The Temperature-Density Relation in the Intergalactic Medium at Redshift <z>=2.4

TL;DR

This study measures the temperature-density relation of the intergalactic medium at redshift 2.4, finding a positive correlation and providing new constraints on the IGM's thermal state through analysis of HI absorbers.

Contribution

It presents the first model-independent evidence that the IGM's temperature-density relation is not inverted, using a large sample of HI absorbers with Voigt profile fitting.

Findings

Positive correlation between HI column density and line width.

Temperature at mean density is approximately 19,400 K.

The temperature-density relation follows a power-law with index ~0.46.

Abstract

We present new measurements of the temperature-density (T-rho) relation for neutral hydrogen in the 2.0 < z < 2.8 intergalactic medium (IGM) using a sample of ~6000 individual HI absorbers fitted with Voigt profiles constrained in all cases by multiple Lyman series transitions. We find model independent evidence for a positive correlation between the column density of HI (NHI) and the minimum observed velocity width of absorbers (bmin). With minimal interpretation, this implies that the temperature-density relation in the IGM is not "inverted", contrary to many recent studies. Fitting bmin as a function of NHI results in line width - column density dependence of the form bdmin = b_0 [NHI/N_(HI,0)]^(Gamma-1) with a minimum line width at mean density rhobar [N_(HI, 0) = 10^13.6 cm^-2] of b_0 = 17.9 +- 0.2 km/s and a power-law index of (Gamma-1) = 0.15 +- 0.02. Using analytic arguments, these measurements imply an "equation of state" for the IGM at <z>= 2.4 of the form T = T_0 (rho/rhobar)^(gamma-1) with a temperature at mean density of T_0 = (1.94 +- 0.05) x 10^4 K and a power-law index (gamma -1) = 0.46 +- 0.05.