Stephen C. Odewahn, Rogier A. Windhorst
Arizona State University, Dept. of Physics & Astronomy,
Tempe, AZ 85287
Simon P. Driver
University of New South Wales, School of Physics, Sydney, NSW 2052,
Australia
William C. Keel
University of Alabama, Dept. of Physics & Astronomy, Tuscaloosa, AL
35487
using bandpass-specific
artificial neural network (ANN) classifiers, which are based on
galaxy surface brightness (SB) and light profile (but not on color
nor directly on scale-length). The ANN distinguishes quite well
between E/S0's, Sabc's, and Sd/Irr's for B 
26.5 mag, with
larger classification errors at fainter magnitudes.
Galaxy types are fairly well-separated in BVI color-magnitude diagrams
down to B
26.5, with E/S0's being the reddest and Sd/Irr's generally
blue down to the formal detection limit for compact objects
(B
27.5 mag). We model the B
band number counts as
a function of morphological type with the E/S0 and Sabc samples following
no/mild-evolution predictions down to B
26.5 mag (z
1).
The Sd/Irr B
band counts are modeled by a moderately steep
local LF undergoing luminosity evolution since z
1.
The way in which galaxies form and evolve is a fundamental question which has
yet to be resolved. Of particular interest is the long standing problem of the
faint blue galaxy (FBG) excess. The majority of explanations
for the FBG excess observed in deep ground-based CCD images (Tyson 1988)
involve Irregular/dwarf populations, or objects with 
fainter than -17.0 mag (H
80, q
0.5,
see Driver et al. 1995; D95).
The superb resolution provided by the refurbished HST Wide Field
Planetary Camera 2 (WFPC2) allows the determination of the morphological
types of individual galaxies over a wide range of epochs and hence
provides important information for studying the nature the FBG population.
In Cycle 4--5, we imaged a single deep dithered WFPC2 field surrounding the
weak radio galaxy 53W002 at z=2.39 (24 orbits in B
, 12 in V and I).
Details of these observations are given by D95
and Odewahn et al. (1996; O96). As a consistency check, we compared
our counts in this 53W002 (hereafter ``W02'') field to those from 3
shallow WFPC2 fields obtained from the HST archive, and those from the
Hubble Deep Field (HDF, Williams et al. 1996) which reach 0.8--1.2
mag deeper than the W02 field in B
VI.
The W02 and shallower WFPC2 images were processed following D95.
The same analysis was made for the HDF images which are closer to
the confusion limit, but we use the HDF data only down to the
detection limit of our W02 sample (B
27.5 mag), where the WFPC2
images are definitely not confusion limited.
An automated software package (MORPHO) was used to determine
type-dependent photometric parameters (e.g., Odewahn & Aldering 1995)
and assign morphological types using an Artificial Neural
Network (e.g., Odewahn et al. 1992).
As network training samples we used 173 WFPC2 galaxies down
to I
22 mag from DWG and 227 galaxies down to I
24.5 mag
from D95. Comparison of the galaxy
types from our V & I band classifiers (for B
26) produced
a scale difference of only 5%, and a scatter of 1.7 steps (rms) on
the 16 step revised Hubble system.
The total B
magnitude versus 
color is shown for each type
in Fig. 1, where ``wide'' 
=
was used to increase the
S/N in colors. While the 50% completeness in the W02 catalogs
occurs at B
27.5 mag, for B
26.0
mag Fig. 1 becomes increasingly incomplete for the bluest
W02 objects because of the red detection limit at 
26.5 mag,
as indicated by the slanted dashed lines (the HDF limits are
again 
1.5 mag fainter).
Of significance is the clear segregation between the early and the late type
galaxies in Fig. 1 down to 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's are generally
blue, at least down to the formal detection and classification limit
(B
27.0).
In Figure 2 we plot the differential B
number counts for the different
morphological subsets in our five WFPC2 fields. Within the errors,
Figs. 2a--2d, show that the B
counts in the W02 field are consistent
with the other 4 WFPC2 B
fields, as well as with
ground-based counts (Metcalfe et al. 1995) down to B
27 mag,
so that field-to-field variations are no larger than the formal errors.
Following the I-band models of D95, we modeled the B
number counts
for the three main morphological types separately.
Fig. 2b shows that the B
counts observed for E/S0's follow the
predictions for passively or mildly evolving models
(solid curves for a non-evolving local Marzke LF and dotted curves for
a non-evolving local Loveday LF).
Fig. 2c shows that Sabc's are also consistent with these models, or
at best 0.5 mag brighter for 22
B
25 mag.
Hence, a scenario invoking strong luminosity evolution is
not required for the early-type galaxies out to B
26
mag (I
24 mag or z
1, see D95), suggesting that their
formation was largely complete by z
1. However, Fig. 2d shows
that non-evolving models are clearly inadequate to model
our B
counts for the Sd/Irr population.
Consistent with the I-band results of D95, our Sd/Irr B-counts (Fig. 2d)
can be described by a population with a moderately steep local LF that
underwent some luminosity evolution since z
1 (dashed curves).
Following the models of D95, a starburst with 
1.5
mag at z
0.5--1 would be required to explain the steep Sd/Irr counts.
Figure: The 
vs. B
(
B
) color-magnitude
diagram for all classified galaxies in the two deep W02 and HDF WFPC2
fields. Symbols indicate membership in the categories E/S0, Sabc,
& Sd/Irr, as classified by the V & I ANN's of O96. Note that a
clear type segregation is present down to the classification
limit, although no color information was used by the ANN classifiers.
Figure: Differential B number counts from the W02 field (filled symbols),
the HDF (open triangles), and three shallower WFPC2 B fields (open
squares), as well as previous ground-based B-band counts (crosses).
Panels show (a) all galaxies, (b) HST ellipticals (E/S0's), and (c)
early-type spirals (Sabc's), as classified by the ANN. Models are
described in the text, and by D95 and O96.
Driver, S. P. et al. 1995, ApJ, 449, L23 (D95)
Driver, S. P. et al. 1995, ApJ, 453, 48 (DWG)
Metcalfe, N., Shanks, T., Fong, R., & Roche, N. 1995, MNRAS, 273, 257
Odewahn, S. C. et al. 1992, AJ, 103, 318
Odewahn, S. C. & Aldering, G. 1995, AJ, 110, 2009
Odewahn, S. C., Windhorst, R. A., Driver, S. P., & Keel, W. C. 1996, ApJ, submitted (O96)
Tyson, J. A. 1988, AJ, 96, 1
Williams, R.E. et al. 1996, this volume, p. 