The morphological mix of field galaxies to mi = 24.25 magnitudes (bj ˜ 26 magnitudes) from a deep hubble space telescope1 wfpc2 image2

We determine the morphological mix of field galaxies down to mJ 3 24.25 mag (mB ˜ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the K606 and the j814 filters. In total, we find 227 objects with mj < 24.5 mag and classify these into three types: Ellipticals (16%), early-type spirals (37%), and late-type spirals/irregulars (47%). The differential number counts for each type are compared with simple models in a standard flat cosmology. We find that both the elliptical and the early-type spiral number counts are well described by little-or-no-evolution models, but only when normalized at b} = 18.0 mag. Given the uncertainties in the luminosity function (LF) normalization, both populations are consistent with a mild evolutionary scenario based on a normal/low rate of star formation. This constrains the end of the last major star formation epoch in the giant galaxy populations to 2 > 0.8. Conversely, the density of the observed late-type/irregular population is found to be a factor of 10 in excess of the conventional no-evolution model. This large population might be explained by a modified local dwarf-rich LF and/or strong evolution acting on the local LF. For the dwarf-rich case, a steep faint-end Schechter slope (a 3 —1.8) is required, plus a fivefold increase in the dwarf normalization. For a purely evolving model based on a flat Loveday et al. LF (a 3 —1.0), a ubiquitous starburst of AJ ˜ 2.0 mag is needed at 2 3 0.5 for the entire late-type population. We argue for a combination of these possibilities, and show that for a steep Marzke et al. LF (a 3 —1.5) a starburst of ˜1.3 mag is required at 2 3 0.5 in the entire late-type population, or ˜2.0 mag in ˜20% of the population.