered a third quasar absorption system which is consistent with a low deuterium to hydrogen abundance ratio, D/H = 3.4 times 10^-5. The z ~ 2.8 partial Lyman limit system towards QSO 0130-4021 provides the strongest evidence to date against large D/H ratios because the H I absorption, which consists of a single high column density component with unsaturated high order Lyman series lines, is readily modeled -- a task which is more complex in other D/H systems. We have obtained twenty-two hours of spectra from the HIRES spectrograph on the W.M. Keck telescope, which allow a detailed description of the Hydrogen. We see excess absorption on the blue wing of the H I Lyman alpha line, near the expected position of Deuterium. However, we find that Deuterium cannot explain all of the excess absorption, and hence there must be contamination by additional absorption, probably H I. This extra H I can account for most or all of the absorption at the D position, and hence D/H = 0 is allowed. We find an upper limit of D/H < 6.7 times 10^-5 in this system, consistent with the value of D/H ~ 3.4 times 10^-5 deduced towards QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). This absorption system shows only weak metal line absorption, and we estimate [Si/H] < -2.6 -- indicating that the D/H ratio of the system is likely primordial. All four of the known high redshift absorption line systems simple enough to provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 times 10^-5. Conversely, this QSO provides the third case which is inconsistent with much larger values.Comment: 18 pages, 5 figures, submitted to Ap

Prevalence, heterogeneity of asymptomatic malaria infections and associated factors in a high transmission region

Background: Although current reports have shown a reduction in malaria cases, the disease still remains a major public health problem in Kenya. In most endemic regions, the majority of infections are asymptomatic which means those infected may not even know and yet they remain infectious to the mosquitoes. Asymptomatic infections are a major threat to malaria control programs since they act as silent reservoirs for the malaria parasites.Objective: The study sought to determine the prevalence of asymptomatic malaria infections, whether they show heterogeneity spatially, across age groups and across time as well as their determinants in a high transmission region.Study Design: This was part of a larger prospective cohort study on malaria indices in the HDSS.Study Setting: The study was conducted in the Webuye Health and Demographic Surveillance Site in Bungoma East Sub-County.Study Subjects: Quarterly parasitological surveys were conducted for a cohort of 400 participants from randomly selected households located in known fever “hotspots” and “coldspots”. Follow-up of all the participants continued for a period of one year. Generalized estimating equations were used to model risk factors associated with asymptomatic parasitemia.Results: Of the total 321 malaria infections detected during the five cross-sectional surveys conducted over the period of one year, almost half (46.3%) of these were asymptomatic. Overall, most of the asymptomatic cases (67%) were in households within known fever “hotspots”. The proportion of infections that were asymptomatic in the coldspots were 73.1%, 31.8%, 13.3%, 55.6% and 48.2% during the first, second, third, fourth and fifth visits respectively. In the known fever “hotspots”, the proportion of infections without symptoms was 47.7%, 48.5%, 35%, 41.3% and 47.5% during the first, second, third, fourth and fifth visits respectively. Factors associated with asymptomatic malaria include; the village one lives: people living in village M were twice likely to be asymptomatic (A.O.R: 2.141, C.I: 0.03 - 1.488), age: children aged between 6 to 15 years were more than twice likely to be asymptomatic (A.O.R: 2.67, C.I. 0.434 - 1.533) and the season: infections during the dry season (January) were less likely to be asymptomatic (A.O.R: 0.26, C.I: -2.289 - 0.400).Conclusion: The prevalence of asymptomatic infections in this region is still very high. The highest proportion of asymptomatic infections was registered in a fever coldpspot village which may explain why the village is a fever coldspot in the first place. There is a need for active surveillance to detect the asymptomatic cases as well as treat them in-order to reduce the reservoir. Targeting interventions to the asymptomatic individuals will further reduce the transmission within this region

Malaria “hotspots” within a larger hotspot; what’s the role of behavioural factors in fine scale heterogeneity in western Kenya?

Background: Malaria remains a major public health problem in Kenya accounting for the highest morbidity and mortality especially among children. Previous reports indicate that infectious agents display heterogeneity in both space and time and malaria is no exception. Heterogeneity has been shown to reduce the effectiveness of interventions. Previous studies have implicated genetic (both human and parasite) and environmental factors as mainly responsible for variation in malaria risk. Human behaviour and its potential risk for contributing to variation in malaria risk has not been extensively explored.Objective: To determine if there were behavioural differences between the people living in hotspots (high malaria burden) and cold spots (low malaria burden) within a geographically homogeneous and high malaria transmission region.Design: A prospective closed cohort study.Setting: The study was conducted in the Health and Demographic Surveillance Site in Bungoma East sub-County.Subjects: A total of 400 people in randomly selected households in both the fever hotspots and cold spots were tested for malaria at quarterly intervals using malaria rapid diagnostic tests (RDTs).Results: Significant heterogeneity in malaria incidence and prevalence was observed between villages. Incidence of malaria was significantly higher in the hotspots (high malaria burden areas) compared to the cold spots (low malaria burden) (49 episodes per 1000 person months compared to 26/1000, t test p &lt; 0.001). The incidence also varied significantly among the individual villages by season (P: 0.0071). Knowledge on malaria therapy was significantly  associated with whether one was in the cold spot or hotspot (P: 0.033). Behavioural practices relating to ITN use were significantly associated with region during particular seasons (P: 0.0001 and P: 0.0001 respectively).Conclusion: There is marked and significant variation in the incidence of malaria among the villages creating actual hotspots of malaria within the larger hotspot. There is a significant difference in malaria infections between the hotspots and cold spots. Knowledge on malaria therapy and behavioural factors such as ITN use may contribute to the observed differences during some seasons

On the galloping instability of two-dimensional bodies having elliptical cross sections.

Galloping, also known as Den Hartog instability, is the large amplitude, low frequency oscillation of a structure in the direction transverse to the mean wind direction. It normally appears in the case of bodies with small stiffness and structural damping, when they are placed in a flow provided the incident velocity is high enough. Galloping depends on the slope of the lift coefficient versus angle of attack curve, which must be negative. Generally speaking this implies that the body is stalled after boundary layer separation, which, as it is known in non-wedged bodies, is a Reynolds number dependent phenomenon. Wind tunnel experiments have been conducted aiming at establishing the characteristics of the galloping motion of elliptical cross-section bodies when subjected to a uniform flow, the angles of attack ranging from 0° to 90°. The results have been summarized in stability maps, both in the angle of attack versus relative thickness and in the angle of attack versus Reynolds number planes, where galloping instability regions are identified

The Keck+Magellan Survey for Lyman Limit Absorption I: The Frequency Distribution of Super Lyman Limit Systems

We present the results of a survey for super Lyman limit systems (SLLS; defined to be absorbers with 19.0 <= log(NHI) <= 20.3 cm^-2) from a large sample of high resolution spectra acquired using the Keck and Magellan telescopes. Specifically, we present 47 new SLLS from 113 QSO sightlines. We focus on the neutral hydrogen frequency distribution f(N,X) of the SLLS and its moments, and compare these results with the Lyman-alpha forest and the damped Lyman alpha systems (DLA; absorbers with log(NHI) >= 20.3 cm^-2). We find that that f(N,X) of the SLLS can be reasonably described with a power-law of index alpha = -1.43^{+0.15}_{-0.16} or alpha = -1.19^{+0.20}_{-0.21} depending on whether we set the lower N(HI) bound for the analysis at 10^{19.0} cm^-2 or 10^{19.3}$ cm^-2, respectively. The results indicate a flattening in the slope of f(N,X) between the SLLS and DLA. We find little evidence for redshift evolution in the shape of f(N,X) for the SLLS over the redshift range of the sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth moment of f(N,X), the line density l_lls(X). We introduce the observable distribution function O(N,X) and its moment, which elucidates comparisons of HI absorbers from the Lyman-alpha through to the DLA. We find that a simple three parameter function can fit O(N,X) over the range 17.0 <= log(NHI) <=22.0. We use these results to predict that f(N,X) must show two additional inflections below the SLLS regime to match the observed f(N,X) distribution of the Lyman-alpha forest. Finally, we demonstrate that SLLS contribute a minor fraction (~15%) of the universe's hydrogen atoms and, therefore, an even small fraction of the mass in predominantly neutral gas.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journal. Revision includes updated reference

PREDOMINANTLY LOW METALLICITIES MEASURED IN A STRATIFIED SAMPLE OF LYMAN LIMIT SYSTEMS AT Z = 3.7

We measured metallicities for 33 z = 3.4–4.2 absorption line systems drawn from a sample of H i-selected-Lyman limit systems (LLSs) identified in Sloan Digital Sky Survey (SDSS) quasar spectra and stratified based on metal line features. We obtained higher-resolution spectra with the Keck Echellette Spectrograph and Imager, selecting targets according to our stratification scheme in an effort to fully sample the LLS population metallicity distribution. We established a plausible range of H i column densities and measured column densities (or limits) for ions of carbon, silicon, and aluminum, finding ionization-corrected metallicities or upper limits. Interestingly, our ionization models were better constrained with enhanced α-to-aluminum abundances, with a median abundance ratio of [α/Al] = 0.3. Measured metallicities were generally low, ranging from [M/H] = −3 to −1.68, with even lower metallicities likely for some systems with upper limits. Using survival statistics to incorporate limits, we constructed the cumulative distribution function (CDF) for LLS metallicities. Recent models of galaxy evolution propose that galaxies replenish their gas from the low-metallicity intergalactic medium (IGM) via high-density H i "flows" and eject enriched interstellar gas via outflows. Thus, there has been some expectation that LLSs at the peak of cosmic star formation (z ≈ 3) might have a bimodal metallicity distribution. We modeled our CDF as a mix of two Gaussian distributions, one reflecting the metallicity of the IGM and the other representative of the interstellar medium of star-forming galaxies. This bimodal distribution yielded a poor fit. A single Gaussian distribution better represented the sample with a low mean metallicity of [M/H] ≈ −2.5.Massachusetts Institute of Technology. Undergraduate Research Opportunities ProgramNational Science Foundation (U.S.) (Award AST-1109915

The Deuterium to Hydrogen Abundance Ratio Towards a Fourth QSO: HS0105+1619

We report the measurement of the primordial D/H abundance ratio towards QSO \object. The column density of the hydrogen in the $z \simeq 2.536$ Lyman limit system is high, \lnhi $= 19.422 \pm 0.009$ \cmm, allowing for the deuterium to be seen in 5 Lyman series transitions. The measured value of the D/H ratio towards QSO \object is found to be D/H$= 2.54 \pm 0.23 \times 10^{-5}$. The metallicity of the system showing D/H is found to be $\simeq 0.01$ solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas which shows D/H is neutral, unlike previous D/H systems which were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H$= 3.0 \pm 0.4 \times 10^{-5}$, which is 10% lower than the previous value. The new values for the baryon to photon ratio, and baryonic matter density derived from D/H are $\eta = 5.6 \pm 0.5 \times 10^{-10}$ and \ob $=0.0205 \pm 0.0018$ respectively.Comment: Minor text and reference changes. To appear in the May 10, 2001 issue of the Astrophysical Journa