The major sources of the cosmic reionizing background at z ≃ 6

In this Letter, we address which sources contributed most of the reionizing photons. Our argument assumes that the reionization ended around z ≃ 6 and that it was a relatively quick process, i.e., that there was a non-negligible fraction of neutral hydrogen in the universe at somewhat earlier epochs. Starting from our earlier estimate of the luminosity function (LF) of galaxies at z ≃ 6, we quantitatively show that the major sources of reionization are most likely galaxies with L < L*. Our approach allows us to put stronger constraints to the LF of galaxies at z ≃ 6. To have the universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than α = -1.6, which is the value measured at lower redshifts (z ≃ 3), unless either the normalization (Φ*) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Φ* is actually lower than what we assumed by a factor of 2, a steep slope close to α = -2.0 is required. Our LF predicts a total of 50-80 z ≃ 6 galaxies in the Hubble Space Telescope Ultra Deep Field to a depth of AB = 28.4 mag, which can be used to constraint both φ* and α. We conclude that the least luminous galaxies existing at this redshift should reach as low as some critical luminosity in order to accumulate the entire reionizing photon budget. On the other hand, the existence of significant amounts of neutral hydrogen at slightly earlier epochs, e.g., z ≃ 7, requires that the least luminous galaxies should not be fainter than another critical value (i.e., the LF should cut off at this point).