Passive Evolution: Are the Faint Blue Galaxy Counts Produced by a Population of Eternally Young Galaxies?

A constant age population of blue galaxies, postulated in the model ofGronwall & Koo (1995), seems to provide an attractive explanation of the excessof very blue galaxies in the deep galaxy counts. Such a population may begenerated by a set of galaxies with cycling star formation rates, or at theother extreme, be maintained by the continual formation of new galaxies whichfade after they reach the age specified in the Gronwall and Koo model. For bothof these hypotheses, we have calculated the luminosity functions including therespective selection criteria, the redshift distributions, and the numbercounts in the B_J and K bands. We find a substantial excess in the number ofgalaxies at low redshift (0 < z < 0.05) over that observed in the CFH redshiftsurvey (Lilly et al. 1995) and at the faint end of the Las Campanas luminosityfunction (Lin et al. 1996). Passive or mild evolution fails to account for thedeep galaxy counts because of the implications for low redshift determinationsof the I-selected redshift distribution and the r-selected luminosity functionin samples where the faded counterparts of the star-forming galaxies would bedetectable.Comment: 11 pages, LaTeX type (aaspp4.sty), 3 Postscript figures, submitted to ApJ Letter