AUTOMATED MORPHOLOGICAL CLASSIFICATION IN DEEP HUBBLE-SPACE-TELESCOPEUBVI FIELDS - RAPIDLY AND PASSIVELY EVOLVING FAINT GALAXY POPULATIONS

We analyze deep Hubble Space Telescope Wide Field Planetary Camera 2 ( WFPC2) images in U, B, V, I using artificial neural network (ANN) clas sifiers, which are based on galaxy surface brightness and light profil e (but not on color nor on scale length, r(hl)). The ANN distinguishes quite well between E/SO, Sabc, and Sd/Irr+M galaxies (M for merging s ystems) for B-J less than or similar to 27 mag. We discuss effects fro m the cosmological surface brightness (SB) dimming and from the redshi fted UV morphology on the classifications, and we correct for the latt er. We present classifications in UBVI from (a) four independent human classifiers; (b) ANNs trained on V-606 and I-814 images; and (c) an A NN trained on images in the rest-frame UBV according to the expected r edshift distribution as a function of B-J. For each of the three metho ds, we find that the fraction of galaxy types does not depend signific antly on wavelength, and that they produce consistent counts as a func tion of type. The median scale length at B-J similar or equal to 27 ma g is r(hl) similar or equal to 0 ''.25-0 ''.3 (1-2 kpc at z approximat e to 1-2). fariy- and late-type galaxies are fairly well separated in BM color-magnitude diagrams for B less than or similar to 27 mag, with E/SO galaxies being the reddest and Sd/Irr+M galaxies generally blue. We present the B-band galaxy counts for five WFPC2 fields as a functi on of morphological type for B-J less than or similar to 27 mag. E/SO galaxies are only marginally above the no-evolution predictions, and S abc galaxies are at most 0.5 dex above the nonevolving models for B-J greater than or similar to 24 mag. The faint blue galaxy counts in the B band are dominated by Sd/Irr+M galaxies and can be explained by a m oderately steep local luminosity function (LF) undergoing strong lumin osity evolution. We suggest that these faint late-type objects (24 mag less than or similar to B-J less than or similar to 28 mag) are a com bination of low-luminosity lower redshift dwarf galaxies, plus compact star-forming galaxies and merging systems at z similar or equal to 1- 3, possibly the building blocks of the luminous early-type galaxies se en today.