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 γ = 0.27, 0.21, and 0.26 in B 450 , V 606 , and I 814 , respectively.