The Non-Linear Power Spectrum of the Lyman Alpha Forest

TL;DR

This paper models the non-linear power spectrum of the Lyman Alpha forest using hydrodynamic simulations, introduces a new fitting formula, and compares predictions with observations to improve understanding of small-scale structures in the intergalactic medium.

Contribution

It provides the first detailed simulation-based predictions of the non-linear Lyman Alpha power spectrum, including a new fitting formula and analysis of bias factors and their evolution.

Findings

The non-linear power spectrum has a characteristic shape with a transition scale.

Predicted redshift distortion parameter agrees with recent measurements.

Density bias factor is lower than observed in preliminary comparisons.

Abstract

The Lyman alpha forest power spectrum has been measured on large scales by the BOSS survey in SDSS-III at $z\sim 2.3$, has been shown to agree well with linear theory predictions, and has provided the first measurement of Baryon Acoustic Oscillations at this redshift. However, the power at small scales, affected by non-linearities, has not been well examined so far. We present results from a variety of hydrodynamic simulations to predict the redshift space non-linear power spectrum of the Lyman Alpha transmission for several models, testing the dependence on resolution and box size. A new fitting formula is introduced to facilitate the comparison of our simulation results with observations and other simulations. The non-linear power spectrum has a generic shape determined by a transition scale from linear to non-linear anisotropy, and a Jeans scale below which the power drops rapidly. In addition, we predict the two linear bias factors of the Lyman Alpha forest and provide a better physical interpretation of their values and redshift evolution. The dependence of these bias factors and the non-linear power on the amplitude and slope of the primordial fluctuations power spectrum, the temperature-density relation of the intergalactic medium, and the mean Lyman Alpha transmission, as well as the redshift evolution, is investigated and discussed in detail. A preliminary comparison to the observations shows that the predicted redshift distortion parameter is in good agreement with the recent determination of Blomqvist et al., but the density bias factor is lower than observed. We make all our results publicly available in the form of tables of the non-linear power spectrum that is directly obtained from all our simulations, and parameters of our fitting formula.