Broadband distortion modeling in Lyman-$\hat{I}\pm$ forest BAO fitting

Abstract

In recent years, the Lyman-$\hat{I}\pm$ absorption observed in the spectra of high-redshift quasars has been used as a tracer of large-scale structure by means of the three-dimensional Lyman-$\hat{I}\pm$ forest auto-correlation function at redshift $z\simeq 2.3$, but the need to fit the quasar continuum in every absorption spectrum introduces a broadband distortion that is difficult to correct and causes a systematic error for measuring any broadband properties. We describe a $k$-space model for this broadband distortion based on a multiplicative correction to the power spectrum of the transmitted flux fraction that suppresses power on scales corresponding to the typical length of a Lyman-$\hat{I}\pm$ forest spectrum. Implementing the distortion model in fits for the baryon acoustic oscillation (BAO) peak position in the Lyman-$\hat{I}\pm$ forest auto-correlation, we find that the fitting method recovers the input values of the linear bias parameter $b_F$ and the redshift-space distortion parameter ${\hat{I}}_F^2$ for mock data sets with a systematic error of less than 0.5\%. Applied to the auto-correlation measured for BOSS Data Release 11, our method improves on the previous treatment of broadband distortions in BAO fitting by providing a better fit to the data using fewer parameters and reducing the statistical errors on ${\hat{I}}_F^2$ and the combination $b_F(1+{\hat{I}}_F^2)$ by more than a factor of seven. The measured values at redshift $z=2.3$ are ${\hat{I}}_F^2=1.39_{-0.10-0.19-0.28}^{+0.11+0.24+0.38}$ and $b_F(1+{\hat{I}}_F^2)=-0.374_{-0.007-0.014-0.022}^{+0.007+0.013+0.020}$ (1$\ddot{I}ƒ$, 2$\ddot{I}ƒ$ and 3$\ddot{I}ƒ$ statistical errors). Our fitting software and the input files needed to reproduce our main results are publicly available.

28 pages, 15 figures, matches the published version