Lyα Forest Simulation and BAO Detection Lin Qiufan Apr.2 nd, 2015

Viel et al. Testing the accuracy of the Hydro-PM approximation in numerical simulations of the Lyman-alpha forest (Apr, 2006) Zhan et al. Lyman Alpha Flux Power Spectrum and Its Covariance (Aug, 2005) McDonald et al. Physical effects on the Lyman-alpha forest flux power spectrum, damping wings, ionizing radiation fluctuations, and galactic winds (Jul, 2004) Viel et al. The effect of (strong) discrete absorption systems on the lyman alpha forest flux power spectrum (Aug, 2003) Meiksin et al. The effects of UV background correlations on Ly-α forest flux statistics (Jul, 2003)  Reference

Delubac et al. Baryon Acoustic Oscillations in the Ly-α forest of BOSS DR11 quasars (Apr, 2014) Busca et al. Baryon Acoustic Oscillations in the Ly-α forest of BOSS quasars (Feb, 2013) Font-Ribera et al. Quasar-Lyman α Forest Cross-Correlation from BOSS DR11 ： Baryon Acoustic Oscillations (May, 2014)  Reference

 Lyα Forest Basics Image from Image from A series of absorption lines in the spectrum of a luminous source, produced from Lyα transition of neutral Hydrogen atoms Used to map the underlying matter distribution Depicting smooth fluctuations of the continuous intergalactic medium, rather than individual discrete absorption systems (theoretical prejudice) Image from

 Lyα Forest Basics Lyα line profile of a cloud of atoms Lorentzian profile Linear broadening Gaussian profile Nonlinear broadening Convolution of two profiles: Voigt profile

 Lyα Forest Basics Optical depth Transmission flux Other variables: temperature, density, neutral fraction of atoms, photoionization rate, radioactive recombination coefficient, etc.

 Lyα Forest Basics An important assumption: ionization equilibrium Photoionization rate + Collisional ionization rate = Radioactive recombination rate A Hydrogen atom absorbs a photon and emits an electron Cosmic ionizing background: radiation emitted by luminous sources at a frequency above 13.6eV Collisions of electrons and atoms A free electron interacts with a proton, emitting a photon and forming an atom Photoionization timescale << Recombination timescale A highly ionized state after Reionization Lyα absorption a small fraction of neutral atoms

 Lyα Forest Basics An important approximation: A consequence of cooling (by Hubble expansion) + Heating (by photoionization), neglecting other mechanisms NOT an equation of state NOT a consequence of thermodynamic equilibrium It is practically unaltered in most situations It breaks down when the gas is shock heated temperature–density relation

 Simulations Full hydrodynamic simulation (precise, time-consuming) Pseudo-hydro techniques (faster, inadequate) Methods Modeling the low-redshift warm-hot gas Considering additional physical effects Improvements Developed from N-body simulations or simplified full- hydro simulation Consistent with full-hydro simulation at high redshifts Departing from full-hydro simulation at low redshifts (shock heating)

 Simulations Physical effects —— strong absorption systems

 Simulations Physical effects —— strong absorption systems Plot from Viel et al. (2003) Power is enhanced on large scales

 Simulations Fluctuations of the ionizing radiation field result in the spatial variations of the neutral density Fluctuations resulting from quasars enhance the power on large scales Fluctuations resulting from other additional sources (like Lyman Break Galaxies) suppress the power on large scales Fluctuations resulting from quasars show large spatial correlations, while the introduction of additional sources would dilute the correlations. Physical effects —— ionizing background fluctuations

 BAO Detection in the Lyα Forest Delta field Flux auto-correlation function Covariance matrix Constraints on cosmological models (Delubac et al., 2014) Measured flux Mean transmitted flux

 BAO Detection in the Lyα Forest Broadband term: take into account imperfect knowledge of non-BAO cosmology and distortions introduced by analysis method Fitting Model Dilation factors: relate the observed peak position to the fiducial peak position

 BAO Detection in the Lyα Forest

Results

 BAO Detection in the Lyα Forest Damped-Lyα absorbers are masked in the observed spectra if the absorption > 20%, the rest are corrected using a Voigt profile Effect of ionizing background fluctuations Inaccurate estimate of the mean transmitted flux Errors introduced by flux calibration Systematic uncertainties the result is made only sensitive to the position of BAO peak rather than the smooth non-BAO components of the correlation function. Would these effects result in a significant shift of the position of BAO peak? Broadband term The flux is a nonlinear transform of the underlying one dimensional density field. It is not clear to what extent the flux correlation can appropriately represent the underlying matter correlation. Beyond these effects...

 BAO Detection in the Lyα Forest The power spectrum would be a promising approach in the BAO detection in the Lyα forest. Difference between the flux power spectrum and matter power spectrum. What’s more... Plot from Zhan et al. (2003)

 Summary Lyα Forest Basics Simulations BAO detection Ionization equilibrium Temperature–density relation Full and pseudo-hydro techniques Improvements: modeling shock heating, adding possible physical effects Systematic uncertainties The nonlinear relation between the flux and the density field (correlation function, power spectrum)