FAINT GALAXY COUNTS AS A FUNCTION OF MORPHOLOGICAL TYPE IN A HIERARCHICAL MERGER MODEL

The unprecedented resolution of the refurbished Wide Field and Planeta ry Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to maj or advances in our understanding of galaxy formation. The high image q uality in the Medium Deep Survey and Hubble Deep Field has made it pos sible, for the first time, to classify faint distant galaxies accordin g to morphological type. These observations have revealed a large popu lation of galaxies classed as irregulars or which show signs of recent merger activity. Their abundance rises steeply with apparent magnitud e, providing a likely explanation for the large number of blue galaxie s seen at faint magnitudes. We demonstrate that such a population aris es naturally in a model in which structure forms hierarchically and wh ich is dynamically dominated by cold dark matter. The number counts of irregular, spiral and elliptical galaxies as a function of magnitude seen in the HST data are well reproduced in this model. We present det ailed predictions for the outcome of spectroscopic follow-up observati ons of the HST surveys. By measuring the redshift distributions of fai nt galaxies of different morphological types, these programmes will pr ovide a test of the hierarchical galaxy formation paradigm and might d istinguish between models with different cosmological parameters.