THE LOCAL-DENSITY OF LOW-LUMINOSITY STAR-FORMING GALAXIES

Deep redshift surveys have recently revealed that a large population of relatively low-luminosity galaxies (approximately 0.1L(B)*) at moderate redshifts (z < 0.5) is responsible for the excess in the galaxy counts over no-evolution models at B approximately 24. The present-day counterparts of these faint blue galaxies have yet to be identified. To produce the comoving galaxy densities observed at moderate redshift, the faint-end slope of the luminosity function must be considerably steeper than observed in surveys of local field galaxies. In this paper, we recalculate the local density of low-luminosity star-forming galaxies from the emission-line-selected sample of Salzer. Our analysis differs from previous work in that we attempt to recover the space density of the parent population of the emission-line sample using two simple assumptions: (1) that the distribution of [O III] 5007-angstrom line equivalent widths can be adequately described by a power law, and (2) that the distribution of equivalent width is independent of luminosity. These assumptions are clearly only first approximations, as they ignore the known variations of line excitation with metallicity and any possible variations of star formation rate with luminosity. Nevertheless, the assumptions appear to be consistent with the available data. With these assumptions we find that the local space density of galaxies with [O III] equivalent widths greater than 5 angstrom and absolute magnitudes M(B) > - 18.5 to be up to a factor of approximately 10 higher than Salzer's estimate without these assumptions. The space density is comparable to that estimated for normal field galaxies, and we suspect that, to some degree, we have simply recovered the luminosity function of field dwarf irregular galaxies from the extreme tail of the emission-line equivalent-width distribution. However, our reconstructed luminosity function has a much steeper faint-end slope than the canonical field-galaxy slope. Detailed examination of the field-galaxy luminosity function and HI mass function shows that such a steep slope is not ruled out by current surveys. However, this additional population of star-forming dwarf galaxies is insufficient to explain the B-band number counts for q0 = 0.5 without invoking some form of evolution.