On the intergalactic temperature-density relation

TL;DR

This paper explains why a tight power-law temperature-density relation exists in the low-density intergalactic medium, clarifying its origin and implications for cosmic reionization and structure formation.

Contribution

It provides a new understanding of the physical reasons behind the power-law relation, addressing limitations of previous explanations and including effects of cooling processes.

Findings

The power-law relation holds over two decades in density.

Recombination and Compton cooling do not disrupt the relation.

Fluctuations in temperature decay rapidly after reionization.

Abstract

Cosmological simulations of the low-density intergalactic medium exhibit a strikingly tight power-law relation between temperature and density that holds over two decades in density. It is found that this relation should roughly apply Delta z ~ 1-2 after a reionization event, and this limiting behavior has motivated the power-law parameterizations used in most analyses of the Ly-alpha forest. This relation has been explained by using equations linearized in the baryonic overdensity (which does not address why a tight power-law relation holds over two decades in density) or by equating the photoheating rate with the cooling rate from cosmological expansion (which we show is incorrect). Previous explanations also did not address why recombination cooling and Compton cooling off of the cosmic microwave background, which are never negligible, do not alter the character of this relation. We provide an understanding for why a tight power-law relation arises for unshocked gas at all densities for which collisional cooling is unimportant. We also use our results to comment on (1) how quickly fluctuations in temperature redshift away after reionization processes, (2) how much shock heating occurs in the low-density intergalactic medium, and (3) how the temperatures of collapsing gas parcels evolve.