Luminosity functions and evolution of blue galaxies in a deep multicolor CCD field survey

A complete sample of 659 field galaxies with 17.0 < U less than or equal to 21.1, each with UBVR-17500-I8600 photometry, has been selected from a deep held survey that covers 0.83 deg(2) along six lines of sight. Each galaxy's spectral type and redshift have been estimated using a multicolor photometric technique of Liu & Green. Total number counts of the galaxies in the U band give a count slope d(logN)/ dM = 0.55 +/- 0.05, consistent with previous studies. The 545 galaxies in the sample classified as spectral type Sbc or bluer are analyzed for signs of evolution with redshift and for unusual star formation histories. The U-band luminosity function of these blue galaxies at 0.02 < z < 0.15 has a steep alpha similar or equal to -1.85 down to M(B) similar or equal to -14. The luminosity functions at 0.15 less than or equal to z < 0.3 and 0.3 less than or equal to z less than or equal to 0.5 show significant evolution in M* and phi* at levels consistent with those found in the Canada-France and Autofib redshift surveys. A significant population of very blue (rest-frame U-B < -0.35) galaxies, with spectral energy distributions indicating strong starburst activity, is observed at z greater than or similar to 0.3 but not at z < 0.3. This population is confirmed via spectroscopy of part of the sample. These may be galaxies temporarily brightened by global starbursts, which subsequently fade and redden at lower redshifts.