Joint Bayesian Estimation of Quasar Continua and the Lyman-Alpha Forest Flux Probability Distribution Function

Abstract

We present a new Bayesian algorithm making use of Markov Chain Monte Carlo sampling that allows us to simultaneously estimate the unknown continuum level of each quasar in an ensemble of high-resolution spectra, as well as their common probability distribution function (PDF) for the transmitted Ly$\alpha$ forest flux. This fully automated PDF regulated continuum fitting method models the unknown quasar continuum with a linear Principal Component Analysis (PCA) basis, with the PCA coefficients treated as nuisance parameters. The method allows one to estimate parameters governing the thermal state of the intergalactic medium (IGM), such as the slope of the temperature-density relation $\gamma -1$, while marginalizing out continuum uncertainties in a fully Bayesian way. Using realistic mock quasar spectra created from a simplified semi-numerical model of the IGM, we show that this method recovers the underlying quasar continua to a precision of $\simeq 7\%$ and $\simeq 10\%$ at $z=3$ and $z=5$, respectively. Given the number of principal component spectra, this is comparable to the underlying accuracy of the PCA model itself. Most importantly, we show that we can achieve a nearly unbiased estimate of the slope $\gamma -1$ of the IGM temperature-density relation with a precision of $\pm 8.6\%$ at $z=3$, $\pm 6.1\%$ at $z=5$, for an ensemble of ten mock high-resolution quasar spectra. Applying this method to real quasar spectra and comparing to a more realistic IGM model from hydrodynamical simulations would enable precise measurements of the thermal and cosmological parameters governing the IGM, albeit with somewhat larger uncertainties given the increased flexibility of the model.

21 pages (+ Appendix), accepted at ApJ

References in corpus (10)

• HeII Reionization and its Effect on the IGM
• A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2
• Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
• An improved measurement of the flux distribution of the Ly-alpha forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties
• Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations
• An unbiased measurement of the UV background and its evolution via the proximity effect in quasar spectra
• IGM Constraints from the SDSS-III/BOSS DR9 Ly-alpha Forest Flux Probability Distribution Function
• Inhomogeneous Helium Reionization and the Equation of State of the Intergalactic Medium
• Exploring the Thermal State of the Low-Density Intergalactic Medium at z=3 with an Ultra-High Signal-to-Noise QSO Spectrum
• The Probability Distribution of the Lya transmitted flux from a sample of SDSS quasars

Cited by in corpus (4)

• Detecting and Characterizing Young Quasars II: Four Quasars at $z\sim 6$ with Lifetimes $<10^4$ years
• Quasar Factor Analysis -- An Unsupervised and Probabilistic Quasar Continuum Prediction Algorithm with Latent Factor Analysis
• Improving IGM temperature constraints using wavelet analysis on high-redshift quasars
• Forecasting constraints on the mean free path of ionizing photons at $z\ge 5.4$ from the Lyman-$\alpha$ forest flux auto-correlation function