Constraining the Temperature-Density Relation of the Inter-Galactic Medium from Analytically Modeling Lyman-alpha Forest Absorbers

Understand

The absorption by neutral hydrogen in the intergalactic medium (IGM) produces the Ly$\alpha$ forest in the spectra of quasars.

• The Ly$\alpha$ forest absorbers have a broad distribution of neutral hydrogen column density $N_{\rm HI}$ and Doppler $b$ parameter.
• The narrowest Ly$\alpha$ absorption lines (of lowest $b$) with neutral hydrogen column density above $\sim 10^{13}{\rm cm^{-2}}$ are dominated by thermal broadening, which can be used to constrain the thermal state of the IGM.
• Here we constrain the temperature-density relation $T=T_0(\rho/\bar{\rho})^{\gamma-1}$ of the IGM at $1.6<z<3.6$ by using $N_{\rm HI}$ and $b$ parameters measured from 24 high-resolution and high-signal-to-noise quasar spectra and by employing an analytic model to model the $N_{\rm HI}$-dependent low-$b$ cutoff in the $b$ distribution.