The Space Density of Galaxies through μB(0)=25.0 Magnitudes per Inverse Arcsecond Squared

Using the catalog of O'Neil, Bothun, & Schombert, we examine the central surface brightness distribution (φ[μ_B(0)]) of galaxies in the 22.0<=μ_B(0)<=25.0 mag arcsec^-2 range. Taking advantage of having a catalog in which each galaxy has a known central surface brightness, scale length, and redshift, we apply a bivariate volume correction to the data and extend the surface brightness distribution function by 1 magnitude, to 25.0 B mag arcsec^-2. The result is a flat (slope=0) space density of galaxies as a function of μ_B(0) from the Freeman value of 21.65+/-0.30 to the survey limit of 25.0 mag arcsec^-2, more than 10 σ away. The galaxies that comprise this sample have scale lengths and circular velocities similar to L_* spiral galaxies (i.e., these are not dwarf galaxies). As such, a significant amount of mass is contained in this population. If the baryonic mass fraction of disk galaxies is independent of μ_B(0), then our results are consistent with a significant percentage of the baryonic content of the universe being contained in potentials only dimly lit by the embedded galaxy.