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

Abstract

We use state of the art hydrodyamical simulations from the Sherwood, EAGLE and Illustris projects to examine the signature of $M_{\mathrm{z}=0}\simeq 10^{14}{M}_{\odot }$ protoclusters observed in Ly-$\alpha$ absorption at $z\simeq 2.4$. We find there is a weak correlation between the mass overdensity, ${\delta }_{\mathrm{m}}$, and the Ly-$\alpha$ effective optical depth relative to the mean, ${\delta }_{{\tau }_{\text{eff}}}$, averaged over $15h^{-1}\mathrm{cMpc}$ scales, although scatter in the ${\delta }_{\mathrm{m}}$--${\delta }_{{\tau }_{\text{eff}}}$ plane means it is not possible to uniquely identify large scale overdensities with strong Ly-$\alpha$ absorption. Although all protoclusters are associated with large scale mass overdensities, most sight lines through protoclusters in a $\sim 10^6$ $\mathrm{cMp}{\mathrm{c}}^3$ volume probe the low column density Ly-$\alpha$ forest. A small subset of sight lines that pass through protoclusters exhibit coherent, strong Ly-$\alpha$ absorption on $15h^{-1}\mathrm{cMpc}$ scales, although these correspond to a wide range in mass overdensity. Assuming perfect removal of contamination by Ly-$\alpha$ absorbers with damping wings, more than half of the remaining sight lines with ${\delta }_{{\tau }_{\mathrm{eff}}}>3.5$ trace protoclusters. It is furthermore possible to identify a model dependent ${\delta }_{{\tau }_{\mathrm{eff}}}$ threshold that selects only protoclusters. However, such regions are rare: excluding absorption caused by damped systems, less than 0.1 per cent of sight lines that pass through a protocluster have ${\delta }_{{\tau }_{\mathrm{eff}}}>3.5$, meaning that any protocluster sample selected in this manner will also be highly incomplete. On the other hand, coherent regions of Ly-$\alpha$ absorption also provide a promising route for identifying and studying filamentary environments at high redshift.

18 pages, 18 figures, 3 tables; accepted for publication in MNRAS

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• The AGN fraction in high-redshift protocluster candidates selected by Planck and Herschel
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• Connection Between Galaxies and HI in the Circumgalactic and Intergalactic Media: Variation According to Galaxy Stellar Mass and Star-formation Activity
• The Lyman-limit photon mean free path at the end of late reionisation in the Sherwood-Relics simulations
• LATIS: A Sample of IGM-selected Protoclusters and Protogroups at $z\sim 2.5$