Interstellar and Intergalactic Medium

The diffuse intergalactic medium (DIM) is photoionized gas at temperature T < 105 K that lies outside galaxies and clusters. The absence of the Gunn–Peterson effect (optically thick absorption by Lyman alpha) at low redshifts indicates that the DIM is almost entirely ionized today. The hydrogen gas filling the universe was also almost entirely ionized soon after the Big Bang; however, at a redshift z ∼ 1400, the hydrogen went from being ionized to being neutral. The end of the era of neutrality came at a redshift z ∼ 7, when the earliest hot massive stars had emitted enough UV photons to reionize the intergalactic gas. Today, within the mostly ionized DIM, there exist regions of higher neutral hydrogen density; these regions give rise to the Lyman alpha forest of absorption lines seen in the spectra of relatively low-redshift quasars.