Constraining the UV background with the proximity effect

After the epoch of reionisation the intergalactic medium (IGM) is kept at a high photoionisation level by the cosmic UV background radiation field. Primarily composed of the integrated contribution of quasars and young star forming galaxies, its intensity is subject to spatial and temporal fluctuations. In particular in the vicinity of luminous quasars, the UV radiation intensity grows by several orders of magnitude. Due to an enhanced UV radiation up to a few Mpc from the quasar, the ionised hydrogen fraction significantly increases and becomes visible as a reduced level of absorption in the HI Lyman alpha (Ly-alpha) forest. This phenomenon is known as the proximity effect and it is the main focus of this thesis ...thesi

Analysis of methods for detecting the proximity effect in quasar spectra

Using numerical simulations of structure formation, we investigate multiple methods of determining the strength of the proximity effect in the HI Lyalpha forest. We analyze three high resolution (~10kpc) redshift snapshots (z=4,3,2.25) of a Hydro-Particle-Mesh simulation to obtain realistic absorption spectra of the HI Lyalpha forest. We begin our analysis investigating the intrinsic biases thought to arise in the widely adopted standard technique of combining multiple lines of sight when searching for the proximity effect. We confirm the existence of this biases. We then concentrate on the analysis of the proximity effect along individual lines of sight. We construct the proximity effect strength distribution (PESD) and confirm that the PESD inferred from a simple averaging technique accurately recovers the input strength of the proximity effect at all redshifts. Moreover, the PESD closely follows the behaviors found in observed samples of quasar spectra. However, the PESD obtained from our new simulated sight lines presents some differences to that of simple Monte Carlo simulations. After developing three new theoretical methods of recovering the strength of the proximity effect on individual lines of sight, we compare their accuracy to the PESD from the simple averaging technique. All our new approaches are based on the maximization of the likelihood function, albeit invoking some modifications. The new techniques presented here fail to recover the input proximity effect in an un-biased way. Thus, employing complex 3D simulations, we provide strong evidence in favor of the proximity effect strength distribution obtained from the simple averaging technique, as method of estimating the UV background intensity, free of any biases.Comment: 12 pages, 14 figures, accepted by the Astrophysical journa

High-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows

We present a study of 15 long-duration gamma-ray burst (GRB) host galaxies at z>2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB050820 and GRB060206 and place 2-sigma upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of =0.1 L*; (2) there exists a moderate correlation between UV luminosity and SiII 1526 absorption width, which together with the observed large line widths of W(1526)>1.5 Ang for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies; (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z=2-4; (4) the interstellar UV radiation field is found ~ 35-350 times higher in GRB hosts than the Galactic mean value; and (5) additional galaxies are found at < 2" from the GRB host in all fields with known presence of strong MgII absorbers, but no additional faint galaxies are found at < 2" in fields without strong MgII absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z>2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.Comment: 24 emulateapj pages, 24 figures, ApJ in press; full-resolution version available at http://lambda.uchicago.edu/public/tmp/ghost.pd

The end of Helium reionization at z ~= 2.7 inferred from cosmic variance in HST/COS He II Lya absorption spectra

We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z em ~= 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from \langle \tau _{eff,He\,\mathsc{ii}}\rangle \simeq 1 at z ~= 2.3 to \langle \tau _{eff,He\,\mathsc{ii}}\rangle \gtrsim 5 at z ~= 3.2, but with a large scatter of 2\lesssim \tau _{eff,He\,\mathsc{ii}}\lesssim 5 at 2.7 < z < 3 on scales of ~10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval H I forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z >~ 2.7, probably indicating He II reionization was incomplete at z reion >~ 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z reion ~= 2.7 or even below, as suggested by a large \tau _{eff,He\,\mathsc{ii}}\gtrsim 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 <~ z <~ 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555 (programs 7575, 9350, 11528, 11742). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA; it was made possible by the generous financial support of the W.M. Keck Foundation. Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile (programs 166.A.-0106, 071.A-0066, 083.A-0421)

Effects on the incidence of cardiovascular events of the addition of pioglitazone versus sulfonylureas in patients with type 2 diabetes inadequately controlled with metformin (TOSCA.IT): a randomised, multicentre trial

Background The best treatment option for patients with type 2 diabetes in whom treatment with metformin alone fails to achieve adequate glycaemic control is debated. We aimed to compare the long-term effects of pioglitazone versus sulfonylureas, given in addition to metformin, on cardiovascular events in patients with type 2 diabetes. Methods TOSCA.IT was a multicentre, randomised, pragmatic clinical trial, in which patients aged 50\ue2\u80\u9375 years with type 2 diabetes inadequately controlled with metformin monotherapy (2\ue2\u80\u933 g per day) were recruited from 57 diabetes clinics in Italy. Patients were randomly assigned (1:1), by permuted blocks randomisation (block size 10), stratified by site and previous cardiovascular events, to add-on pioglitazone (15\ue2\u80\u9345 mg) or a sulfonylurea (5\ue2\u80\u9315 mg glibenclamide, 2\ue2\u80\u936 mg glimepiride, or 30\ue2\u80\u93120 mg gliclazide, in accordance with local practice). The trial was unblinded, but event adjudicators were unaware of treatment assignment. The primary outcome, assessed with a Cox proportional-hazards model, was a composite of first occurrence of all-cause death, non-fatal myocardial infarction, non-fatal stroke, or urgent coronary revascularisation, assessed in the modified intention-to-treat population (all randomly assigned participants with baseline data available and without any protocol violations in relation to inclusion or exclusion criteria). This study is registered with ClinicalTrials.gov, number NCT00700856. Findings Between Sept 18, 2008, and Jan 15, 2014, 3028 patients were randomly assigned and included in the analyses. 1535 were assigned to pioglitazone and 1493 to sulfonylureas (glibenclamide 24 [2%], glimepiride 723 [48%], gliclazide 745 [50%]). At baseline, 335 (11%) participants had a previous cardiovascular event. The study was stopped early on the basis of a futility analysis after a median follow-up of 57\uc2\ub73 months. The primary outcome occurred in 105 patients (1\uc2\ub75 per 100 person-years) who were given pioglitazone and 108 (1\uc2\ub75 per 100 person-years) who were given sulfonylureas (hazard ratio 0\uc2\ub796, 95% CI 0\uc2\ub774\ue2\u80\u931\uc2\ub726, p=0\uc2\ub779). Fewer patients had hypoglycaemias in the pioglitazone group than in the sulfonylureas group (148 [10%] vs 508 [34%], p&lt;0\uc2\ub70001). Moderate weight gain (less than 2 kg, on average) occurred in both groups. Rates of heart failure, bladder cancer, and fractures were not significantly different between treatment groups. Interpretation In this long-term, pragmatic trial, incidence of cardiovascular events was similar with sulfonylureas (mostly glimepiride and gliclazide) and pioglitazone as add-on treatments to metformin. Both of these widely available and affordable treatments are suitable options with respect to efficacy and adverse events, although pioglitazone was associated with fewer hypoglycaemia events. Funding Italian Medicines Agency, Diabete Ricerca, and Italian Diabetes Society