Morphological Number Counts and Redshift Distributions to I < 26 from the Hubble Deep Field: Implications for the Evolution of Ellipticals, Spirals, and Irregulars

We combine the photometric redshift data of Fernández-Soto et al. with the morphological data of Odewahn et al. for all galaxies with I<26.0 detected in the Hubble Deep Field. From this combined catalog we generate the morphological galaxy number counts and corresponding redshift distributions and compare these to the predictions of high-normalization zero- and passive-evolution models. From this comparison we conclude the following: 1. E/S0's are seen in numbers and over a redshift range consistent with zero-evolution or minimal passive-evolution to I=24. Beyond this limit, fewer E/S0's are observed than predicted implying a net negative evolutionary process—luminosity dimming, disassembly or masking by dust—at I>24. The breadth of the redshift distribution at faint magnitudes implies strong clustering or an extended epoch of formation commencing at z>3. 2. Spiral galaxies are present in numbers consistent with zero-evolution predictions to I=22. Beyond this magnitude some net positive evolution is required. Although the number counts are consistent with the passive-evolution predictions to I=26.0, the redshift distributions favor number and luminosity evolution, although few obvious mergers are seen (possibly classified as irregulars). We note that beyond z~2 very few ordered spirals are seen suggesting a formation epoch of spiral galaxies at z~1.5-2. 3. There is no obvious explanation for the late-type/irregular class, and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e., true irregulars), the majority lie at redshifts 11.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z>2, the logical inference is that they represent the progenitors of normal spirals that form via hierarchical merging.