Three- and two-point spatial correlations of IGM at z ∼ 2: cloud-based analysis using simulations

Abstract

Lyα forest absorption spectra decomposed into multiple Voigt profile components (clouds) allow us to study clustering of intergalactic medium (IGM) as a function of H i column density (NHI). Here, we explore the transverse three-point correlation (ζ) of Lyα clouds using mock triplet spectra from simulations at z ∼ 2 on scales of 1-5 h−1cMpc. We find ζ to depend strongly on NHI and scale and weakly on angle of the triplet configuration. We show that the ”hierarchical ansatz” is applicable for scales ≥ 3h−1cMpc, and obtain a median reduced three-point correlation (Q) in the range 0.2-0.7. We show, ζ is influenced strongly by the thermal and ionization state of the gas. As in the case of galaxies, the influence of physical parameters on Q is weaker compared to that on ζ. We show difference in ζ and Q between different simulations are minimized if we use appropriate NHI cut-offs corresponding to a given baryon over-density (Δ) using the NHI vs Δ relationship obtained from simulations. We study the effect of pressure broadening on ζ using a model with artificially boosted heating rates. However, for models with realistic thermal and ionization histories the effect of pressure broadening on ζ is sub-dominant compared to other local effects. We find the strong redshift evolution shown by ζ, mainly originating from the redshift evolution of thermal and ionization state of the IGM. We discuss the observational requirements for the detection of three-point correlation, specifically, in small intervals of configuration parameters and redshift.

Measurement of redshift-space two- and three-point correlation of Ly$α$ absorbers at $1.7< z <3.5$: Implications on evolution of the physical properties of IGM

“…Note that in Maitra et al (2020b), we found the relative difference in Δ 𝐵 is very small when we consider UVB models with allowed range in power-law spectral index for the quasar SED (see their figure 14 for details). From the measurement presented in Gaikwad et al (2020a) the value of 𝑇 0 is in the range (0.95-1.45) ×10 4 K. Our Hot model typically produces higher 𝑇 0 values and hence the actual pressure smoothing effects may be smaller than that seen in our "Enhanced 𝛼 = 1.8" model.…”

“…In what follows we call the individual Voigt profile component as "absorbers". In the series of studies we have highlighted the advantage of using "absorbers" for the clustering analysis (refer to discussions in Maitra et al 2019Maitra et al , 2020b compared to most frequently used flux based statistics.…”

“…The modified code self-consistently incorporates radiative heating and cooling processes in the thermal and ionization evolution equation, thereby simulating the physical properties of the IGM more accurately. The description of the simulation run and the procedure of forward modelling the simulated particle properties along sightlines to produce the transmitted flux is given in the following subsections (see also, Maitra et al 2020b).…”

“…Viel et al (2004a) had used a large sample of Ultraviolet Echelle Spectrograph (UVES) QSO absorption spectra (LUQAS) and found that the errorbars in the observed bispectrum (redshift space) of 𝑧 − 2 − 2.4 Ly𝛼 forest transmitted flux was too large to distinguish them from a set of absorption spectra having randomized absorber positions. For the past three years we have been exploring using the Voigt profile components of the Ly𝛼 absorption to study two-and three-point correlation function of the IGM (Maitra et al 2019(Maitra et al , 2020b. It is found that such an approach allow us to study the clustering properties as a function of 𝑁 HI and b-parameters.…”

“…Maitra et al (2020a) had explored low-𝑧 Ly𝛼 forest (𝑧 < 0.48) using HST-COS data (of Danforth et al 2016) and had detected longitudinal three-point correlation in Ly𝛼 absorbers for the first time with a 2.8𝜎 significance. While there are upcoming redshift space study of three-point correlation due to relatively large number of single quasars, any transverse study of three-point correlation using close projected quasar triplets are still at theoretical stages (Tie et al 2019;Maitra et al 2020b). There are still some attempts at studying higher order clustering properties of Ly𝛼 forest using few projected quasar triplets at high-𝑧 (Cappetta et al 2010;Maitra et al 2019).…”