The Spin Temperature of Intergalactic Neutral Hydrogen.

Three mechanisms (collisions, radio-frequency radiation, and Lyman-a radiation) which affect the spin temperature (Ts) of intergalactic hydrogen are studied in the hope of being able to draw conclusions concerning density from the upper limit on the 21-cm opacity derived in the preceding paper (Field 1959a), hereinafter referred to as "Paper I." In Section II it is shown that the density of material can be limited enough from the observations themselves to insure that collisions cannot raise T3 above about 3 K, a lower limit imposed by radio-frequency radiation. A strong upper limit on density (1.3 x 10-30 gm cm-3) would follow for a mostly neutral gas if this were the whole story. In Appendix A a study of the transfer of Lyman-a radiation in an expanding gas is carried out. Using the intensity of Lyman-a predicted by this study, we show in Section III that Lyman-a from galaxies alone is sufficient to raise Ts to 32 K. In Section IV we consider recombinations in a partially ionized medium-it is found that if the ionization is greater than 13 per cent, Ts rapidly approaches the kinetic temperature as a result of the copious Lyman-a generated by recombinations. We conclude in Section V that ionization greater than 13 per cent makes it impossible to draw deductions from the observations of opacity, while if it is less than 13 per cent, the total density is not likely to exceed 5.9 X 10- gm . This may be compared with a value of 3 X 10- needed to explain a sizable deceleration of the expansion. Some discussion of ways to detect ionized gas which may be present is also given.