Photoheating

Photoheating is the heating of gas due to the absorption of photons. In astrophysical contexts it most often results from photoionization, where an ultraviolet photon with energy above an ionization threshold ejects an electron from an atom or ion, and the excess energy beyond the binding energy becomes kinetic energy of the free electron. That kinetic energy then thermalizes with the gas, raising its temperature. A related mechanism is the photoelectric effect on dust grains, in which UV photons eject electrons from grains and transfer energy to the surrounding gas through collisions and grain–gas interactions.

Photoheating requires a source of photons and a medium that can absorb them. Sources include young, massive stars and active galactic nuclei. In H II regions around hot stars, photoionization dominates and heats gas to roughly 10,000 kelvin. In the interstellar and intergalactic media, the ultraviolet background produced by stars and quasars photoionizes hydrogen and helium, elevating the gas temperature to tens of thousands of kelvin during reionization and thereafter. The heating rate depends on the radiation spectrum and intensity, the gas density, and its ionization state; cooling processes such as line cooling, recombination cooling, and Compton cooling balance heating to establish a thermal state.

Photoheating is a key feedback mechanism in galaxy formation and cosmic evolution, increasing the Jeans mass and suppressing gas accretion in low-mass halos. It shapes the thermal history of the intergalactic medium, influences the appearance of the Lyman-alpha forest, and affects emission lines from H II regions and planetary nebulae. Observationally, temperatures inferred from absorption and emission features reflect the balance between photoheating and cooling.