Temperature of Intergalactic Matter.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Temperature of Intergalactic Matter. Gould, Robert J. ; Ramsay, William Abstract The presence of a cosmic abundance of helium in the intergalactic medium is of considerable importance because of the associated strong cooling due to excitation of discrete levels in He and He+ through inelastic electron collisions. This cooling reaches a maximum at 8x 10~ 0K where the main contribution is due to electron collision-induced production of He Ly-a, and the cooling is almost two orders of magnitude larger than that for bremsstrahlung and radiative and dielectronic recombination in ionized hydrogen and helium. It can be shown that heating by a universal cosmic ray flux can maintain steady state temperatures in the gas of 1-5 x 10~ 0K against cooling by electron collisions with H, He, and He for a cosmic He/H abundance and local hydrogen densities of 10-'- 10-~ particles/cm'. However, the relaxation time for hydrogen and singly ionized helium recombination is about 10 times the cosmic expansion time of 10" sec for hydrogen densities of 10-' cm-' at these temperatures, while doubly ionized helium recombines in about 3 x 10" sec. Thus, unless intergalactic matter exists in "clouds" in which the hydrogen density is 10 times the usually assumed mean of 10-' cm-', ionization steady state conditions may not exist. Moreover, for temperatures >2 x 10' 0K no process is effective in cooling the gas in a time less than 10" sec, so that no quasi-steady state temperature can exist. However, due to the strong cooling by He+ at temperatures lower than 2 X 10' 0K, an intergalactic gas at this temperature may be unstable to condensations, perhaps leading to the formation of galaxies or galaxy clusters. Publication: The Astronomical Journal Pub Date: February 1966 DOI: 10.1086/110010 Bibcode: 1966AJ.....71Q.163G full text sources ADS |