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The Astronomical Journal, 128:1990 ­ 2012, 2004 November
# 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE LUMINOSITY FUNCTION OF EARLY-TYPE FIELD GALAXIES AT
1 2 1 1 1 1

Z

% 0.75
2

N. J. G. Cross, R. J. Bouwens, N. Benitez, J. P. Blakeslee, F. Menanteau, H. C. Ford, T. Goto,1 B. Holden, ´ A. R. Martel,1 A. Zirm,3 R. Overzier,3 C. Gronwall,4 N. Homeier,1 M. Clampin,5 G. F. Hartig,6 G. D. Illingworth,2 D. R. Ardila,1 F. Bartko,7 T. J. Broadhurst,8 R. A. Brown,6 C. J. Burrows,6 E. S. Cheng,5 P. D. Feldman,1 M. Franx,3 D. A. Golimowski,1 L. Infante,9 R. A. Kimble,5 J. E. Krist,6 M. P. Lesser,10 G. R. Meurer,1 G. K. Miley,3 M. Postman,1,6 P. Rosati,11 M. Sirianni,6 W. B. Sparks,6 H. D. Tran,12 Z. I. Tsvetanov,13 R. L. White,1,6 and W. Zheng1
Received 2004 April 5; accepted 2004 July 29

ABSTRACT We measure the luminosity function of morphologically selected E/S0 galaxies from z ¼ 0:5 to 1.0 using deep high-resolution Advanced Camera for Surveys (ACS) imaging data. Our analysis covers an area of 48 arcmin2 (8 times the area of the Hubble Deep Field North) and extends 2 mag deeper (I $ 24 mag) than was possible in the Deep Groth Strip Survey ( DGSS). Our fields were observed as part of the ACS Guaranteed Time Observations. At ö 0:5 < z < 0:75, we find MB þ 5log h0:7 ¼ þ21:1 ô 0:3 and ¼ þ0:53 ô 0:2, and at 0:75 < z < 1:0, we find ö MB þ 5log h0:7 ¼ þ21:4 ô 0:2, consistent with 0.3 mag of luminosity evolution (across our two redshift intervals). These luminosity functions are similar in both shape and number density to the luminosity function using morphological selection (e.g., DGSS), but are much steeper than the luminosity functions of samples selected using morphological proxies such as the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The difference is due to the ``blue,'' (U þ V )0 < 1:7, E/S0 galaxies, which make up to $30% of the sample at all magnitudes and an increasing proportion of faint galaxies. We thereby demonstrate the need for both morphological and structural information to constrain the evolution of galaxies. We find that the blue E/S0 galaxies have the same average sizes and Sersic parameters as the ``red,'' (U þ V )0 > 1:7, E/S0 galaxies at brighter luminosities (MB < þ20:1), but are increasingly different at fainter magnitudes, where blue galaxies are both smaller and have lower Sersic parameters. We find differences in both the size-magnitude relation and the photometric plane offset for red and blue E/S0s, although neither red nor blue galaxies give a good fit to the size-magnitude relation. Fits of the colors to stellar population models suggest that most E /S0 galaxies have short star formation timescales ( < 1 Gyr), and that galaxies have formed at an increasing rate from z $ 8 until z $ 2, after which there has been a gradual decline. Key words: galaxies: elliptical and lenticular, cD -- galaxies: evolution -- galaxies: fundamental parameters -- galaxies: luminosity function, mass function

1. INTRODUCTION The luminosity function ( LF ) of galaxies is the number density of galaxies as a function of absolute magnitude. The shape of the LF can be used to constrain galaxy formation models. The LF is often described by three numbers: M ö , the
1 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 2 UCO/ Lick Observatory, University of California, Santa Cruz, CA 95064. 3 Leiden Observatory, Postbus 9513, 2300 RA Leiden, Netherlands. 4 Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802. 5 Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771. 6 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. 7 Bartko Science and Technology, P.O. Box 670, Mead, CO 80542-0670. 8 Racah Institute of Physics, The Hebrew University, Jerusalem, Israel 91904. 9 ´ Departmento de Astronomia y Astrof ´sica, Pontificia Universidad Catolica ´ i de Chile, Casilla 306, Santiago 22, Chile. 10 Steward Observatory, University of Arizona, Tucson, AZ 85721. 11 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany. 12 W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743. 13 NASA Headquarters, Washington, DC 20546-0001.

magnitude at which the number of bright galaxies rapidly decreases; ö , the space density at M ö ; and the faint-end slope , which characterizes the ratio of dwarf galaxies to giant galaxies. Models of galaxy formation and evolution must be able to account for these parameters, which vary with galaxy type. Over the past few years, the LF of high-redshift (z > 0:5) galaxies have been studied extensively through the use of deep, wide-area surveys. Some of the more notable efforts include the Canada-France Redshift Survey (CFRS; Lilly et al. 1995), the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2; Lin et al. 1999), the Calar Alto Deep Imaging Survey (CADIS; Fried et al. 2001), the Deep Groth Strip Survey ( DGSS; Im et al. 2002), the Subaru Deep Survey ( Kashikawa et al. 2003), the Classifying Objects by Medium Band Observations (COMBO-17; Wolf et al. 2003), and from a combination of Hubble Space Telescope (HST ) and Very Large Telescope ( VLT ) images, Poli et al. (2003). Most of these use deep, ground-based images with spectroscopic or photometric redshifts to construct the LF, but do not have the spatial resolution to measure the structural properties of galaxies at higher redshifts. Without information on the structural properties, groundbased surveys have resorted to using color information as a proxy for morphologies, whether this information comes in the form of a best-fit spectral energy distribution (e.g., Wolf 1990


LF OF EARLY-TYPE FIELD GALAXIES AT z % 0.75 et al. 2003), or a rest-frame color cut (e.g., Lilly et al. 1995). This can result in apparent discrepant results. For example, Wolf et al. (2003) found that the elliptical /S0 ( E/S0) galaxies that produce $50% of the current B -band luminosity density only contributed $5% at z ¼ 1. By contrast, using morphological classification, van den Bergh (2001) found that the fraction of elliptical galaxies has remained constant at $17% over 0:25 < z < 1:2, implying that either the luminosity of ellipticals has increased over time relative to other types of galaxies, or the differences in color selection and morphological selection have produced apparently inconsistent results between these surveys. Surveys using the HST, such as the DGSS ( Im et al. 2002; Simard et al. 2002) and the Medium Deep Survey (Griffiths et al. 1994) have been able to reliably morphologically classify and measure structural parameters for galaxies with IAB < 22 mag, but over much smaller areas of sky than the deep ground-based surveys. These HST surveys have discovered a population of 0:3 < z < 1 blue E/S0 galaxies (e.g., Menanteau et al. 1999; Im et al. 2001; Gebhardt et al. 2003) that have luminosities similar to standard red E/S0 galaxies. Im et al. (2001) find these make up $15% of the E/S0 sample, whereas Menanteau et al. (1999) find a much higher fraction, 30% ­ 50% of the sample. Objects such as these demonstrate the inherent weakness of using color as a proxy for morphology. At low redshifts, almost all of the bright E/S0 galaxies are red, with blue ellipticals (dwarf ellipticals) many magnitudes fainter. From the work of Menanteau et al. (2004) and Im et al. (2001), it appears that most of these blue E /S0 galaxies have blue cores and red exteriors, with the exteriors having the same colors as red E/S0 galaxies, which have constant colors at all radii. Im et al. (2001) concluded that these blue E /S0 galaxies were less massive than the red E /S0 galaxies based on the dynamical masses calculated from the velocity dispersions. However, because the velocity dispersions were measured much closer to the core of the galaxy for the low-redshift red ellipticals, the high-redshift blue ellipticals may be more massive than the measurements suggest. Even if the measurements give accurate dynamical masses, the blue E/S0 galaxies have masses equivalent to the lower mass red E/S0s, so they may still yet evolve into high-mass red E/S0s through a combination of luminosity evolution that reddens the stellar population over time and mergers that increase the mass. Luminosity evolution occurs when there is new star formation, or when the stellar population ages, and does not necessarily imply any change in the mass or number of stars in a galaxy. Structural parameters such as the size and shape are better indicators of the morphological evolution, since they are only weakly dependent on the age of the stellar population and are mainly determined by dynamical characteristics such as total mass and angular momentum. Within the half-light radius of a giant elliptical galaxy the dynamical timescale is very short, less than 108 yr, so dynamical equilibrium is reached very quickly. The size and shape of the galaxy will not change significantly unless mass is added via mergers or accretion; a close encounter changes the angular momentum; tidal forces disrupt the outer layers. Small changes in the apparent shape and size do occur when star formation is localized in the center, in bars, rings, or spiral arms, but these are much weaker changes than the variation in SED or color. Therefore morphology is a more robust indicator of the nature of a galaxy, but it requires good resolution to use. Previous studies have differed in the way they have utilized size information to make inferences about evolution. Several

1991

surveys have assumed that galaxy size and shape are constant with redshift. Schade et al. (1997) showed that cluster ellipticals evolve as à M ¼ þ2:85 log10 (1 × z), assuming they maintain a constant size, and Schade et al. (1999) demonstrated that field ellipticals show a similar evolution. Using a sample of 44 galaxies with z < 2, Roche et al. (1998) discovered that ellipticals show significant luminosity evolution but little size evolution from z ¼ 1:0 to 0.2. They found that most size evolution appears to happen at z > 1:5. Graham (2002) compared the scatter in the ``photometric plane,'' which only requires parameters measured from galaxy images, to the scatter in the ``fundamental plane,'' which requires dispersion velocities measured from high-resolution spectra. Graham showed that the photometric plane could be used to constrain distances to elliptical galaxies. In Cross et al. (2001) and Cross & Driver (2002), the effects of surface brightness selection on the z ¼ 0 galaxy LF were discussed. In this paper we look at the LF of morphological early types at 0:5 < z 1. We then examine the effect that color selection has on this LF. Finally, we use structural parameters to test whether blue E/S0 galaxies are progenitors of red E /S0 galaxies and what evolution has taken place from z ¼ 1 t o 0.5. The Advanced Camera for Surveys (ACS; Ford et al. 2002) significantly improves on WFPC2 in terms of sensitivity (a factor of 5), field of view (a factor of 2), and resolution (a factor of 2), giving well-sampled PSFs in the i and z bands. This leads to significant improvements in both the accuracy of the size measurements and the overall sample size. In this paper we use data from five fields observed as part of the ACS GTO program. The total area is over 8 times the Hubble Deep Field North ( HDFN ). These fields were selected to observe very nearby (z < 0:03) galaxies or very distant (z > 4) galaxies, so galaxies in the redshift range 0:5 < z < 1:0 should be representative of the universe at that redshift. The fields are in various parts of the sky, sampling a large volume in each redshift range ($1:6 ; 104 Mpc3 0:5 < z < 0:75 and $2:4 ; 104 Mpc3 0:75 < z < 1:0), so the effects of cosmic variance should be much smaller than in the Hubble Deep Fields. In fact, the relative independence of our fields makes this survey more competitive with larger surveys than one might think based upon the areal coverage alone. We express all magnitudes in the AB system and use a M ¼ 0:3, ö ¼ 0:7 cosmology with H0 ¼ 70 km sþ1 Mpcþ1. We define h0:7 ¼ H0 =70: 2. DATA The data were extracted from five fields observed by the ACS Wide Field Camera ( WFC) between 2002 April and 2003 June. The fields were selected to give accurate photometric redshifts (three or more filters), to not have any primary targets in the range 0:5 < z < 1:0, and to not contain any clusters at lower redshifts. While the HDFN was only imaged in two ACS bands ( F775W and F850LP), it has been imaged extensively in seven optical and near-infrared bands, and has a large amount of spectroscopic follow-up. The combined area of these fields is 47.9 arcmin2, over 8 times the area of the HDFN. The extinction values, E(BþV ), are taken from the Schlegel et al. (1998) dust maps, and the total extinction in each filter, A(Blter), is calculated using the method described in Schlegel et al. (1998). A summary of the data properties in each field is given in Table 1, which lists the ACS filters, field of view, I-band exposure time, E(BþV ), I-band extinction, I-band zero point, and the number of E/S0 galaxies in our sample.


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TABLE 1 Su mmary of Data fr om Diff erent F ield s Area (arcsec2) 7.8 10.7 11.7 11.7 5.8 Texp, (s)
a

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I

Field NGC 4676 ............ UGC 10214 .......... TN 1338 ............... TN 0924 ............... HDFN ...................
a

Filters g, V, I g, V, I g, r, i, z V, i, z i+FLY99

E(B þV ) 0.017 0.009 0.096 0.057 0.012

A(I )a 0.030 0.017 0.193 0.115 0.024

ZP(I )a,b 25.947 25.947 25.655 25.655 25.655

N( E /S0) 12 17 14 19 10

4070 8180 11700 11800 5600

The exposure time, extinction and zero point are given for the F775W or F814W filter, since this was used for measurements of the structural parameters. b This is the zero point for a 1 s exposure.

2.1. NGC 4676 NGC 4676 is a low-redshift pair of merging spiral galaxies and was observed as part of the ACS Early Release Observations ( ERO) program ( Ford et al. 2002). We mask out NGC 4676 and use galaxies in the background field. It was observed for 6740 s in the F475W ( g) filter, 4000 s in the F606W (V ) filter, and 4070 s in the F814W (I ) filter. The area remaining after masking out the two prominent foreground galaxies is 7.8 arcmin2. 2.2. UGC 10214 UGC 10214 is a low-redshift spiral galaxy that is merging with a much smaller dwarf galaxy and has an extended tidal tail as a result ( Tran et al. 2003). As with NGC 4676, it was selected as part of the ERO program. We mask out UGC 10214 and use galaxies in the background field (see Benitez et al. 2004). It was ´ observed in two separate pointings, giving a combined exposure of 13600 s in F475W ( g), 8040 s in F606W (V ), and 8180 s in F814W (I ). The area remaining after masking out the prominent foreground galaxy is 10.7 arcmin2. 2.3. TN 1338 TN J1338þ1942 ( TN 1338) is a radio galaxy at z ¼ 4:1 that was observed as part of our ACS/GTO program to study protoclusters around high-redshift radio galaxies (see Miley et al. 2004; R. Overzier et al., in preparation). It was observed for 9400 s in F475W ( g), 9400 s in F625W (r), 11,700 s in F775W (i ), and 11,800 s in F850LP (z). The total observed area is 11.7 arcmin2. 2.4. TN 0924 TN J0924þ2201 ( TN 0924), a radio galaxy at z ¼ 5:2, was also observed as part of the high-redshift radio galaxy protocluster program ( R. Overzier et al., in preparation). It was observed for 9400 s in F606W (V ), 11,800 s in F775W (i ), and 11,800 s in F850LP (z). The total observed area is 11.7 arcmin2. 2.5. HDFN The Hubble Deep Field North ( HDFN ) was observed with the ACS to find supernovae and test the ACS Grism ( Blakeslee et al. 2003b). It was observed for 5600 s in the F775W (i ) filter and 10,300 s in the F850LP (z)filter. We use theACS i-band for measurements of the structural parameters, but we do not have enough ACS filters for accurate photometric redshifts. However, there are deep seven-filter data available for the portion of the ACS image already observed by WFPC2 ( Williams et al.

´ 1996). We use the photometric catalog from Fernandez-Soto et al. (1999, hereafter FLY99), which has very deep F300W (U ), F450W (B), F606W (V ), F814W (I ), WFPC2, and Kitt Peak National Observatory ( KPNO) J, H, and K band photometry. There are 146 spectroscopic redshifts from Cohen et al. (2000). We only use ACS data coincident with the deep WFPC2 image and take our photometric redshifts and colors from the FLY99 data. The observed area is 5.8 arcmin2. 2.6. Catalogs g Each set of images was run through the ACS Science Data Analysis Pipeline ( Blakeslee et al. 2003a). The data in each field were selected from the detection images produced from combining the filter images, weighted by the inverse noise squared. This aids in the detection of extremely faint objects by combining the signal from the different filters to produce a more significant detection. SExtractor ( Bertin & Arnouts 1996) was run first on the detection image and then in dual mode on the detection image and each filter image, to produce catalogs of the same objects, with photometry in matched apertures. We use these source catalogs as the starting point for selecting our sample and measuring the photometric properties. 3. MEASUREMENTS 3.1. Photometric Redshifts We use the Bayesian photometric redshift code ( BPZ; Ben´tez 2000) to calculate the photometric redshifts of galaxies i in the fields of NGC 4676, UGC 10214, TN 1338, and TN 0924. This takes advantage of both the color information and a magnitude prior to constrain the redshift. The magnitude prior distinguishes nearby red galaxies (e.g., giant ellipticals) from distant, redshifted blue galaxies, which while having similar colors when seen through a small set of filters, will have very different magnitudes. We use the template spectra described in Ben´tez et al. (2004), which are based upon a subset of the temi plates from Coleman et al. (1980) and Kinney et al. (1996). The template set consists of El, Sbc, Scd, Im, SB3, and SB2. These represent the typical spectral energy distributions (SED) of elliptical, early/intermediate type spiral, late-type spiral, irregular, and two types of starburst galaxies. These templates have been modeled using Chebyshev polynomials to remove differences between the predicted colors and those of real galaxies. The final ``calibrated'' templates have been found to give better BPZ results on the HDFN ( Benitez et al. 2004). We use ´ extinction-corrected isophotal magnitudes to maximize the signal-to-noise ratio (S/ N ) on the color input to BPZ. In each case, the aperture is the same for each filter. The magnitude


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Fig. 1.--Filter sets used in these observations. The top panel shows the three filters used in TN 0924, the upper middle panel shows the griz filters used in TN 1338, the lower middle panel shows the gVI filters used in UGC 10214 and NGC 4676 and the bottom panel shows the HDFN, with seven bands from U to K. The dotted, short-dashed, and long-dashed curves show the ``El'' SED (N. Ben´tez et al. 2004, in preparation) at z ¼ 0:5, 0.75, and 1.0, respectively. i The arrows mark the position of the 4000 8 break at these three redshifts. The 4000 8 break is well within our filter coverage at all redshifts.

prior is based on the HDFN database ( Williams et al. 1996), which uses deep ($27 mag arcsecþ2) isophotal magnitudes. 3.2. Testing BPZ g To test our photometric redshift catalogs for completeness, contamination, and systematic and random errors, we compare them to spectroscopic data in the HDFN and to simulations. Figure 1 shows the spectral energy distribution of an elliptical galaxy against the throughput of the filters used. The bottom panel shows the HDFN filter set, consisting of the UBVI WFPC2filters andthe JHK KPNO filters. The El SED is plotted three times, at z ¼ 0:5 (dotted line), 0.75 (short-dashed line), and 1.0 (long-dashed line). The main feature of this spectrum is the 4000 8 break, which is indicated by the bold arrow at each of these redshifts. The 4000 8 break is prominent in galaxies where there is very little ultraviolet radiation produced by hot, young stars, compared to the optical flux produced by an older stellar population. This break falls within the V or I filters at every redshift in the range that we use. The drop in flux per wavelength from one side of the break to the other produces a significant change in magnitude from one filter to the next, leading to an accurate measurement of the photometric redshift. The lower middle panel of Figure 1 shows the same plot for the ACS g, V and I filters used in the UGC 10214 and NGC 4676 fields. The upper middle panel shows the g, r, i, and z filters used in the TN 1338 field. The top panel shows the V, i, and z filters used in the TN 0924 field. We use the HDFN photometric and spectroscopic redshifts to estimate the errors for three-color BPZ measurements of real galaxies seen through the WFPC2 filters and then use simulations to determine any biases in the BPZ measurements through ACS filters at the noise limits of our data. The g, V,and

Fig. 2.--Errors in BPZ derived from the HDFN. In the top panel we show the three-color BPZ redshifts plotted against the seven-color BPZ redshifts for all IAB < 25 galaxies with 0:3 < z < 1:2. The squares surround morphological elliptical galaxies that have 0:5 < zspec < 1:0or 0:5 < zBPZ < 1:0 in the sevencolor BPZ catalog. There are no outliers in our sample, and the systematic offset and error in the redshift each galaxy are small, à z=(1 × z) ¼ 0:010 and (à z=(1 × z)) ¼ 0:074, respectively. In the middle panel, we compare seven-color BPZ photometric redshifts to the smaller sample of objects with spectroscopic redshifts. We find that there is a significant offset between the seven-color BPZ and the spectroscopic redshifts. We correct for this offset (see eq. [1]) and calculate zbest , which is plotted in the lower panel.

I filters used in the UGC 10214 and NGC 4676 fields are similar in wavelength coverage to the B, V, and I filters used in the HDFN data set. Therefore we can test the accuracy of the photometric redshifts in these fields by calculating three-color photometric redshifts for ellipticals in the HDFN. In the top panel of Figure 2, we plot the three-color photometric redshifts calculated using the B, V, and I filters against the seven-color photometric redshifts. The offset, Ïz3BPZ þ z7BPZ ÷=Ï1 × z7BPZ ÷ ¼ 0:010 ô 0:074, is low and there are no outliers. We calibrate the seven-color photomþ etric redshift à þthe spectà to roscopic sample and find a deviation z7BPZ þ zspec = 1 × zspec ¼ þ0:045 ô 0:026, shown in the middle panel of Figure 2. There is one outlier, a galaxy with zBPZ ¼ 0:87 and zspec ¼ 0:67. As expected from the poor fit, this object has (V þ I ) colors that are much redder and (B þ V ) colors that are slightly bluer than one would expect for an elliptical galaxy at this redshift. The bottom panel shows the three-color photometric redshifts corrected for this offset. The correction is described at the end of this section. The quoted error in the above cases and for future BPZ measurements is for a single galaxy, so this offset is significant. Cohen et al. (2000) show that the errors in the spectroscopic data are àv ¼ 200 km sþ1, implying à z ¼ 0:0007. The final error is consistent with the typical scatter found in the overall analysis of all HDF redshifts, à z=(1 × z) ¼ 0:06. The offset between BPZ and spectroscopic redshifts implies some evolution in elliptical galaxies from z ¼ 0:2 (the redshift of the calibration cluster) and z $ 0:75. Given that all of the HDFN ellipticals have good three-band photometric redshifts, we expect that ellipticals in NGC 4676 and UGC 10214 should also have good photometric redshifts.


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Fig. 3.--Results of the four simulations, (zdetection þ zsimulation )=(1 × zsimulation ). The open represent objects with a Scd SED. The circles z ¼ 0:5, 0.75, and 1.0. The dashed lines show 2004, in preparation). There are no significant

TN 0924, TN 1338, NGC 4676, and UGC 10214, as labeled. The y axis is the difference in the redshift, squares represent objects with an El SED, the crosses represent objects with a Sbc SED, and the filled triangles with error bars represent the 3 clipped mean for the El and Sbc SEDs. The dotted lines mark out the samples at the expected mean and standard deviations based on the measurements against spectroscopic data ( N. Ben´tez et al. i systematic errors, but galaxies in UGC 10214 and NGC 4676 have large random errors for z > 0:85.

However, the noise in these fields is somewhat greater than the HDFN, so there may be some missing objects. We test the reliability of BPZ in each of the fields using Bouwens' Universe Construction Set ( BUCS; R. Bouwens et al., in preparation; Bouwens et al. 2003, 2004) simulations of r 1=4 elliptical galaxies with three different SEDs: El, Sbc, and Scd ( Benitez et al. 2004). These simulations are designed to ´ have the same noise characteristics as the observed ACS data sets and are processed in the same way as the data (x 2.6). Therefore, the UGC 10214 simulation, with double the exposure time, has 1.4 times the S/ N of the NGC 4676 simulation. We use the three SEDs to test the reliability of redshifts for early-type galaxies with a range of colors. All the simulations are made up of galaxies with elliptical morphologies (  ¼ 4) and a Schechter LF with parameters ö ¼ 0:00475, M ö ¼ þ20:87, and ¼ þ0:48. The density of galaxies was increased by a factor of 5 over the normal elliptical galaxy density to give a large sample of galaxies at each redshift. In these simulations elliptical galaxies are placed at random in four fields, each 2000 ; 2000 pixels. Each of these fields is approximately the area of a single amplifier on the Wide Field Camera.

Once the images had been processed, we compared the simulation input catalog and the catalog of detected objects. The results are shown in Figure 3. In each of the fields we find small differences between the measured redshift and the input redshift. The only major differences occur in the NGC 4676 and UGC 10214 simulations, in the zsimulation ¼ 0:95 bin. In both cases zdetection is overestimated. Figure 1 shows that at this redshift, the 4000 8 break is in the middle of the F814W filter, with no redder filter to compare to. This is also the redshift range at which there is increased scatter in three-band photometric redshifts in the HDFN, which had a similar combination of filters. The offsets are due to the increased scatter and are not a systematic effect. We find that the TN 1338 simulation has a mean scatter z ¼ 0:023, TN 0924 has z ¼ 0:028, NGC 4676 has z ¼ 0:045, and UGC 10214 has z ¼ 0:046. Since the HDFN has filters similar to those used for NGC 4676 and UGC 10214 and is deeper, we would expect z to be lower. The additional noise is due to the real galaxy spectral energy distributions varying from the ideal templates used in our simulations. There is a large increase in the scatter for all galaxy types in the HDFN, UGC 10214, and NGC 4676 fields at


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z > 0:85, with the rms in the HDFN increasing from z ¼ 0:029 (z < 0:85) to z ¼ 0:068 (z > 0:85), and the rms in the UGC 10214 and NGC 4676 fields increasing from z ¼ 0:036 (z < 0:85) to z ¼ 0:050 (z > 0:85). We can use the simulations to check for incompleteness. All of the galaxies with Bz ¼ 0 24:5 mag (Bz ¼ 0 24:0 mag at z > 0:75) were detected, apart from one or two galaxies close to the edge of each image, one or two with a nearby neighbor, and a few galaxies at z > 1:2 in TN 0924. At fainter magnitudes the errors become very large for galaxies in NGC 4676 in particular. Altogether, 15% of 0:5 < z < 1:0 objects have þ0:06 < à z=(1 × z) < 0:06, and only 6% have þ0:12 < à z=(1 × z) < 0:12. There is also around 2% contamination from lower or higher redshift objects (z < 0:3 and z > 1:2). We correct the BPZ redshift estimates to account for the difference between the spectroscopic and BPZ measurements for elliptical galaxies, z z
best best

a and b are only weakly dependent on the Sersic profile. The best-fit parameters for an exponential profile (  ¼ 1) are a ¼ 0:38 and b ¼ 0:28, whereas a de Vaucouleur 's profile (  ¼ 4) is well fitted by a ¼ 0:24 and b ¼ 0:21. Therefore, if ¼ 0:8, r ell =r cir ¼ 1:11 and 1.09 for  ¼ 1 and 4, respectively, and if e e ¼ 0:6, r ell =r cir ¼ 1:26 and 1.22, respectively. These two exe e amples demonstrate the weak dependence on . Once the bestfit parameters are found, a new total flux is calculated, and the process is iterated until the new flux is no longer larger than the old flux. We use the output from GALFIT for the rest of our analysis, since it is corrected for the PSF, which is important for galaxies with re < 0B4, but use the growth curve to identify outliers (see Fig. 4). The scatter in the two measurements is linear with size: àre ¼ 0:25re þ 0:013: Ï 3÷

¼

zBPZ × 0:045 ; (1 þ 0:045)

Ï 1÷

is plotted against zspec in the lower panel of Figure 2. This changes the input BPZ redshift range to 0:43 < zBPZ < 0:91. It also considerably reduces the errors associated with zBPZ > 0:85 galaxies in UGC 10214 and NGC 4676. We use the Benitez et al. (2004) errors (z ¼ 0:06) for our BPZ measure´ ments. We find that a few (seven) of our objects have significantly broader probability density functions. The width of these PDFs are added in quadrature to the initial z ¼ 0:06. The objects in UGC 10214 and NGC 4676 with zBPZ > 0:85 are given an uncertainty z ¼ 0:09. This takes into account both template error (errors related to mismatches between the real and assumed templates) and random errors (due to the noise). In summary, our final sample contains 72 galaxies, 10 of which have spectroscopic redshifts. The completeness is expected to be in excess of 95% (P3 ­ 4 missing galaxies), with a contamination of less than 2 ­ 3 galaxies ( from redshift uncertainties). We list the properties of all our galaxies in Table 2, in two redshift intervals (0:5 < z 0:75 and 0:75 < z 1:0). Within each interval they are listed in order of increasing restframe (U þ V )0 color (see x 5.1). 3.3. Measuring the Half-Light Radius and Total Magnitude g g g We calculate the half-li