UV background fluctuations and three-point correlations in the large-scale clustering of the Lyman $\alpha$ forest

Suk Sien Tie , David H. Weinberg , Paul Martini , Wei Zhu , Sébastien Peirani (1, 2) , Teresita Suarez , Stéphane Colombi (1)

1 IAP - Institut d'Astrophysique de Paris
2 LAGRANGE - Joseph Louis LAGRANGE

Using the Lyman α (Lyα) Mass Association Scheme, we make theoretical predictions for the three-dimensional three-point correlation function (3PCF) of the Lyα forest at redshift z = 2.3. We bootstrap results from the (100 h^−1 Mpc)^3 Horizon hydrodynamic simulation to a (1 h^−1 Gpc)^3N-body simulation, considering both a uniform ultraviolet background (UVB) and a fluctuating UVB sourced by quasars with a comoving n_q ≈ 10^−5h^3 Mpc^−3 placed either in massive haloes or randomly. On scales of 10–30 h^−1 Mpc, the flux 3PCF displays hierarchical scaling with the square of the two-point correlation function (2PCF), but with an unusual value of Q ≡ ζ_123/(ξ_12ξ_13 + ξ_12ξ_23 + ξ_13ξ_23) ≈ −4.5 that reflects the low bias of the Lyα forest and the anticorrelation between mass density and transmitted flux. For halo-based quasars and an ionizing photon mean free path of λ = 300 h^−1 Mpc comoving, UVB fluctuations moderately depress the 2PCF and 3PCF, with cancelling effects on Q. For λ = 100 or 50 h^−1 Mpc, UVB fluctuations substantially boost the 2PCF and 3PCF on large scales, shifting the hierarchical ratio to Q ≈ −3. We scale our simulation results to derive rough estimate of the detectability of the 3PCF in current and future observational data sets for the redshift range z = 2.1–2.6. At r = 10 and 20 h^−1 Mpc, we predict a signal-to-noise ratio (SNR) of ∼9 and ∼7, respectively, for both Baryon Oscillation Spectroscopic Survey (BOSS) and extended BOSS (eBOSS), and ∼37 and ∼25 for Dark Energy Spectroscopic Instrument (DESI). At r = 40 h^−1 Mpc the predicted SNR is lower by a factor of ∼3–5. Measuring the flux 3PCF would provide a novel test of the conventional paradigm of the Lyα forest and help separate the contributions of UVB fluctuations and density fluctuations to Lyα forest clustering, thereby solidifying its foundation as a tool of precision cosmology.

Mots clés: ultraviolet: general; large-scale structure of Universe; cosmology: theory; intergalactic medium

Domaines: Astrophysique [astro-ph]

Suk Sien Tie, David H. Weinberg, Paul Martini, Wei Zhu, Sébastien Peirani, et al.. UV background fluctuations and three-point correlations in the large-scale clustering of the Lyman $\alpha$ forest. Monthly Notices of the Royal Astronomical Society, 2019, 487 (4), pp.5346-5362. ⟨10.1093/mnras/stz1632⟩. ⟨hal-02268941⟩