A comparison of quasar emission reconstruction techniques for z ≥ 5.0 Lyman α and Lyman β transmission

Abstract

Reconstruction techniques for intrinsic quasar continua are crucial for the precision study of Lyman α (Ly α) and Lyman β (Ly β) transmission at z > 5.5, where the λ < 1215 Å emission of quasars is nearly completely absorbed. While the number and quality of spectroscopic observations have become theoretically sufficient to quantify Ly α transmission at 5.0 < z < 6.0 to better than $1{{\ \rm per\ cent}}$, the biases and uncertainties arising from predicting the unabsorbed continuum are not known to the same level. In this paper, we systematically evaluate eight reconstruction techniques on a unified testing sample of 2.7 < z < 3.5 quasars drawn from the Extended Baryon Oscillation Spectroscopic Survey. The methods include power-law extrapolation, stacking of neighbours, and six variants of principal component analysis (PCA) using direct projection, fitting of components, or neural networks to perform weight mapping. We find that power-law reconstructions and the PCA with fewest components and smallest training sample display the largest biases in the Ly α forest (${-}9.58{{\ \rm per\ cent}}/{+}8.22{{\ \rm per\ cent}}$, respectively). Power-law extrapolations have larger scatters than previously assumed of ${+}13.1{{\ \rm per\ cent}}/{-}13.2{{\ \rm per\ cent}}$ over Ly α and ${+}19.9{{\ \rm per\ cent}}/{-}20.1{{\ \rm per\ cent}}$ over Ly β. We present two new PCAs that achieve the best current accuracies of $9{{\ \rm per\ cent}}$ for Ly α and $17{{\ \rm per\ cent}}$ for Ly β. We apply the eight techniques after accounting for wavelength-dependent biases and scatter to a sample of 19 quasars at z > 5.7 with IR X-Shooter spectroscopy, obtaining well-characterized measurements for the mean flux transmission at 4.7 < z < 6.3. Our results demonstrate the importance of testing and, when relevant, training, continuum reconstruction techniques in a systematic way.

Measuring the Density Fields around Bright Quasars at $z\sim 6$ with XQR-30 Spectra

“…We reconstruct the un-absorbed quasar emission in the Lyman-α line by performing a Principal Component Analysis (PCA) on a large set of quasars at lower z. Our procedure follows the log-PCA approach of Davies et al (2018) with the improvements introduced in Bosman et al (2021b). The specific PCA used in this paper differs from previous work mainly on the wavelength range we predict and the training/testing sample.…”

“…This step enables the PCA to be less biased by random noise in the spectra. The spline is fit automatically following the procedure of Young et al (1979); Carswell et al (1982) as implemented by Dall'Aglio et al (2008) and refined by Bosman et al (2021b). We then divide the sample randomly into a training and a testing sample of equal size.…”

IGM damping wing constraints on reionisation from covariance reconstruction of two $z\gtrsim7$ QSOs

“…Here, spectral features of the QSO continuum are deconstructed into principal component vectors. These principal components are separated redward and blueward of λ = 1280 Å with either a linear (Davies et al 2018b;Bosman et al 2021b) or neural network based ( Ďurovčíková et al 2020) mapping used to reconstruct the blue-side from the measured red-side components. These mappings are learned from ∼ 13, 000 moderatez QSOs.…”

Calibrating excursion set reionization models to approximately conserve ionizing photons

“…Bouwens et al 2016Bouwens et al , 2015Oesch et al 2017), Lyman alpha forest statistics (e.g. Bosman et al 2018Bosman et al , 2021, and various probes of the EoR history (e.g. McGreer et al 2015;Planck Collaboration 2016;Pagano et al 2020).…”