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

Abstract

ABSTRACT We use state-of-the-art hydrodyamical simulations from the Sherwood, EAGLE, and Illustris projects to examine the signature of Mz = 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, δm, and the Ly α effective optical depth relative to the mean, $\delta _{\tau _\textrm{eff}}$, averaged over $15~h^{-1}\, \textrm{cMpc}$ scales, although scatter in the δm–$\delta _{\tau _\textrm{eff}}$ 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 ∼106$\rm cMpc^{3}$ 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 $15h^{-1}\rm \, cMpc$ 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 $\delta _{\tau _{\rm eff}}\gt 3.5$ trace protoclusters. It is furthermore possible to identify a model-dependent $\delta _{\tau _{\rm eff}}$ 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 $\delta _{\tau _{\rm eff}}\gt 3.5$, 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.