2.2. WFPC2 B-band results and discussion

2.2.1. The scale-length - magnitude relation

Fig. 1 shows the scale-length r_hl (averaged over the BVI filters in which the object was detected to increase S/N) versus B-magnitude for both the W02 and the HDF galaxies. The three main galaxy classes resulting from the rest-frame ANN are indicated with different colors or shades, as are the formal 50% completeness limits for both fields computed for nearby RC3 galaxies of the same types (see O96). Also plotted are the best fit models to the local r_hl vs. M_B relations for RC3 mid-type spirals and ellipticals for a median r_hl = 4.7 and 3.2 kpc, respectively, assuming model redshift distributions (cf. CC92, NW95a) and K-corrections (BC93, D95a & b) as a function of galaxy type (using q₀ = 0.5; but in one case also for q₀ = 0.01). Generally, r_hl is smallest at a given flux for E/S0's. The median scale-length at B 27 mag is r_hl 0".25-0".3, which corresponds to ~ 1.2-2.5 kpc for the redshift range z ~ 0.5-2.5. These values appear to be smaller than the characteristic scale-lengths of mid to late-type galaxies measured locally, as well as those measured with HST for z 0.8 (or B 23 mag; cf. Mu94). A possible explanation is that the faint galaxy population becomes progressively more dominated by lower luminosity and therefore smaller late-type objects at fainter fluxes (cf. D94, D95a, D95b & Section 3).

2.2.2. The color-magnitude diagram

The total B₄₅₀-magnitude versus (B - R) color is shown for ANN each type in Fig. 2, where "wide R" (V + I)/2 was used to increase S/N in the colors. Fig. 2 becomes increasingly incomplete at B 26.0 mag for the bluest W02 objects, because of the red detection limit at R 26.5 mag, as indicated by the slanted dashed lines (the HDF limits are ~ 1.5 mag fainter due to its finer substepping and the r_hl vs. B-mag relation of Fig. 1). Only objects detected in all three filters are plotted here, but the fraction of objects seen only in B₄₅₀ (at B 27 mag) and not in V₆₀₆ and/or I₈₁₄ - as well as those seen only in V₆₀₆ & I₈₁₄ ( 26 mag) and not in B₄₅₀ - is 5%. Of significance is the clear segregation between the early and the late type galaxies for B 27 mag, even though no color information was used by the ANN classifier. E/S0's are almost without exception the reddest galaxies at any flux level, and Sd/Irr+M's are generally blue, at least down to the formal detection limit. Mid-type spirals (Sabc's) have colors with large dispersion and about as blue as Sd/Irr's, as expected from the (slightly) different colors measured for these populations locally and the K-corrections for galaxies with ongoing star formation at z 1-2 (BC93). The increase in the relative fraction of late-type and merging galaxies (Sd/Irr+M) in Fig. 3 towards fainter B₄₅₀ is quite remarkable. They exist essentially down to the very detection limit, despite their larger relative scale-lengths (Fig. 1). The redder objects-mostly classified as E/S0 galaxies do not increase as rapidly towards the completeness limits (B 27 mag) as the late-types. Since early-type galaxies do not generally have the largest scale-lengths (Fig. 1) nor the lowest SB, this is probably not due to large differences in their redshift distribution - and therefore due to a larger cosmological SB-dimming. Instead, it may suggest that the formation of early-type galaxies was largely complete by the median redshift corresponding to B 26-27 mag (probably z 1, see NW95a), so that their counts should indeed be converging for B 26 mag (see O96 and Fig. 3b).

2.2.3. The galaxy counts versus type for B 27 mag

Fig. 3 gives the differential B number counts for the different morphological types (E/S0's, Sabc's, Sd/Irr+M's) in the ~ 36 WFPC2 B₄₅₀-fields discussed in 2.1.1. The plotted counts are based on mean types from the visually classified U, B, V, & I images (lines coded by colors or shades), from the I₈₁₄ + V₆₀₆ single-filter ANN types (data points), and the rest-frame ANN types (heavy solid lines). Following D95a, D95b & O96, the B number counts were modeled for the three main morphological types separately. The best fit non-evolving models in Figs. 3b-3d are based on the local LF's of Ma94 (long-dashed curve) and Lo92 (dotted curve). Fig. 3b shows that the B-band counts of E/S0's follow the predictions for passively or mildly evolving models for B 24 mag, and for B 24 mag the observed counts are at most 0.3 dex higher than these models. Fig. 3c shows that Sabc's are consistent with these models for B 24 mag, and are at most 0.5 dex higher for 24 B 27 mag. Hence, a scenario invoking strong luminosity evolution is not required for the early-type galaxies out to B 27 mag (I₈₁₄ 25 mag or z ~ 1), suggesting that their formation was largely complete by z ~ 1. From a spectroscopic survey for z 1, L95 deduce a similar lack of evolution for early-type galaxies as found in D95a and O96. The morphological HST studies can push this work now another 3 magnitudes fainter than can be done spectroscopically from the ground.

Fig. 3d shows that non-evolving models are inadequate to explain the high WFPC2 B number counts for the Sd/Irr+M population. The fraction of visually classified Mergers are plotted as blue or filled squares. The fraction of mergers rises strongly for B₄₅₀ 23 mag, with up to one third of the late-type systems possibly being mergers. For B₄₅₀ 22 mag, the number counts of the classical Sd/Irr galaxies from both the ANN and visual types are well above the nonevolving models. It is clear that the FBG excess is dominated by galaxies with late-types, with a non-negligible fraction of merging morphologies. A non-evolving dwarf-rich population fits the FBG counts quite well [steep LF with = -2.0 (short-dash-dot) in Fig. 3d], but is inconsistent with the redshift distributions observed for B 24.5 mag (cf. PD95), and therefore cannot be the only explanation. Consistent with the I₈₁₄-band counts of D95a, the Sd/Irr+M B₄₅₀-counts in Fig. 3d is best described by an evolving Sd/Irr+M population with a moderately steep local LF [starburst with Lum = 1.5 mag at z 0.5 and a local Marzke LF (long-dash-dot), with the high normalization of D95b], that underwent significant luminosity evolution since z 1 (cf. PD95). The late-type galaxy population therefore must have evolved strongly and/or be dwarf-dominated, and causes most of FBG excess.

In such a scenario, a non-negligible fraction of the FBG's with 25 B 29 could be at z 1-3. In Section 3, we argue that these objects may have been the reservoir of building blocks from which the luminous early-type galaxies were formed through repeated merging - a process that must have been largely completed by z 1 for the early-type galaxies (Fig. 3b & c), with perhaps some residual epoch-dependent merger rate (cf. B94). The fact that 20-35% of the galaxies classified as late-type by the ANN appear to be merging systems - with that fraction increasing at fainter magnitudes tends to support this possibility. With the completion of ~ 100 B-band parallel fields in Cycle 6-7, we hope to extend the B₄₅₀ counts with sufficient statistics at the bright end, covering the entire range 18 B 28 mag, thereby significantly constraining the "LF normalization problem" (Ma94) and the evolution for each population separately.