On the effect of the ionising background on the Ly{\alpha} forest autocorrelation function

TL;DR

This paper develops an analytical model to understand how fluctuations in the cosmic ionising background affect the Lyα forest autocorrelation function, revealing scale-dependent biases and potential observational signatures.

Contribution

It introduces a new analytical framework accounting for ionising background fluctuations and their impact on Lyα forest correlations, including shot noise effects from sources.

Findings

Ionising background fluctuations induce scale dependence in bias parameters.

Observations can measure the scale dependence to infer source properties.

Shot noise from sources significantly affects the Lyα spectrum.

Abstract

An analytical framework is presented to understand the effects of a fluctuating intensity of the cosmic ionising background on the correlations of the Ly{\alpha} forest transmission fraction measured in quasar spectra. In the absence of intensity fluctuations, the Ly{\alpha} power spectrum should have the expected cold dark matter power spectrum with redshift distortions in the linear regime, with a bias factor b_{\delta} and a redshift distortion parameter {\beta} that depend on redshift but are independent of scale. The intensity fluctuations introduce a scale dependence in both b_{\delta} and {\beta}, but keeping their product b_{\delta}{\beta} fixed. Observations of the Ly{\alpha} correlations and cross-correlations with radiation sources like those being done at present in the BOSS survey of SDSS-III (Busca et al. 2013; Slosar et al. 2013; Font-Ribera et al. 2014) have the potential to measure this scale dependence, which reflects the biasing properties of the sources and absorbers of the ionising background. We also compute a second term affecting the Ly{\alpha} spectrum, due to shot noise in the sources of radiation. This term is very large if luminous quasars are assumed to produce the ionising background and to emit isotropically with a constant luminosity, but should be reduced by a contribution from galaxies, and by the finite lifetime and anisotropic emission of quasars.