A high space density of L* Active Galactic Nuclei at z~4 in the COSMOS
  field
  Identifying the source population of ionizing radiation, responsible for the
reionization of the universe, is currently a hotly debated subject with
conflicting results. Studies of faint, high-redshift star-forming galaxies, in
most cases, fail to detect enough escaping ionizing radiation to sustain the
process. Recently, the capacity of bright quasi-stellar objects to ionize their
surrounding medium has been confirmed also for faint active galactic nuclei
(AGNs), which were found to display an escaping fraction of ~74% at z~4. Such
levels of escaping radiation could sustain the required UV background, given
the number density of faint AGNs is adequate. Thus, it is mandatory to
accurately measure the luminosity function of faint AGNs (L~L*) in the same
redshift range. For this reason we have conducted a spectroscopic survey, using
the wide field spectrograph IMACS at the 6.5m Baade Telescope, to determine the
nature of our sample of faint AGN candidates in the COSMOS field. This sample
was assembled using photometric redshifts, color, and X-ray information. We
ended up with 16 spectroscopically confirmed AGNs at 3.6<z<4.2 down to a
magnitude of i$_{AB}$=23.0 for an area of 1.73 deg$^{2}$. This leads to an AGN
space density of ~1.6$\times10^{-6} Mpc^{-3}$ (corrected) at z~4 for an
absolute magnitude of M$_{1450}$=-23.5. This is higher than previous
measurements and seems to indicate that AGNs could make a substantial
contribution to the ionizing background at z~4. Assuming that AGN physical
parameters remain unchanged at higher redshifts and fainter luminosities, these
sources could be regarded as the main drivers of cosmic reionization.
