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arXiv · 1909.02513

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

Joel S. A. Miller·James S. Bolton·Nina Hatch
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Abstract

We use state of the art hydrodyamical simulations from the Sherwood, EAGLE and Illustris projects to examine the signature of $M_{\rm z=0}\simeq 10^{14}M_{\odot}$ protoclusters observed in Ly-$α$ absorption at $z\simeq 2.4$. We find there is a weak correlation between the mass overdensity, $δ_{\rm m}$, and the Ly-$α$ effective optical depth relative to the mean, $δ_{τ_\textrm{eff}}$, averaged over $15~h^{-1}\rm\,cMpc$ scales, although scatter in the $δ_{\rm m}$--$δ_{τ_\textrm{eff}}$ 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 sight lines through protoclusters in a $\sim 10^{6}$ $\rm cMpc^{3}$ volume probe the low column density Ly-$α$ forest. A small subset of sight lines 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 sight lines with $δ_{τ_{\rm eff}}>3.5$ trace protoclusters. It is furthermore possible to identify a model dependent $δ_{τ_{\rm eff}}$ threshold that selects only protoclusters. However, such regions are rare: excluding absorption caused by damped systems, less than 0.1 per cent of sight lines that pass through a protocluster have $δ_{τ_{\rm eff}}>3.5$, 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.

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Joel S. A. Miller, James S. Bolton, Nina Hatch. 2019-09-12. Searching for the shadows of giants: characterising protoclusters with line of sight Lyman-α absorption. https://doi.org/10.1093/mnras%2Fstz2504

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