UC San Diego

The Lyman Alpha Forest (LAF) has been extensively studied observationally and by simulations. Good agreement between data and simulations has been reached in some statistics describing the LAF. However, there are still significant discrepancies in the magnitude of the flux power spectrum (FPS) and the shape of the flux probability distribution function (FPDF). Here we explore a possible resolution by adding new physical ingredients into the standard LAF simulations.

Firstly, we repeat the standard simulation. Here the intergalactic medium (IGM) is assumed to be photoionized and photoheated by a homogeneous ultraviolet background. Simulations with various box sizes and He II photoheating rates all confirm the mismatch.

Then we introduce simulations that treat quasars as point sources, and calculate the radiation transfer using multigroup flux-limited diffusion. Our simulations here fall into two categories. Simulations with larger volumes (126 Mpc/h)3 sample the more luminous quasars and exhibit a more reliable thermal and ionization history. Due to the finite time it takes He III bubbles to expand, our simulations reionize later (z~2.76) than standard simulations, and show better agreement with observations.

Simulations with higher spatial resolutions (80 Mpc/h)3 are used to generate synthetic spectra. Our mean H I effective optical depths and rescaled b ’s agree with data well, but the discrepancies in FPS and FPDF still exist. We therefore conclude that the inhomogeneous heating due to He II reionization is not the missing physics needed to reconcile simulations with data. We also generate He II LAF spectra and successfully reproduce the large scatter in the He II effective optical depth before He II reionization completes. We also calculate its FPS and FPDF, which could be used to compare with future observations.

On a related topic, we conduct simulations of the hydrogen reionization to examine the effect of the smallest galaxies. We find that reionization begins substantially earlier when galaxies in the mass range 10^7