Measurements of effective optical depth in the Ly$\alpha$ forest from the BOSS DR12 Quasar sample

Understand

We measure the effective optical depth in the Lyman-alpha (Ly$\alpha$) forest using 40,035 quasar spectra from the Twelfth Data Release (DR12) of the Baryon Oscillation Spectroscopic Survey (BOSS) of SDSS-IV.

• A rigorous selection based on spectral index and the equivalent width of the C IV emission line is applied to choose seven uniform samples with minimal intrinsic variations across redshifts.
• Modeling the redshift evolution of the effective optical depth with a power-law, $\tau_\mathrm{eff}=\tau_0 (1 + z)^\gamma$ produces $\tau_0=(5.54 \pm 0.64)\times 10^{-3}$ and $\gamma=3.182 \pm 0.074$.
• The 2.2% precision estimate on $\gamma$ is dominated by systematic errors, likely arising from the bias and uncertainties in spectral index estimates.