Automated morphological classification in deep Hubble Space Telescope UBVI 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) classifiers, which are based on galaxy surface brightness and light profile (but not on color nor on scale length, r_hl). The ANN distinguishes quite well between E/S0, Sabc, and Sd/Irr+M galaxies (M for merging systems) for B_J ≲ 27 mag. We discuss effects from the cosmological surface brightness (SB) dimming and from the redshifted UV morphology on the classifications, and we correct for the latter. We present classifications in UBVI from (a) four independent human classifiers; (b) ANNs trained on V₆₀₆ and I₈₁₄ images; and (c) an ANN trained on images in the rest-frame UBV according to the expected redshift distribution as a function of B_J. For each of the three methods, we find that the fraction of galaxy types does not depend significantly on wavelength, and that they produce consistent counts as a function of type. The median scale length at B_J ≃ 27 mag is r_hl = 0″.25-0″.3 (1-2 kpc at z ≈ 1-2). Early- and late-type galaxies are fairly well separated in BVI color-magnitude diagrams for B ≲ 27 mag, with E/S0 galaxies being the reddest and Sd/Irr+M galaxies generally blue. We present the B-band galaxy counts for five WFPC2 fields as a function of morphological type for B_J ≲ 27 mag. E/S0 galaxies are only marginally above the no-evolution predictions, and Sabc galaxies are at most 0.5 dex above the nonevolving models for B_J ≳ 24 mag. The faint blue galaxy counts in the B band are dominated by Sd/Irr+M galaxies and can be explained by a moderately steep local luminosity function (LF) undergoing strong luminosity evolution. We suggest that these faint late-type objects (24 mag ≲ B_J ≲ 28 mag) are a combination of low-luminosity lower redshift dwarf galaxies, plus compact star-forming galaxies and merging systems at z ≃ 1-3, possibly the building blocks of the luminous early-type galaxies seen today.