The impact and mitigation of broad absorption line quasars in Lyman$-\alpha$ forest correlations

TL;DR

This study investigates how including broad absorption line quasars in Lyman-alpha forest correlation analyses can recover lost data, improve statistical precision, and maintain accuracy without significantly altering the correlation measurements.

Contribution

The paper introduces a method to incorporate BAL quasars into Lyman-alpha correlation analyses by masking absorption troughs, reducing data loss and increasing sightline density.

Findings

Including BAL quasars reduces the fractional uncertainty in the covariance matrix.

The shape of the correlation function remains consistent when BAL quasars are included.

The method recovers over 95% of the data lost by excluding BAL quasars.

Abstract

Correlations in and with the flux transmission of the Lyman$-\alpha$ (Ly$\alpha$) forest in the spectra of high-redshift quasars are powerful cosmological tools, yet these measurements can be compromised if the intrinsic quasar continuum is significantly uncertain. One particularly problematic case is broad absorption line (BAL) quasars, which exhibit blueshifted absorption associated with many spectral features that are consistent with outflows of up to $\sim0.1c$. As these absorption features can both fall in the forest region and be difficult to distinguish from Ly$\alpha$ absorption, cosmological analyses eliminate the 12 - 16% of quasars that exhibit BALs. In this paper we explore an alternate approach that includes BALs in the Ly$\alpha$ auto correlation function, with the exception of the expected locations of the BAL absorption troughs. This procedure returns over 95% of the pathlength that is lost by the exclusion of BALs, as well as increases the density of sightlines. We show that including BAL quasars reduces the fractional uncertainty in the covariance matrix and correlation function and does not significantly change the shape of the correlation function relative to analyses that exclude BAL quasars. We also evaluate different definitions of BALs, masking strategies, and potential differences in the quasar continuum in the forest region for BALs with different amounts of absorption.