Searching for the shadows of giants: characterizing protoclusters with line of sight Lyman-α absorption

We use state-of-the-art hydrodyamical simulations from the Sherwood, EAGLE, and Illustris projects to examine the signature of M_{z = 0} ≃ 10¹⁴ M_☉ protoclusters observed in Ly α absorption at z ≃ 2.4. We find that there is a weak correlation between the mass overdensity, δ_m, and the Ly α effective optical depth relative to the mean, δ _{τ_{eff}}, averaged over 15 h^{-1} cMpc scales, although scatter in the δ_m-δ _{τ_{eff}} plane means it is not possible to uniquely identify large-scale overdensities with strong Ly α absorption. Although all protoclusters are associated with large-scale mass overdensities, most sightlines through protoclusters in a ̃10⁶cMpc³ volume probe the low column density Ly α forest. A small subset of sightlines that pass through protoclusters exhibit coherent, strong Ly α absorption on 15h^{-1} cMpc scales, although these correspond to a wide range in mass overdensity. Assuming perfect removal of contamination by Ly α absorbers with damping wings, more than half of the remaining sightlines with δ _{τ_eff}> 3.5 trace protoclusters. It is furthermore possible to identify a model-dependent δ _{τ_eff} threshold that selects only protoclusters. However, such regions are rare: excluding absorption caused by damped systems, less than 0.1 per cent of sightlines that pass through a protocluster have δ _{τ_eff}> 3.5, meaning that any protocluster sample selected in this manner will also be highly incomplete. On the other hand, coherent regions of Ly α absorption also provide a promising route for identifying and studying filamentary environments at high redshift.