Models of the Thermal Evolution of the Intergalactic Medium After Reionization

TL;DR

This paper introduces a semi-analytical, multizone model for the IGM's thermal history post-reionization, aligning well with recent temperature data and constraining reionization scenarios and additional heating sources.

Contribution

The model improves upon previous approaches by accurately capturing inhomogeneous heating during patchy reionization and compares favorably with recent measurements to constrain reionization timelines.

Findings

He II reionization likely occurred at z=3-4 by quasars.

High-temperature, low-redshift hydrogen reionization scenarios are inconsistent with data.

Additional heating sources beyond photoheating are limited to <~1 eV per baryon by z=2.

Abstract

Recent years have brought more precise temperature measurements of the low-density intergalactic medium (IGM). These new measurements constrain the processes that heated the IGM, such as the reionization of H I and of He II. We present a semi-analytical model for the thermal history of the IGM that follows the photoheating history of primordial gas. Our model adopts a multizone approach that, compared to previous models, more accurately captures the inhomogeneous heating and cooling of the IGM during patchy reionization. We compare our model with recent temperature measurements spanning z = 1.6-4.8, finding that these measurements are consistent with scenarios in which the He II was reionized at z = 3-4 by quasars. Significantly longer duration or higher redshift He II reionization scenarios are ruled out by the measurements. For hydrogen reionization, we find that only low-redshift and high-temperature scenarios are excluded. For example, a model in which the IGM was heated to 30 000 K when an ionization front passed, and with hydrogen reionization occurring over 6 < z < 9, is ruled out. Finally, we place constraints on how much heating could owe to TeV blazars, cosmic rays, and other non-standard mechanisms. We find that by z = 2, a maximum of 1 eV of additional heat could be injected per baryon over standard photoheating-only models, with this limit becoming <~0.5 eV at z > 3.