Ly\alpha\ emission line reconstruction for high-$z$ QSOs

TL;DR

This paper presents a novel statistical method to reconstruct the intrinsic Ly extalpha\ emission line profiles of high-redshift quasars using a covariance matrix derived from moderate-redshift, high-quality spectra, enabling better analysis of obscured Ly extalpha\ emission.

Contribution

The authors develop a covariance-based reconstruction technique for Ly extalpha\ lines in high-z quasars, improving upon previous methods by leveraging correlations among emission lines.

Findings

Reconstructed Ly extalpha\ flux within 15% of measured flux 85-90% of the time.

Covariance matrix well approximated by an N-dimensional Gaussian distribution.

Method enables Ly extalpha\ profile recovery even when obscured.

Abstract

We introduce an intrinsic Ly\alpha\ emission line profile reconstruction method for high-$z$ quasars (QSOs). This approach utilises a covariance matrix of emission line properties obtained from a large, moderate-$z$ ($2 \leq z \leq 2.5$), high signal to noise (S/N > 15) sample of BOSS QSOs. For each QSO, we complete a Monte Carlo Markov Chain fitting of the continuum and emission line properties and perform a visual quality assessment to construct a large database of robustly fit spectra. With this dataset, we construct a covariance matrix to describe the correlations between the high ionisation emission lines Ly\alpha, C IV, Si IV + O IV] and C III], and find it to be well approximated by an $N$-dimensional Gaussian distribution. This covariance matrix characterises the correlations between the line width, peak height and velocity offset from systemic while also allowing for the existence of broad and narrow line components for Ly\alpha\ and C IV. We illustrate how this covariance matrix allows us to statistically characterise the intrinsic Ly\alpha\ line solely from the observed spectrum redward of 1275\AA. This procedure can be used to reconstruct the intrinsic Ly\alpha\ line emission profile in cases where Ly\alpha\ may otherwise be obscured. Applying this reconstruction method to our sample of QSOs, we recovered the Ly\alpha\ line flux to within 15 per cent of the measured flux at 1205\AA\ (1220\AA) ~85 (90) per cent of the time.