The number density of low-surface brightness galaxies with 23<μ₀<25 V mag/arcsec²

We present results of a large area CCD survey for low-surface brightness galaxies (LSBs) that reaches central surface brightnesses of 25 mag/arcsec² in V. We have analyzed 17.5 degree² of transit scan data, and identified a statistical subset of 7 pure disk LSBs with central surface brightnesses fainter than μ₀ = 23 V mag/arcsec² and with angular exponential scale lengths larger than α = 2.5″. The LSB detection is entirely automated, and the selection efficiency of the survey is well-quantified. After correcting for the selection efficiency, we find a surface density of 4.1^+2.6_-2.1 galaxies/degree² for LSBs in the considered range of μ₀ and α (90% confidence levels), with the largest correction being due to the area lost behind bright stars, and the difficulty in detecting LSBs with small angular sizes. We have measured redshifts to the final sample of LSBs, and find them to be at distances comparable to those probed by large galaxy catalogs, and to have intrinsic scale lengths of 1.7 - 3.6 h^-1₅₀ kpc, also comparable to normal galaxies. We use the redshifts and the selection efficiency to calculate the number density in LSBs with 23<μ₀<25 V mag/arcsec² and find script N sign = 0.01+^0.006_-0.005 galaxies h³₅₀ Mpc^-3, with 90% confidence. The measurement of the absolute number density of LSBs probably represents a lower limit, due to very strong biases against LSBs with bulges or edge-on LSBs in our sample. Comparing the LSB number density to the number density of normal galaxies with either similar scale lengths or similar luminosities, we find that the number density of LSBs with 23<μ₀<25 V mag/arcsec² is comparable to or greater than the number density of normal galaxies. The luminosity density in LSBs is comparable to the luminosity density of normal galaxies with similar luminosities, but is a factor of 3-10 smaller than the luminosity density of normal galaxies with similar scale lengths. The relative LSB number density and luminosity density agree well with the theoretical predictions of Dalcanton et al. (1997, ApJ, 482, in press). The redshift-space distribution of the LSBs suggests that the trend for low-surface brightness galaxies to have weak small-scale correlations may continue to the fainter surface brightnesses covered in this survey.