Helium is reionised
Quasars finish the job starlight could not, stripping helium bare across intergalactic space.
- When
- Known to
- ± 300 million years
- Era
- Galaxy Assembly, Stelliferous
Hydrogen reionisation was driven by ordinary stars, but ionising helium twice over takes far harder radiation. As quasars grew to peak abundance around redshift 3, their ultraviolet output completed helium reionisation and heated the intergalactic medium to tens of thousands of degrees. It is the last major change in the state of ordinary matter between galaxies.