The properties and luminosity function of extremely low luminosity galaxies

We examine a sample of low-redshift (10 h^-1 Mpc < d < 150 h^-1 Mpc) field galaxies including galaxies with luminosities as low as M_r - 5 log h ∼ -12.5, selected from the Sloan Digital Sky Survey Data Release 2 (SDSS DR2). The sample is unique in containing galaxies of extremely low luminosities in a wide range of environments, selected with uniform and well-understood criteria. We present the luminosity function, as well as the broadband properties, of low-luminosity galaxies in this sample. A Schechter function is an insufficient parameterization of the r-band luminosity function; there is an upturn in the slope for M_r - 5 log h > -18. The resulting slope at low luminosities in this sample is α₂ ∼ -1.3. However, we almost certainly miss a large number of galaxies at very low luminosities due to low surface brightness selection effects, and we estimate that the true low-luminosity slope may be as steep as or steeper than α₂ ∼ -1.5. The results here are consistent with previous SDSS results and, in the g band, roughly consistent with the results of the Two Degree Field Galaxy Redshift Survey. Extremely low luminosity galaxies are predominantly low surface brightness, exponential disks, the majority of which are red.