Beyond the Hubble Deep Field limiting magnitude:: Faint galaxy number counts from surface brightness fluctuations

The faint end of the differential galaxy number counts n(m) in the Hubble Deep Field - North (HDF-N) has been determined for the F450W, F606W, and F814W filters by means of surface brightness fluctuation measurements. This technique allows us to explore n(m) beyond the limiting magnitude of the HDF, providing new, stronger constraints on the faint end of n(m). This has allowed us to test the validity of previous number count studies, to produce a new determination of the faint end of n(m) for magnitudes fainter than 28.8 in the AB system, and to extend this estimate down to 31 mag. This value represents an extension of more than 2 mag beyond the limits of previous photometric studies. The obtained n(m) slopes are gamma = 0.27, 0.21, and 0.26 in B-450, V-606, and I-814, respectively.