IGM Constraints from the SDSS-III/BOSS DR9 Ly-alpha Forest Flux Probability Distribution Function

TL;DR

This study measures the Lyα forest transmission PDF from SDSS BOSS data, compares it with hydrodynamical simulations, and constrains the IGM temperature-density relationship, finding a best-fit slope of γ=1.6 and disfavoring isothermal or inverted models.

Contribution

It provides the first detailed measurement of the transmission PDF from BOSS data and constrains the IGM temperature-density relationship using advanced modeling of LLSs and uncertainties.

Findings

Best-fit γ=1.6 for the IGM temperature-density relation.

Partial Lyman-limit systems are essential to explain low-transmission data.

Isothermal or inverted temperature-density relations are statistically disfavored.

Abstract

The Ly$\alpha$ forest transmission probability distribution function (PDF) is an established probe of the intergalactic medium (IGM) astrophysics, especially the temperature-density relationship of the IGM. We measure the transmission PDF from 3393 Baryon Oscillations Spectroscopic Survey (BOSS) quasars from SDSS Data Release 9, and compare with mock spectra that include careful modeling of the noise, continuum, and astrophysical uncertainties. The BOSS transmission PDFs, measured at $\langle z \rangle = [2.3,2.6,3.0]$, are compared with PDFs created from mock spectra drawn from a suite of hydrodynamical simulations that sample the IGM temperature-density relationship, $\gamma$, and temperature at mean-density, $T_0$, where $T(\Delta) = T_0 \Delta^{\gamma-1}$. We find that a significant population of partial Lyman-limit systems with a column-density distribution slope of $\beta_\mathrm{pLLS} \sim -2$ are required to explain the data at the low-transmission end of transmission PDF, while uncertainties in the mean Ly$\alpha$ forest transmission affect the high-transmission end. After modelling the LLSs and marginalizing over mean-transmission uncertainties, we find that $\gamma=1.6$ best describes the data over our entire redshift range, although constraints on $T_0$ are affected by systematic uncertainties. Within our model framework, isothermal or inverted temperature-density relationships ($\gamma \leq 1$) are disfavored at a significance of over 4$\sigma$, although this could be somewhat weakened by cosmological and astrophysical uncertainties that we did not model.