Searching for the shadows of giants: characterizing 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 z=0 10 14 M protoclusters observed in Ly-α absorption at z 2.4. We find 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 sight lines through protoclusters in a ∼ 10 6 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 cMpc scales, although these correspond to a wide range in mass overdensity. Assuming perfect removal of contamination by Lyα absorbers with damping wings, more than half of the remaining sight lines 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 sight lines 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.

Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

“…The transmission contrast of this spatial offset is further enhanced by the proximity effect associated with a M 1450 = −24.7 AGN hosted within the halo. We furthermore find that the incidence of CoSLAs within protoclusters is sensitive to changes in our local ionization models, largely as a result of changes in the number of self-shielded, damped Ly α absorbers with N H I ≥ 10 19 cm −2 (see also Miller et al 2019).…”

“…In particular, the dynamic range of the hydrodynamical simulations should ideally be larger. A mass resolution of M gas ∼ 10 6 M is required to resolve Ly α absorption from the IGM at z 2 (Bolton & Becker 2009;Miller et al 2019). While the Ly α forest in the TNG100-1 simulation is therefore well resolved, the statistics are somewhat limited with only 22 protoclusters with M z=0 ≥ 10 14 M .…”

“…X-ray and mm-wavelength observations -the latter via the Sunyaev-Zel'dovich effect (Sunyaev & Zeldovich 1972) -are E-mail: joel.miller@nottingham.ac.uk used to identify the T ∼ 10 7 K intracluster medium within massive collapsed clusters and groups (Ebeling et al 2010;Finoguenov et al 2010;Bleem et al 2015), leading to the detection of clusters up to redshifts of z = 1.7 (Strazzullo et al 2019). Yet the majority of the gas by volume within protoclusters has not yet been shock heated to X-ray emitting temperatures and instead has T ∼ 10 4 K (Miller, Bolton & Hatch 2019).…”

Searching for the shadows of giants II: the effect of local ionisation on the Lyman-$α$ absorption signatures of protoclusters at redshift $z\sim2.4$

“…X-ray and mm-wavelength observations -the latter via the Sunyaev-Zel'dovich effect (Sunyaev & Zeldovich 1972) -are used to identify the 𝑇 ∼ 10 7 K intracluster medium within massive collapsed clusters and groups (Ebeling et al 2010;Finoguenov et al 2010;Bleem et al 2015), leading to the detection of clusters up to redshifts of 𝑧 = 1.7 (Strazzullo et al 2019). Yet the majority of the gas by volume within protoclusters has not yet been shock heated to X-ray emitting temperatures and instead has 𝑇 ∼ 10 4 K (Miller et al 2019).…”

“…In particular, Cai et al (2017) have used spectral regions with strong Ly-𝛼 transmission decrements over a scale of 15ℎ −1 cMpc -which they name Coherently Strong Ly𝛼 Absorption systems (CoSLAs) -to locate a protocluster at 𝑧 = 2.3 (see also Cai et al 2016;Zheng et al 2021;Shi et al 2021). However, using cosmological hydrodynamical simulations, Miller et al (2019) (hereafter Paper I) also showed that such CoSLAs are rare 1 and are not an exclusive probe of protoclusters. It is possible to adopt a rather strict (and model dependent) CoSLA detection threshold that removes contamination from coherent structures originating in the diffuse IGM, but any such protocluster sample is then incomplete.…”