The contribution of faint AGNs to the ionizing background at z~4

Understand

Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in cosmology.

• Bright QSOs are known to ionize their surrounding neighborhood, but they are too few to ensure the required HI ionizing background.
• A significant contribution by faint AGNs, however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei at z>4.
• We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U-I color interval, with I~21-23 and 3.6<z<4.2.