On the intergalactic temperature–density relation

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 Δz ∼ 1–2 after a reionization event, and this limiting behaviour...

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Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2016-02, Vol.456 (1), p.47-54
Main Authors: McQuinn, Matthew, Upton Sanderbeck, Phoebe R.
Format: Article
Language:English
Subjects:
Collapsing
Cooling
Cosmology
Density
Intergalactic media
Ionization
Mathematical analysis
Parametrization
Simulation
Star & galaxy formation
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Summary: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 Δz ∼ 1–2 after a reionization event, and this limiting behaviour has motivated the power-law parameterizations used in most analyses of the Ly α 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.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/stv2675