Resurrection of traditional luminosity evolution models to explain faint field galaxies

We explore the nature of the evolution of faint field galaxies by assuming that the local luminosity function is not well-defined. We use a non-negative least-squares technique to derive a near optimal set of local luminosity functions for different spectral types of galaxies by fitting to the observed optical and near-infrared counts, B-R colors, and redshift distributions for galaxies with 15 ≤ B ≤ 27. Our previous work showed that a no-evolution model for the luminosity functions was able to match the observed blue galaxy counts to within a factor of less than 50% by B ∼ 25 versus the 5 × to 15 × (e.g., Tyson 1988) of other nonevolving models. We report here the results of using only traditional luminosity evolution (i.e., the photometric evolu tion of stars in a galaxy over time given reasonable assumptions of the form of the star-formation history for various galaxy types), and find excellent fits to the observed data to B ∼ 23. The addition of simple reddening with an SMC extinction law to our model spectral energy distributions extends the almost perfect fits to the faintest limits. While prior luminosity evolution models required both a low q₀ and a high galaxy formation redshift to fit the observed data, the quality of our fits is not significantly degraded by changing the value of q₀ to 0.5. We conclude that models more exotic than traditional luminosity evolution are not yet required to explain existing faint galaxy data and thus the need for contributions by mergers or new populations of galaxies is at least 5 × less than previously estimated (e.g., Broadhurst et al. 1992).