Detection of Lyβ auto-correlations and Lyα-Lyβ cross-correlations in BOSS Data Release 9

The Lyman-beta forest refers to a region in the spectra of distant quasars that lies between the rest-frame Lyman-beta and Lyman-gamma emissions. The forest in this region is dominated by a combination of absorption due to resonant Ly alpha and Ly beta scattering. When considering the 1D Ly beta forest in addition to the 1D Lya forest, the full statistical description of the data requires four 1D power spectra: Lya and Ly beta auto-power spectra and the Ly alpha-Ly beta real and imaginary cross-power spectra. We describe how these can be measured using an optimal quadratic estimator that naturally disentangles Ly alpha and Ly beta contributions. Using a sample of approximately 60,000 quasar sight-lines from the BOSS Data Release 9, we make the measurement of the one-dimensional power spectrum of fluctuations due to the Ly beta resonant scattering. While we have not corrected our measurements for resolution damping of the power and other systematic effects carefully enough to use them for cosmological constraints, we can robustly conclude the following: i) Ly beta power spectrum and Ly alpha-Ly beta cross spectra are detected with high statistical significance; ii) the cross-correlation coefficient is approximate to 1 on large scales; iii) the Ly beta measurements are contaminated by the associated OVI absorption, which is analogous to the SiIII contamination of the Ly alpha forest. Measurements of the Ly beta forest will allow extension of the usable path-length for the Ly alpha measurements while allowing a better understanding of the physics of intergalactic medium and thus more robust cosmological constraints.