Perioperative Hemostasis in Neurosurgery

Picard, J.; Bouzat, Pierre; Francony, Gilles; Payen, Jean François; Schoettker, Patrick

doi:10.1007/978-3-642-55004-1_19

Cited by 8 publications

(7 citation statements)

References 89 publications

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“…The agreement between the empirical model and the data is excellent. These diagrams clearly demonstrate that the UV flux of z 1 and z 2.3 quasars, for FUV and NUV respectively, is suppressed because of absorption below the Lyman limit (λ912Å) by intervening systems (Møller & Jakobsen 1990;Picard & Jakobsen 1993;Worseck & Prochaska 2011). UV observations are an excellent tool to distinguish z ≈ 0.8 quasars from z ≈ 2.3 quasars, which have degenerate ugriz colors and plague medium-redshift quasar selection (e.g., Ross et al 2011), even at low UV signal-to-noise ratio.…”

Section: Comparison Of the Model And Observationsmentioning

confidence: 88%

Photometric Redshifts and Quasar Probabilities From a Single, Data-Driven Generative Model

Bovy

Myers

McMahon

et al. 2012

ApJ

134

169

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We describe a technique for simultaneously classifying and estimating the redshift of quasars. It can separate quasars from stars in arbitrary redshift ranges, estimate full posterior distribution functions for the redshift, and naturally incorporate flux uncertainties, missing data, and multi-wavelength photometry. We build models of quasars in flux-redshift space by applying the extreme deconvolution technique to estimate the underlying density. By integrating this density over redshift one can obtain quasar flux-densities in different redshift ranges. This approach allows for efficient, consistent, and fast classification and photometric redshift estimation. This is achieved by combining the speed obtained by choosing simple analytical forms as the basis of our density model with the flexibility of non-parametric models through the use of many simple -2components with many parameters. We show that this technique is competitive with the best photometric quasar classification techniques-which are limited to fixed, broad redshift ranges and high signal-to-noise ratio data-and with the best photometric redshift techniques when applied to broadband optical data. We demonstrate that the inclusion of UV and NIR data significantly improves photometric quasar-star separation and essentially resolves all of the redshift degeneracies for quasars inherent to the ugriz filter system, even when included data have a low signal-to-noise ratio. For quasars spectroscopically confirmed by the SDSS 84 and 97 percent of the objects with GALEX UV and UKIDSS NIR data have photometric redshifts within 0.1 and 0.3, respectively, of the spectroscopic redshift; this amounts to about a factor of three improvement over ugriz-only photometric redshifts. Our code to calculate quasar probabilities and redshift probability distributions is publicly available.

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Section: Comparison Of the Model And Observationsmentioning

confidence: 88%

Photometric Redshifts and Quasar Probabilities From a Single, Data-Driven Generative Model

Bovy

Myers

McMahon

et al. 2012

ApJ

134

169

View full text Add to dashboard Cite

show abstract

“…Becker et al 2001;White et al 2003;Fan et al 2006), models predict a transition from a forest of He II absorption lines at z ∼ 2 to troughs of complete absorption as the number of IGM patches with significant He II fractions rises with redshift, signaling the epoch of helium reionization. However, for the first decade of HST operations only seven He II sightlines probed this transition, because for most z em > 2.7 quasars the spectral range covering He II absorption is extinguished by intervening optically thick H I absorbers (Picard & Jakobsen 1993;Worseck & Prochaska 2011). HST low-resolution (R = λ /∆λ < 2, 000) spectra of the four sightlines covering z > 3 (Q 0302−003, PKS 1935−692, SDSS J2346−0016, SDSS J1711+6052) generally reveal large He II effective optical depths τ eff,HeII > 3, indicating an incomplete He II reionization (Jakobsen et al 1994;Hogan et al 1997;Heap et al 2000;Anderson et al 1999;Zheng et al 2004aZheng et al , 2008.…”

Section: Introductionmentioning

confidence: 99%

Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time*

Worseck

Prochaska

McQuinn

2016

ApJ

155

223

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We measure the effective optical depth of He II Lyα absorption τ eff,HeII at 2.3 < z < 3.5 in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope (HST). The median τ eff,HeII values increase gradually from 1.95 at z = 2.7 to 5.17 at z = 3.4, but with a strong sightline-to-sightline variance. Many ≃ 35 comoving Mpc regions of the z > 3 intergalactic medium (IGM) remain transmissive (τ eff,HeII < 4), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM. These modest optical depths imply average He II fractions of x HeII < 0.01 and He II ionizing photon mean free paths of ≃ 50 comoving Mpc at z ≃ 3.4, thus requiring that a substantial volume of the helium in the Universe was already doubly ionized at early times; this stands in conflict with current models of He II reionization driven by luminous quasars. Along 10 sightlines we measure the coeval H I Lyα effective optical depths, allowing us to study the density dependence of τ eff,HeII at z ∼ 3. We establish that the dependence of τ eff,HeII on increasing τ eff,HI is significantly shallower than expected from simple models of an IGM reionized in He II. This requires higher He II photoionization rates in overdense regions or underdense regions being not in photoionization equilibrium. Moreover, there are very large fluctuations in τ eff,HeII at all τ eff,HI which greatly exceed the expectations from these simple models. These data present a distinct challenge to scenarios of He II reionization -an IGM where He II appears to be predominantly ionized at z ≃ 3.4, and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities.

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“…The discovery and systematic study of the He II Lyα forest at z 2.7 has been a science driver for HST since its inception (Bahcall 1979). However, despite extensive efforts during the first 15 years of HST operations, only seven He II sightlines had been successfully probed, because for most z em > 2.7 quasars the spectral range covering the He II Lyman series is blacked out by optically thick H I Lyman limit systems in the foreground IGM (Picard & Jakobsen 1993;Worseck & Prochaska 2011). Early HST spectra of varying spectral resolution and quality revealed strong evolution of the He II Lyα effective optical depth τ eff from Gunn-Peterson troughs (τ eff 3) measured toward four z em > 3 quasars (Jakobsen et al 1994;Hogan et al 1997;Anderson et al 1999;Heap et al 2000;Zheng et al 2004aZheng et al , 2008 to fluctuating He II absorption at 2.7 z 2.9 (Reimers et al 1997(Reimers et al , 2005Heap et al 2000;Smette et al 2002).…”

Section: Introductionmentioning

confidence: 99%

The Evolution of the He ii-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He ii Lyα Absorption Spectra*

Worseck

Davies

Prochaska

2019

ApJ

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We present measurements of the large-scale (≈ 40 comoving Mpc) effective optical depth of He II Lyα absorption, τ eff , at 2.54 < z < 3.86 toward 16 He II-transparent quasars observed with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of τ eff measurements in six signal-to-noise ratio 3 He II sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of τ eff in 24 He II sightlines. We confirm an increase of the median τ eff from 2 at z = 2.7 to τ eff 5 at z > 3, and a scatter in τ eff that increases with redshift. The z > 3.5 He II absorption is predominantly saturated, but isolated narrow (∆v < 650 km s −1 ) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc) 3 hydrodynamical simulation by forward-modeling our sample's quality and size. At z > 2.74 the variance in τ eff significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He II photoionization rate Γ HeII of a fluctuating UV background from the median τ eff , finding a factor 5 decrease in Γ HeII between z 2.6 and z 3.1. At z 3.1 a Γ HeII = 9.1 +1.1 −1.2 (stat.) +2.4 −3.4 (sys.) × 10 −16 s −1 corresponds to a median He II fraction of 2.5%, indicating that our data probe the tail end of He II reionization.

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Perioperative Hemostasis in Neurosurgery

Cited by 8 publications

References 89 publications

Photometric Redshifts and Quasar Probabilities From a Single, Data-Driven Generative Model

Photometric Redshifts and Quasar Probabilities From a Single, Data-Driven Generative Model

Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time*

The Evolution of the He ii-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He ii Lyα Absorption Spectra*

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