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 similar or equal to 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 similar or equal to 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 similar or equal to 6. To have the universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than alpha = -1.6, which is the value measured at lower redshifts (z similar or equal to 3), unless either the normalization (Phi) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Phi(*) is actually lower than what we assumed by a factor of 2, a steep slope close to alpha = -2.0 is required. Our LF predicts a total of 50 - 80 z similar or equal to 6 galaxies in the Hubble Space Telescope Ultra Deep Field to a depth of AB = mag, which can be used to constraint both Phi(*) and alpha. We conclude that the least luminous galaxies existing 28.4 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 similar or equal to 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).