Multicolor Observations of the Hubble Deep Field South

Eros Vanzella, Stefano Cristiani, Paolo Saracco, Stephane Arnouts, Simone Bianchi, Sandro D'odorico, Adriano Fontana, Emanuele Giallongo, Andrea Grazian

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Eros Vanzella, Stefano Cristiani, Paolo Saracco, Stephane Arnouts, Simone Bianchi, et al. Multicolor Observations of the Hubble Deep Field South. The Astronomical Journal, 2001, 122 (5), pp.2190-2204. 10.1086/323544. hal-05358160

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Distributed under a Creative Commons Attribution 4.0 International License
MULTICOLOR OBSERVATIONS OF THE HUBBLE DEEP FIELD SOUTH

EROS VANZELLA,<fnref n="1" />,<fnref n="2" /> STEFANO CRISTIANI,<fnref n="2" />,<fnref n="3" /> PAOLO SARACCO,<fnref n="4" /> STEPHANE ARNOUTS,<fnref n="1" /> SIMONE BIANCHI,<fnref n="1" /> SANDRO D'ODORICO,<fnref n="1" /> ADRIANO FONTANA,<fnref n="5" /> EMANUELE GIALLONGO,<fnref n="5" /> AND ANDREA GRAZIAN<fnref n="1" />,<fnref n="2" />

Received 2001 June 5; accepted 2001 July 23

ABSTRACT

We present a deep multicolor (U, B, V, I, Js, H, Ks) catalog of galaxies in the Hubble Deep Field South, based on observations obtained with the HST WFPC2 in 1998 and VLT-ISAAC in 1999. The photometric procedures were tuned to derive a catalog optimized for the estimation of photometric redshifts. In particular we adopted a “conservative” detection threshold, which resulted in a list of 1611 objects. The behavior of the observed source counts is in general agreement with the previous results of Casertano et al. in the Hubble Deep Field South and Williams et al. in the Hubble Deep Field North, while the corresponding counts in the Hubble Deep Field North provided by Fernández-Soto, Lanzetta, & Yahil are systematically lower by a factor 1.5 beyond $I_{AB} = 26$. After correcting for the incompleteness of the source counts, the object surface density at $I_{AB} \le 27.5$ is estimated to be $220 \ \text{arcmin}^{-2}$, in agreement with the corresponding measure of Volonteri et al. and providing an estimate of the extragalactic background light in the I band consistent with the work of Madau & Pozzetti. The comparison between the median $V-I$ color in the Hubble Deep Field North and South shows a significant difference around $I_{AB} \sim 26$, possibly due to the presence of large-scale structure at $z \sim 1$ in the HDF-N. High-redshift galaxy candidates (90 U dropouts and 17 B dropouts) were selected by means of color diagrams, down to a magnitude $I_{AB} = 27$, with a surface density of $(21 \pm 1)$ and $(3.9 \pm 0.9) \ \text{arcmin}^{-2}$, respectively. Eleven extremely red objects [with $(I-K)_{AB} > 2.7$] were selected down to $K_{AB} = 24$, plus three objects whose upper limit to the Ks flux is still compatible with the selection criterion. The corresponding surface density of EROs is $(2.5 \pm 0.8) \ \text{arcmin}^{-2}$ [$(3.2 \pm 0.9) \ \text{arcmin}^{-2}$ if we include the three Ks upper limits]. They show a remarkably nonuniform spatial distribution and are classified with roughly equal fractions in the categories of elliptical and starburst galaxies.

Key words: cosmology: observations — galaxies: evolution — galaxies: statistics

On-line material: color figures, machine-readable table

1. INTRODUCTION

The Hubble Deep Field South (hereafter HDF-S) consists of a large set of observations of an otherwise unremarkable field around the QSO J2233−606 (z = 2.24), taken in parallel by three instruments aboard the *Hubble Space Telescope* (HST): the Wide Field Planetary Camera 2 (WFPC2), the Space Telescope Imaging Spectrograph (STIS), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The observations are described in a series of papers: Williams et al. (2000) provide details on the field selection and the overall strategy, Fruchter et al. (2001) the NICMOS observations, Gardner et al. (2000) the STIS imaging observations, Savaglio et al. (1999) the STIS spectroscopic observations, and Casertano et al. (2000) the WFPC2 observations (the main field).

The WFPC2 observations were centered around $\alpha =$

$22^h32^m56.22, \delta = -60^\circ33'02".7$ (J2000.0) and consist of a total of approximately 450 ks worth of exposures in the four bands F300, F450, F606, F814 (in the following indicated as U, B, V, and I, respectively), like the original HDF (Williams et al. 1996).

The HDFs represent an important part of the frontier studies of the distant universe. They have revolutionized the understanding of high redshift galaxies, providing resolved images of faint objects, and have contributed in a wide variety of ways to shaping the debate over issues such as the origin of elliptical galaxies and the importance of obscured star formation. The addition of deep near-infrared images to the database provided by the WFPC2 is essential to monitor the SEDs of the objects on a wide baseline and address a number of key issues, such as the total stellar content of baryonic mass, the effects of dust extinction, the dependence of morphology on the rest-frame wavelength, the photometric redshifts, and the detection and nature of extremely red objects (EROs).

For these reasons deep NIR imaging of the HDF-S was carried out with VLT-ISAAC and the present paper describes a new catalog built from the combined WFPC2-ISAAC database.

2. THE OBSERVATIONS

2.1. *Optical: WFPC2*

In Table 1, we summarize the observations in the filters U, B, V, and I and list the estimated magnitude limits at 10

<footnote n="1">¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany.</footnote>

<footnote n="2">² Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, I-35122 Padova, Italy.</footnote>

<footnote n="3">³ Space Telescope European Coordinating Facility, Karl-Schwarzschild-Strasse 2, D-85748, Garching bei München, Germany.</footnote>

<footnote n="4">⁴ Osservatorio Astronomico di Brera, via E. Bianchi 46, I-22055 Merate, Italy.</footnote>

<footnote n="5">⁵ Osservatorio Astronomico di Roma, via dell'Osservatorio 2, I-00040 Monteporzio, Italy.</footnote>
<table><caption>TABLE 1
WFPC2 OBSERVATIONS</caption><thead><tr><th>Filter</th><th>Number of Frames</th><th>Total Time (s)</th><th>FWHM (arcsec)</th><th>Zero Point (ABMAG)</th><th>Limiting AB Magnitude (10 σ in 0.2 arcsec²)</th></tr></thead><tbody><tr><td>F300......</td><td>102</td><td>132,585</td><td>0.14</td><td>20.77</td><td>26.8</td></tr><tr><td>F450......</td><td>51</td><td>101,800</td><td>0.14</td><td>21.9</td><td>27.7</td></tr><tr><td>F606......</td><td>49</td><td>97,200</td><td>0.14</td><td>23.02</td><td>28.3</td></tr><tr><td>F814......</td><td>56</td><td>112,200</td><td>0.14</td><td>22.09</td><td>27.7</td></tr></tbody></table>

NOTE.—From Casertano et al. 2000.

<table><caption>TABLE 2
ISAAC OBSERVATIONS</caption><thead><tr><th>Filter</th><th>⟨λ⟩</th><th>Δλ</th><th>Number of Frames</th><th>Total Time (s)</th><th>FWHM (arcsec)</th><th>Zero Point</th><th>Limiting Magnitude (5 σ)</th></tr></thead><tbody><tr><td>Js......</td><td>1.24</td><td>0.16</td><td>210</td><td>25,200</td><td>0.60</td><td>28.35</td><td>23.5</td></tr><tr><td>H......</td><td>1.65</td><td>0.30</td><td>181</td><td>21,720</td><td>0.60</td><td>27.50</td><td>22.0</td></tr><tr><td>Ks......</td><td>2.16</td><td>0.27</td><td>487</td><td>29,220</td><td>0.60</td><td>26.26</td><td>22.0</td></tr></tbody></table>

NOTE.—Magnitudes are provided in the Vega magnitude system.

σ within apertures of 2 × FWHM for the combined frames (more details are given in Casertano et al. 2000).<fnref n="6" />

## 2.2. Infrared: ISAAC

The IR data were obtained with the ISAAC infrared

imager/spectrometer (Moorwood et al. 1999) at the ESO VLT-UT1 telescope. ISAAC is equipped with a 1024 × 1024 pixel Rockwell Hawaii array, providing a pixel scale of 0.147 pixel⁻¹ and a total field of view of about 2.5 × 2.5. The observed field is centered at α = 22h32m55s, −60°33′08″. The observations were carried out over several nights from 1999 September to December under homogeneous seeing conditions: about 0.6. The HDF-S was imaged in the *Js*, *H*, and *Ks* bands: in Table 2 we sum-

<figure><img src="image_2.png" /><figcaption>FIG. 1.—Global scheme of reduction</figcaption></figure>

<footnote n="6">⁶ The optical deep images of WFPC2 are available at ftp://archive.stsci.edu/pub/hdf_south/version1/wfpc2_version2/.</footnote>
<figure><img src="image_3.png" /><figcaption>FIG. 2.—Left: Detection without subframes; global parameter DEBLEND_MINCONT = 0.02. Right: Detection with modified version of SExtractor; inside the rectangle DEBLEND_MINCONT = 1.0. [See the electronic edition of the Journal for color versions of Figs. 2–3.]</figcaption></figure>

marize the number of frames and the relevant total exposure time per band. The *Js* and *Ks* filters were adopted instead of the standard *J* and *K* because: (1) The standard *J* filter of ISAAC (with central wavelength 1.25 µm and width 0.29 µm) has leaks in the *K* band, and the atmosphere defines the red edge of the filter. With the *Js* filter it is possible to obtain more accurate photometry. (2) The *Ks* filter, having a shorter cutoff at long wavelength, makes it possible to reduce the thermal background and increase the signal-to-noise ratio (S/N). Further details on the instrument are available in the ISAAC user manual (Cuby, Lidman, & Moutou 2001).<fnref n="7" />

The photometric calibration of the observations was carried out by observing several standard stars from the list of the infrared NICMOS standard stars (Persson et al. 1998), with magnitudes ranging between 10 and 12.

The reduction and co-adding were performed using the facilities available at the Merate Observatory. Raw frames, corrected for flat field, bias, dark current, and thermal noise were processed with DIMSUM<fnref n="8" /> to produce the sky-

subtracted frames; for each exposure a sky background image is generated as the median of a set (from three to six) of time adjacent frames in which sources were previously masked out.

The sky-subtracted frames were inspected and sky residuals were removed where present, using the IRAF task IMSURFIT. The images were rescaled to the same median value by adding to each of them a suitable constant term. Finally, shifting and co-adding were performed with DIMSUM. The final co-added image is the average of the shifted frames. The infrared data analyzed in this work were presented in a paper of Saracco et al. (2001), to which we refer for further details on the data reduction.

In Table 2, we summarize the observations and list the observed magnitude limits at 5 σ within apertures of 2 × FWHM of the combined frames. The total time exposures in the filters *Js*, *H*, *Ks* are 7^h00^m, 6^h02^m and 8^h07^m, respectively.

### 3. DATA REDUCTION

Figure 1 shows the global scheme of the reduction.

#### 3.1. Object Detection

The catalog was produced using the source extractor package (SExtractor, Bertin & Arnouts 1996) in a slightly modified and more flexible version (see below).

<footnote n="7">⁷ Available at http://www.eso.org/instruments/isaac/index.html.</footnote>

<footnote n="8">⁸ Deep Infrared Mosaicking Software, a package of IRAF scripts by P. Eisenhardt, M. Dickinson, and J. Ward available at ftp://iraf.noao.edu/contrib/dimsumV2.</footnote>
<figure><img src="image_4.png" /><figcaption>FIG. 3.—Left: Detection without subframes; global parameters DEBLEND_MINCONT = 0.02. Right: Detection with modified version of SExtractor, inside the rectangle DEBLEND_MINCONT = 0.0004 (the coordinates X, Y of brighter object split are more accurate).</figcaption></figure>

Source detection and deblending were carried out on the inverse-variance-weighted sum of the *V* and *I* drizzled images. While other techniques for weighting are possible (e.g., Szalay, Connolly, & Szokoly 1999), for most sources this summed image provides the maximum limiting depth. The resulting source position and isophotes are then used for subsequent photometric analysis in each of the individual bands. The combined *V* + *I* image is significantly deeper than any of the individual images, and using it to define the object catalog allows, in practice, the detection of all the objects with “normal” SEDs that are detectable in the individual optical bands.

Source detection with SExtractor follows a standard “connected pixel” algorithm. To identify sources, the detection image is first convolved with a fixed smoothing kernel, for which we use a circular Gaussian with FWHM = 2.5 pixels. Pixels with convolved values higher than a fixed threshold are marked as potential sources. This threshold is in unit of σ<sub>sky</sub>, where σ<sub>sky</sub> as a function of position comes from the background rms map produced by SExtractor. The variation of S/N as a function of position is thus automatically taken into account.

After thresholding, regions consisting of more than a certain number of contiguous pixels are counted as sources. The aim of the present work is to provide a high-quality photometric catalog, sampling the SEDs of the sources

from the UV to the NIR, in order to produce reliable photo-
metric redshifts. Therefore we used a “conservative” detec-
tion strategy: the source detection threshold was set to 1.26
σ<sub>sky</sub> (per pixel in the convolved image) and the minimum
area to 16 drizzled pixels, i.e., 0.025 arcsec<sup>2</sup>. After extensive
tests of SExtractor it became clear that no single set of input
parameters provides the desired detection depth for isolated
objects and faint companions of bright galaxies, and the
required rejection of spurious sources in correspondence
with stellar diffraction spikes. <sup>9</sup> This is a standard problem
with SExtractor and at the moment an ideal solution does
not exist. For example, Casertano et al. (2000) ran SExtrac-
tor two times with different detection thresholds. Objects
from the deeper catalog are selectively removed and substi-
tuted with the corresponding ones from the shallower
catalog. In this way, however, it is not easy to achieve con-
sistent estimates of the fluxes (and in general also of the
colors) with the different thresholds. Moreover, the use of
different detection thresholds can lead to the loss of objects
near bright sources.

In order to obtain the desired detection depth and at the
same time the required rejection of spurious sources with a

<figure><img src="image_5.png" /></figure>

<footnote n="9">⁹ In the reduction seven relatively bright stars were masked using the Weight Watcher package.</footnote>
<figure><img src="image_6.png" /><figcaption>FIG. 4.—Comparison between the object counts in the HDF-S and the HDF-N. The present catalog (HDF-S-2), Casertano et al. (2000) (HDF-S-1), Volonteri et al. (2000) (HDF-S-3), Fernández-Soto et al. (1999) (HDF-N-2), and Williams et al. (1996) (HDF-N-1). All the samples, except the Volonteri et al. (2000), include the planetary camera.</figcaption></figure>

single value of the detection threshold, we introduced a modification of SExtractor that makes the detection strategy more flexible. To solve particular cases of clearly wrong detection (as described above), a table is prepared in which it is possible to define different values of the parameters in different regions of the frame (in the present application the DEBLEND_MINCONT, the minimum contrast parameter for deblending, and the DETECT_MINAREA, the

<figure><img src="image_7.png" /><figcaption>FIG. 5.—Extragalactic background light per half magnitude bin, $i_v = 10^{-0.4(I_{AB}+48.6)}N(I_{AB})$, as a function of *I* magnitude. The open circles are taken from Madau & Pozzetti (2000). The filled circles were obtained in the present work adopting the incompleteness correction estimated for galaxies (see text). The error bars at $I_{AB} \approx 26$ show the lower limit corresponding to the incompleteness correction estimated for objects with a stellar PSF.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIG. 6.—Comparison of the magnitudes and colors estimated in the present work and the results of Casertano et al. (2000). In abscissa the magnitudes, in ordinate the differences between our estimates and those of Casertano et al. (2000).</figcaption></figure>

minimum number of pixels above threshold triggering detection<fnref n="10" />). Outside the specified subframes the program uses the *global* parameters defined in the standard configuration file. In our case the global values are: `DETECT_MINAREA = 16` drizzled pixels and `DEBLEND_MINCONT = 0.02`. Therefore, in a single run we obtained a unique catalog where all objects are detected over the same threshold (`DETECT_THRESHOLD`, the detection threshold). We defined 78 rectangular subframes, 57 with `DEBLEND_MINCONT` ranging from 0 to 1 (maximum deblending and no deblending) and the others with suitable `DETECT_MINAREA` basically to avoid spurious detections. It is also possible to personalize both parameters at the same time in a given subframe and define more subframes one inside the other.

Although such a procedure is in direct contrast to the convenience of a fully automated algorithm able to process a large amount of data, we had to resort to it in order to ensure the highest quality of the result. In Figures 2 and 3 we show two examples of correction through the definition of subframes. The accurate detection is also helpful for the correct estimation of the object coordinates (*X*, *Y*) on the *V*+*I* image. The correct centering of the photometric apertures is in fact a critical issue for the estimation of the colors, when SExtractor is used in *double image mode* (see below).

The final result of the detection procedure is a list of 1611 objects.

The remaining process to obtain the photometry followed the standard SExtractor package.

## 3.2. Magnitudes

### 3.2.1. Optical Catalog

To estimate the total magnitudes [$f(I)_{tot}$] of the objects, we used a procedure described by Djorgovski et al. (1995).

<footnote n="10">¹⁰ For details on the parameters used by SExtractor see the SExtractor user guide at ftp://ftp.iap.fr/pub/from_users/bertin/sextractor/.</footnote>
<figure><img src="image_9.png" /><figcaption>FIG. 7.—Comparison between HDF-S and HDF-N (Fernández-Soto et al. 1999). In the first two panels the V-I color versus I (AB) is shown. The third panel shows the comparison between the median V-I colors in 0.5 mag bins. Beyond $I_{AB} = 28$ the limited depth of the V image (29.5, 2 σ) gives origin to systematic effects on the V-I color.</figcaption></figure>

For brighter objects, where the isophotal area (detected in V+I image) is larger than 0.497 arcsec² (corresponding to an aperture of 20 pixels in diameter), we used the SEExtractor BEST magnitude as defined in the SEExtractor manual. The BEST magnitude is provided by an adaptive aperture method (mag AUTO), except if a neighbor is suspected to bias the measurement by more than 0.1 mag. In the latter

<figure><img src="image_10.png" /><figcaption>FIG. 8.—Color-color plot for the candidate U-band dropouts (above the dashed line) in the HDF-S. The size of the symbols scales with the magnitude in $I_{AB}$. There are two spectroscopic redshift available (from Cristiani et al. 2000).</figcaption></figure>

case, the corrected isophotal magnitude is taken (Bertin & Arnouts 1996).

For objects with an isophotal area smaller than 0.497 arcsec², a magnitude estimated in a circular aperture of 0.8 (corresponding roughly to 5 FWHM) was used, corrected to the total BEST magnitude provided by SEExtractor, assuming that the wings follow a stellar profile. The corrections turned out to be independent of the magnitude and in terms of fluxes correspond to 1.061, 1.012, 1.010, and 1.012 in the U, B, V, and I band, respectively. There are 231 objects (out of 1611) with isophotal area greater than 0.497 arcsec². The flux BEST was computed (by SEExtractor) for 208 objects out of 231 as a flux AUTO and for the remaining 23 as isophotal corrected. These 23 objects are smoothly distributed in the range $22 < I_{AB} < 25$ and the rms difference with respect to the flux AUTO is 0.07 mag.

We selected the I as the fundamental band, and we referred to it for the determination of the object colors. If not otherwise specified, magnitudes are expressed in the AB system, that is, a system based on a spectrum that is flat in $f_v: m = -2.5 \log f_v + 48.59$ (Oke 1974).

In order to apply the photometric redshift codes it is mandatory to obtain a reliable estimation of the colors for each object. The colors were measured using the same fixed aperture (0.8) in the different optical filters.

For the U, B, V bands, the flux is obtained by first estimating the ratio of the flux in each band with respect to the I band within the same fixed aperture and then rescaling it to the total I flux:

$$F(n) = \left[ \frac{f(n)}{f(I)} \right]_{\text{fix.aperture}} \times f(I)_{\text{tot}}, \quad (1)$$

with $n = U, B, V$. The corresponding error is estimated by standard propagation.

We ran SEExtractor in the so-called *double image mode*: the V+I image was used for the detection of the sources and the images in the bands U, B, V, I for the photometric measurements.

### 3.2.2. Infrared Catalog

The infrared frames have different orientation, pixel size, and point-spread function (PSF) with respect to the HST images. In order to determine the color indices between the optical (in particular the I band) and the near infrared bands (Js, H, Ks), we adopted the following strategy:

1. The IR images were rebinned to the same pixel size and orientation as the WFPC2 images.

2. To estimate colors, the I frame was smoothed in turn to the same effective PSF of each of the images Js, H, Ks (seeing ≈ 0.6) using a fast Fourier transform (FFT) approach. In order to obtain the correct transfer function, we extracted the median stellar PSF (obtained from 13 stellar objects) from each IR image and from the I image, and we transformed them in the frequency space through an FFT.

The division of the FFTs of the infrared PSFs (Js, H, Ks) by the FFT of the I-band PSF provided the transformed transfer functions (i.e., the Fourier transform of the kernel). For each band, we inverse-transformed it in order to obtain the shape of the filter to be applied to the I image (in the coordinates domain). The noise due to the presence of the aliasing frequencies in the Fourier space was reduced with a suitable filter. The flux is preserved.
<caption>TABLE 3</caption>

THE OBJECT CATALOG FOR THE HDF-S WFPC2 FIELD

<table><thead><tr><th>ID</th><th>x</th><th>y</th><th>α</th><th>δ</th><th>m<sub>i</sub></th><th>σ(m<sub>i</sub>)</th><th>Flux U</th><th>errU</th><th>Flux B</th><th>errB</th><th>Flux V</th><th>errV</th><th>Flux J</th><th>errJ</th><th>Flux H</th><th>errH</th><th>Flux K</th><th>errK</th><th>s/g</th><th>f<sub>s</sub></th><th>f<sub>we</sub></th></tr></thead><tbody><tr><td>1...</td><td>222.56</td><td>1849.99</td><td>33 05.771</td><td>33 23.32</td><td>27.08</td><td>0.11</td><td>0.0144</td><td>0.0079</td><td>0.0312</td><td>0.0039</td><td>0.0382</td><td>0.0024</td><td>0.3045</td><td>0.2701</td><td>0.1359</td><td>0.4970</td><td>0.2725</td><td>0.4323</td><td>0.01</td><td>0</td><td>0</td></tr><tr><td>2...</td><td>229.87</td><td>1667.82</td><td>33 05.728</td><td>33 30.57</td><td>29.72</td><td>1.29</td><td>0.0009</td><td>0.009</td><td>-0.0032</td><td>0.0041</td><td>0.0071</td><td>0.0024</td><td>0.1733</td><td>0.1826</td><td>-0.0959</td><td>0.3911</td><td>-0.1325</td><td>0.3180</td><td>0.53</td><td>0</td><td>7</td></tr><tr><td>3...</td><td>245.78</td><td>1812.88</td><td>33 05.647</td><td>33 24.79</td><td>27.75</td><td>0.19</td><td>0.0210</td><td>0.0076</td><td>0.0131</td><td>0.0038</td><td>0.0140</td><td>0.0023</td><td>-0.0406</td><td>0.2279</td><td>0.2266</td><td>0.4699</td><td>0.3363</td><td>0.4302</td><td>0.20</td><td>0</td><td>0</td></tr><tr><td>4...</td><td>248.01</td><td>2109.94</td><td>33 05.647</td><td>33 12.95</td><td>28.08</td><td>0.34</td><td>0.0080</td><td>0.0096</td><td>0.0015</td><td>0.0046</td><td>0.0153</td><td>0.0027</td><td>-0.0589</td><td>0.2563</td><td>0.5490</td><td>0.8582</td><td>0.9603</td><td>1.2918</td><td>0.88</td><td>0</td><td>0</td></tr><tr><td>5...</td><td>252.54</td><td>2205.65</td><td>33 05.625</td><td>33 09.13</td><td>26.00</td><td>0.09</td><td>0.0149</td><td>0.0126</td><td>0.0727</td><td>0.0065</td><td>0.0903</td><td>0.0035</td><td>0.2667</td><td>0.1143</td><td>0.4916</td><td>0.2870</td><td>-0.1419</td><td>0.1902</td><td>0.03</td><td>0</td><td>0</td></tr></tbody></table>

NOTES.—Table 3 is presented in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. Units of right ascension are 22°, minutes, and seconds, and units of declination are -60°, arcminutes, and arcseconds. Fluxes and corresponding errors are in units of 10¹⁰ ergs cm⁻² s⁻¹ Hz⁻¹.
At the end we obtained three I-band smoothed images with the “same” PSF as the infrared images, namely: $I_j$, $I_h$, $I_k$.

3. We ran SEExtractor in double image mode, with the detection in the $V+I$ image and the photometry on the three smoothed I band images ($I_j$, $I_h$, $I_k$) and on the three infrared images $Js$, $H$, $Ks$. We then computed the $I_j-Js$, $I_h-H$, and $I_k-Ks$, using a fixed aperture of $2 \times \text{FWHM}$ diameter.

The estimated flux for each IR band is, again,

$$F(n) = \left[ \frac{f(n)}{f(In)} \right]_{\text{fix.aperture}} \times f(I)_{\text{tot}}, \quad (2)$$

where n runs over $Js$, $H$, $Ks$, and $[f(n)/f(In)]_{\text{fix.aperture}}$ is the ratio of the flux in each IR band with respect to the I smoothed image within the same fixed aperture. The $f(I)_{\text{tot}}$ is the total I magnitude obtained as described in § 3.2.1.

SEExtractor was used (in double image mode) to carry out the photometry on all the objects detected in the $V+I$ image. The fraction of isolated objects that are not significantly detected in the IR images can be identified by comparing their fluxes with the corresponding errors.

There are however 317 sources (but only seven with $K_{AB} < 24$), for which a spurious IR flux is generated by contamination from a nearby brighter object. Those cases were identified on the basis of the optical images [using the $f(I)_{tot}/f(In)$ ratio] and flagged as upper limits (see the description of the flag $f_{we}$ below).

### 3.3. Photometric Errors for the Infrared Images

Because neighboring pixels are strongly correlated, the noise in the ISAAC images is not well described by the background rms map produced by SEExtractor, resulting in unrealistic estimates of the photometric uncertainties. In order to determine the correction factor to the errors provided by SEExtractor, we adopted the following strategy.

Synthetic stellar objects with known magnitude were generated with a relatively high signal-to-noise ratio (S/N = 20) and randomly inserted in 35 different relatively isolated regions of the frame. Their fluxes were then measured again with SEExtractor. The dispersion in magnitude provides a reliable estimate of the real error which was used to rescale the formal SEExtractor errors by a constant factor (3.25 in the three bands $Js$, $H$, $Ks$).

## 4. THE CATALOG

The catalog<fnref n="11" /> is presented in Table 3.

For each object in the catalog we report the following parameters:

**ID:** the identification number. The objects in the list are sorted by increasing abscissa (x).

**x, y:** abscissa and ordinate in pixels of the objects in the version 2 WFPC2 images.

**R.A., Decl.:** right ascension and declination coordinates (J2000.0) in the version 2 WFPC2 images.

**$m_i$ and $\sigma(m_i)$:** The magnitude in the I image and its uncertainty. It is the best SEExtractor magnitude if the isophotal area of the object is greater than 0.497 arcsec² (314 pixels); otherwise it is the fixed aperture magnitude (diameter 0.8).

**flux and $\sigma$ (flux):** The aperture fluxes, with their uncertainties (estimated for the IR images according to the procedure described in § 3.2.2).

**$s/g$:** The star-galaxy separation provided by SEExtractor applied to the $V+I$ image to which the best PSF corresponds. This classifier gives output values in the range (0–1) (0 for galaxies and 1 for stars) and represents the degree of membership of an object to one of the two classes.

**$f_s$:** The internal flag provided by SEExtractor (see Bertin & Arnouts 1996). It is a short integer containing (coded in decimal) all the extraction flags as a sum of powers of 2:

1: object has neighbors bright and close enough to significantly bias the photometry, or bad pixel;

2: object was originally blended with another one;

4: at least one pixel of the object is saturated (or very close to saturated);

8: object is truncated (too close to an image boundary);

16: object's aperture data incomplete or corrupted;

32: object's isophotal data incomplete or corrupted;

64: memory overflow occurred during deblending;

128: memory overflow occurred during extraction.

**$f_{we}$:** Flag for infrared fluxes for the possible contamination of the flux estimation from neighboring objects. In these cases the flux measurement is considered as an upper limit. The coding is a sum of powers of 2 (ranging from 0 to 7): for example, 7 corresponds to 111 in binary and means that the infrared fluxes in $Js$, $H$, and $Ks$ are all upper limits, 4 (100) only the $Js$ band is an upper limit, 3 (011) both $H$ and $Ks$ bands are upper limits, 0 (000) all fluxes in the infrared bands have been measured without problems, etc.

# 5. DISCUSSION

## 5.1. Source Counts

Figure 4 shows the behavior of the observed source counts: the result of the present work is consistent with the corresponding estimates in the HDF-S of Casertano et al. (2000) down to $I_{AB} \sim 28$ and with the counts in the HDF-N by Williams et al. (1996). On the other hand, if we compare our source counts with the result in the HDF-N by Fernández-Soto, Lanzetta, & Yahil (1999), beyond $I_{AB} \sim 26$ the HDF-N counts turn out to be systematically lower with respect to the HDF-S by about a factor 1.5, while the source counts in the HDF-S by Volonteri et al. (2000) appear to be higher in this range by about the same factor. These discrepancies are to be ascribed to differences in the approach used to carry out the photometry, in particular the profile-fitting method for Fernández-Soto et al. (1999) and different parameters for the source detection in the case of Volonteri et al. (2000).

In order to compute the incompleteness of the source counts, two different approaches were followed with the aim of obtaining an estimate of the systematic uncertainties. In the first method 46 galaxies in the magnitude interval $24.5 < I_{AB} < 25$ were selected and their extracted $V+I$ images were randomly inserted in the $V+I$ WF image, suitably dimmed by increasing factors in order to cover the magnitude range down to $I_{AB} = 28$. Then the source detection was performed with the same SEExtractor parameters used in the original image. For each 0.5 mag bin 30 simulations were carried out. The resulting incompleteness is listed in column (3) ($c_{gal}$) of Table 4. For the magnitude bins

<footnote n="11">¹¹ The full photometric catalog with a description file is available in fits format at http://www.stecf.org/hstprogrammes/ISAAC/ISAAC.html.</footnote>
<figure><img src="image_11.png" /><figcaption>FIG. 9.—Color-color plot for the candidate B-band dropouts (above the dashed line) in the HDF-S. The size of the symbols scales with the magnitude in $I_{AB}$.</figcaption></figure>

brighter than $I_{AB} = 24.5$, no simulations were carried out and the incompleteness factor corresponds to the masked area due to the brightest objects in the field (see § 3.1).

The same approach was also applied using a reference sample of 46 stellar images in order to estimate a lower limit of the incompleteness. The result is listed in column (4) ($c_{star}$) of Table 4 and is increasingly different from the case of the galaxy simulations beyond $I_{AB} = 26.5$.

The estimated surface density, corrected by the incompleteness factor for galaxies, is listed in column (5) ($N_{c_{gal}}$) and is in very good agreement with the corrected source counts of Volonteri et al. (2000).

Figure 5 shows the extragalactic background light (EBL) per half magnitude bin, $i_v = 10^{-0.4(I_{AB}+48.6)}N(I_{AB})$, as a

<figure><img src="image_12.png" /><figcaption>FIG. 10.—EROs in the HDF-S $[(I-K)_{AB} > 2.7$ and $K_{AB} < 24]$. In the top panel empty symbols indicate the starburst galaxies and stars the ellipticals. The triangle corresponds to a radio-galaxy (see text). In the top panel the error bars are plotted in the upper left corner for an object with $K_{AB} = 24$ and $(I-K)_{AB} = 3.5$. In the bottom panel the classification between elliptical and starburst is carried out with a color-color technique (Pozzetti et al. 2000). Crosses indicate objects morphologically classified as elliptical galaxies. The filled circle in both panels is an object morphologically classified as “late-type.”</figcaption></figure>

function of I magnitude. The agreement with the work of Madau & Pozzetti (2000) is very good down to $I_{AB} \approx 26$. Beyond this limit the Madau & Pozzetti (2000) values are close to our estimates, corrected by the incompleteness factor for stars rather than for galaxies. In any case, the uncertainty of this correction has little effect on the total amount of EBL in I. Adopting the values of Madau & Pozzetti (2000) down to $I_{AB} = 22$ and our counts at fainter magnitudes, the EBL in the I band turns out to be 8.7 and

<table><caption>TABLE 4
COUNTS IN THE F814W BAND MEASURED IN THE INNER AREA OF THE WF CAMERAª</caption><thead><tr><th>I<sub>AB</sub><br>(1)</th><th>n<br>(2)</th><th>c<sub>gal</sub><br>(3)</th><th>c<sub>star</sub><br>(4)</th><th>Nc<sub>gal</sub><br>(0.5 mag<sup>-1</sup> arcmin<sup>-2</sup>)<br>(5)</th><th>Nc<sub>star</sub><br>(0.5 mag<sup>-1</sup> arcmin<sup>-2</sup>)<br>(6)</th><th>n<sub>vol</sub><br>(7)</th><th>c<sub>vol</sub><br>(8)</th><th>Nc<sub>vol</sub><br>(0.5 mag<sup>-1</sup> arcmin<sup>-2</sup>)<br>(9)</th></tr></thead><tbody><tr><td>22.25......</td><td>11</td><td>1.04</td><td>1.04</td><td>2.55</td><td>2.55</td><td>11</td><td>1</td><td>2.51</td></tr><tr><td>22.75......</td><td>17</td><td>1.04</td><td>1.04</td><td>3.94</td><td>3.94</td><td>28</td><td>1</td><td>6.39</td></tr><tr><td>23.25......</td><td>25</td><td>1.04</td><td>1.04</td><td>5.79</td><td>5.79</td><td>26</td><td>1</td><td>5.94</td></tr><tr><td>23.75......</td><td>28</td><td>1.04</td><td>1.04</td><td>6.49</td><td>6.49</td><td>35</td><td>1</td><td>7.99</td></tr><tr><td>24.25......</td><td>35</td><td>1.04</td><td>1.04</td><td>8.11</td><td>8.11</td><td>37</td><td>1</td><td>8.45</td></tr><tr><td>24.75......</td><td>53</td><td>1.04</td><td>1.04</td><td>12.28</td><td>12.28</td><td>65</td><td>1</td><td>14.84</td></tr><tr><td>25.25......</td><td>46</td><td>1.05</td><td>1.05</td><td>10.76</td><td>10.76</td><td>94</td><td>1</td><td>21.46</td></tr><tr><td>25.75......</td><td>79</td><td>1.07</td><td>1.06</td><td>18.83</td><td>18.65</td><td>139</td><td>1</td><td>31.73</td></tr><tr><td>26.25......</td><td>143</td><td>1.11</td><td>1.06</td><td>35.35</td><td>33.76</td><td>173</td><td>1</td><td>39.50</td></tr><tr><td>26.75......</td><td>157</td><td>1.23</td><td>1.07</td><td>43.01</td><td>37.41</td><td>235</td><td>1</td><td>53.65</td></tr><tr><td>27.25......</td><td>205</td><td>1.59</td><td>1.08</td><td>72.59</td><td>49.31</td><td>342</td><td>1.08</td><td>84.33</td></tr><tr><td>27.75......</td><td>222</td><td>2.87</td><td>1.12</td><td>141.90</td><td>55.38</td><td>332</td><td>1.86</td><td>140.99</td></tr></tbody></table>

NOTES.—n and $n_{vol}$ are the observed counts in 0.5 mag bins in the present work and in the catalog by Volonteri et al. (2000), respectively. $c_{gal}$ and $c_{star}$ are the incompleteness correction factors derived from our simulations for galaxies and stars, respectively (see text). $N_{c_{gal}}$ and $N_{c_{star}}$ are the corrected counts derived in the present work adopting the incompleteness correction estimated for galaxies and for stars, respectively. $c_{vol}$ is the incompleteness correction factor, and $N_{c_{vol}}$ the corrected counts per square arcminute as estimated by Volonteri et al. (2000).

ª 4.49 arcmin² in the present work; 4.38 arcmin² in the work of Volonteri et al. 2000).
8.8 nW m⁻² sr⁻¹ using the incompleteness correction for stars and galaxies, respectively. These values are well within the uncertainty of the estimate by Madau & Pozzetti (2000), 8.04<sup>+1.62</sup><sub>-0.92</sub> nW m⁻² sr⁻¹, which arises mostly from field-to-field variations in the number of relatively bright galaxies.

## 5.2. Colors

In Figure 6 we show the comparison between the optical colors of our catalog as a function of the I<sub>AB</sub> magnitude and those obtained by Casertano et al. (2000). The objects were

<figure><img src="image_13.png" /><figcaption>FIG. 11.—EROs in the V + I and Ks images; they are marked with a cross and the identifier [See the electronic edition of the Journal for color versions of Figs. 11–12.]</figcaption></figure>
selected with the magnitude *U*, *B*, *V*, *I* brighter than 27.0, 27.5, 27.5, 27.5, respectively. The main differences (in particular in the *U* band) are due to the different strategy adopted in terms of detection and photometry: we computed the colors using a fixed aperture, while Casertano et al. (2000) computed the isophotal colors.

The comparison between the colors of the HDF-S (our catalog) and HDF-N (Fernández-Soto et al. 1999) is shown in Figure 7. There is a significant difference in the median color *V*−*I* between the HDF-S and the HDF-N around magnitude *I*<sub>AB</sub> ~ 26. This could be due to systematic differences in the distributions of the galaxy redshifts in the two fields, in particular to the presence of large scale structure at *z* ~ 1 (Cohen et al. 2000) and will be discussed in a forthcoming paper. Beyond *I*<sub>AB</sub> = 28 the limited depth of the *V* image (29.5, 2 σ) gives rise to systematic effects in the color *V*−*I*.

Color criteria, which are sensitive to the presence of a Lyman continuum break superposed on an otherwise flat UV spectrum, have been shown to successfully identify high-redshift star-forming galaxies (e.g., Madau et al. 1996). In Figure 8 we show the selection of *U*-band dropouts with *I*<sub>AB</sub> < 27 (according to the criteria defined by Madau et al. 1996). The dashed lines outline the selection region within which candidate 2.5 < *z* < 3.5 objects are identified.

Objects undetected in the *U* band (with S/N < 1) are plotted as triangles at the 1 σ limits to their (*U* − *B*)<sub>AB</sub> colors. We adopted a similar technique to select candidate 3.5 < *z* < 4.5 galaxies with *I*<sub>AB</sub> < 27 (see Fig. 9). The selection area is outlined with dashed lines. The undetected objects in the *B* band (with S/N < 1) are plotted as triangles. Symbol sizes scale with the *I*<sub>AB</sub> magnitude of the object.

Ninety *U*-band dropouts and 17 *B*-band dropouts are identified, equivalent to a surface density of (21 ± 1) and (3.9 ± 0.9) arcmin⁻², respectively. They are listed in Tables 5 and 6.

### 5.3. Extremely Red Objects

The selection of extremely red objects (EROs) is shown in Figure 10 (top). EROs are defined as sources with (*I* − *K*)<sub>AB</sub> > 2.7, corresponding to the color of passively evolving elliptical galaxies at *z* > 1. The selection was carried out down to *K*<sub>AB</sub> < 24. Table 7 lists the identification number and coordinates of 11 EROs in the field (plus three more objects whose upper limit to the *Ks* flux is still compatible with the selection criterion).

Saracco et al. (2001), using the same infrared images as the present work, selected 20 EROs on the basis of a color criterion *Js*−*Ks* > 2.4, corresponding to a surface density

<figure><img src="image_14.png" /><figcaption>FIG. 12.—EROs in the *V* + *I* and *Ks* images marked with a cross and the identifier</figcaption></figure>
<table><caption>TABLE 5
HDF-S U DROPOUTS</caption><thead><tr><th>ID</th><th>I<sub>AB</sub></th><th>(B-I)<sub>AB</sub></th><th>(U-B)<sub>AB</sub></th><th>X</th><th>Y</th><th>R.A.</th><th>Decl.</th></tr></thead><tbody><tr><td>28</td><td>24.20</td><td>0.31</td><td>3.53</td><td>381.90</td><td>1663.84</td><td>22 33 04.90</td><td>-60 33 30.7</td></tr><tr><td>46</td><td>26.26</td><td>0.26</td><td>2.47</td><td>445.65</td><td>1420.50</td><td>22 33 04.55</td><td>-60 33 40.4</td></tr><tr><td>49</td><td>26.44</td><td>-0.01</td><td>2.53</td><td>456.64</td><td>1695.53</td><td>22 33 04.50</td><td>-60 33 29.4</td></tr><tr><td>56</td><td>24.88</td><td>0.83</td><td>2.90</td><td>479.55</td><td>1811.28</td><td>22 33 04.38</td><td>-60 33 24.8</td></tr><tr><td>57</td><td>26.59</td><td>0.42</td><td>1.96</td><td>489.72</td><td>1669.49</td><td>22 33 04.32</td><td>-60 33 30.4</td></tr><tr><td>80</td><td>25.34</td><td>0.73</td><td>2.37</td><td>565.62</td><td>1027.16</td><td>22 33 03.88</td><td>-60 33 56.0</td></tr><tr><td>81</td><td>24.30</td><td>0.56</td><td>2.20</td><td>565.77</td><td>2083.53</td><td>22 33 03.93</td><td>-60 33 13.9</td></tr><tr><td>89</td><td>25.90</td><td>-0.04</td><td>1.54</td><td>596.78</td><td>1023.16</td><td>22 33 03.71</td><td>-60 33 56.1</td></tr><tr><td>109</td><td>25.09</td><td>0.28</td><td>3.15</td><td>669.68</td><td>571.08</td><td>22 33 03.30</td><td>-60 34 14.1</td></tr><tr><td>115</td><td>23.25</td><td>0.68</td><td>2.16</td><td>695.55</td><td>1685.70</td><td>22 33 03.21</td><td>-60 33 29.7</td></tr><tr><td>124</td><td>25.41</td><td>0.58</td><td>2.54</td><td>727.14</td><td>1083.73</td><td>22 33 03.01</td><td>-60 33 53.7</td></tr><tr><td>127</td><td>25.46</td><td>1.10</td><td>2.46</td><td>736.19</td><td>965.96</td><td>22 33 02.96</td><td>-60 33 58.4</td></tr><tr><td>219</td><td>24.74</td><td>0.47</td><td>1.83</td><td>1137.10</td><td>1426.53</td><td>22 33 00.81</td><td>-60 33 39.9</td></tr><tr><td>225</td><td>25.98</td><td>0.76</td><td>2.25</td><td>1152.38</td><td>872.99</td><td>22 33 00.70</td><td>-60 34 02.0</td></tr><tr><td>307</td><td>25.69</td><td>0.21</td><td>1.82</td><td>1402.15</td><td>1216.71</td><td>22 32 59.37</td><td>-60 33 48.2</td></tr><tr><td>329</td><td>26.16</td><td>-0.16</td><td>2.23</td><td>1457.20</td><td>1077.04</td><td>22 32 59.06</td><td>-60 33 53.7</td></tr><tr><td>382</td><td>25.98</td><td>0.54</td><td>2.50</td><td>1567.01</td><td>1102.02</td><td>22 32 58.47</td><td>-60 33 52.7</td></tr><tr><td>385</td><td>25.54</td><td>0.49</td><td>3.00</td><td>1570.16</td><td>1127.06</td><td>22 32 58.46</td><td>-60 33 51.7</td></tr><tr><td>396</td><td>24.74</td><td>0.74</td><td>2.86</td><td>1604.91</td><td>1695.32</td><td>22 32 58.29</td><td>-60 33 29.0</td></tr><tr><td>536</td><td>26.08</td><td>0.10</td><td>1.90</td><td>1986.10</td><td>907.38</td><td>22 32 56.20</td><td>-60 34 00.3</td></tr><tr><td>540</td><td>26.05</td><td>-0.02</td><td>1.66</td><td>1989.17</td><td>620.40</td><td>22 32 56.17</td><td>-60 34 11.7</td></tr><tr><td>583</td><td>24.87</td><td>0.60</td><td>3.29</td><td>2072.45</td><td>3106.80</td><td>22 32 55.83</td><td>-60 32 32.6</td></tr><tr><td>600</td><td>26.72</td><td>0.33</td><td>1.88</td><td>2124.12</td><td>4053.40</td><td>22 32 55.60</td><td>-60 31 54.8</td></tr><tr><td>638</td><td>26.09</td><td>0.46</td><td>1.88</td><td>2213.60</td><td>1214.37</td><td>22 32 54.98</td><td>-60 33 48.0</td></tr><tr><td>646</td><td>26.38</td><td>0.36</td><td>2.15</td><td>2229.45</td><td>1710.16</td><td>22 32 54.92</td><td>-60 33 28.2</td></tr><tr><td>649</td><td>25.70</td><td>-0.10</td><td>3.28</td><td>2234.97</td><td>1359.88</td><td>22 32 54.87</td><td>-60 33 42.2</td></tr><tr><td>663</td><td>26.74</td><td>0.05</td><td>1.55</td><td>2268.45</td><td>1293.80</td><td>22 32 54.69</td><td>-60 33 44.8</td></tr><tr><td>676</td><td>26.11</td><td>0.24</td><td>1.93</td><td>2290.75</td><td>2716.97</td><td>22 32 54.63</td><td>-60 32 48.1</td></tr><tr><td>685</td><td>25.80</td><td>0.87</td><td>2.32</td><td>2308.56</td><td>2887.93</td><td>22 32 54.54</td><td>-60 32 41.2</td></tr><tr><td>693</td><td>26.34</td><td>-0.06</td><td>1.42</td><td>2322.93</td><td>699.27</td><td>22 32 54.37</td><td>-60 34 08.5</td></tr><tr><td>709</td><td>26.82</td><td>0.23</td><td>1.63</td><td>2341.94</td><td>2472.97</td><td>22 32 54.35</td><td>-60 32 57.8</td></tr><tr><td>719</td><td>25.56</td><td>0.08</td><td>2.01</td><td>2354.33</td><td>2476.44</td><td>22 32 54.28</td><td>-60 32 57.6</td></tr><tr><td>723</td><td>24.56</td><td>0.56</td><td>2.25</td><td>2359.55</td><td>682.90</td><td>22 32 54.16</td><td>-60 34 09.1</td></tr><tr><td>738</td><td>25.30</td><td>0.72</td><td>2.63</td><td>2387.22</td><td>3168.21</td><td>22 32 54.13</td><td>-60 32 30.1</td></tr><tr><td>757</td><td>26.57</td><td>0.39</td><td>1.93</td><td>2415.28</td><td>904.51</td><td>22 32 53.88</td><td>-60 34 00.3</td></tr><tr><td>776</td><td>25.85</td><td>0.46</td><td>2.67</td><td>2447.00</td><td>4070.66</td><td>22 32 53.85</td><td>-60 31 54.0</td></tr><tr><td>778</td><td>26.22</td><td>0.10</td><td>1.84</td><td>2450.30</td><td>967.91</td><td>22 32 53.69</td><td>-60 33 57.7</td></tr><tr><td>784</td><td>26.31</td><td>0.71</td><td>1.98</td><td>2455.43</td><td>3453.57</td><td>22 32 53.78</td><td>-60 32 18.7</td></tr><tr><td>802</td><td>24.54</td><td>1.05</td><td>2.77</td><td>2507.67</td><td>2110.67</td><td>22 32 53.43</td><td>-60 33 12.2</td></tr><tr><td>807</td><td>24.67</td><td>0.94</td><td>2.32</td><td>2512.53</td><td>3856.67</td><td>22 32 53.49</td><td>-60 32 02.5</td></tr><tr><td>829</td><td>25.03</td><td>0.79</td><td>2.44</td><td>2544.12</td><td>1906.26</td><td>22 32 53.23</td><td>-60 33 20.3</td></tr><tr><td>847</td><td>26.60</td><td>0.28</td><td>1.59</td><td>2571.80</td><td>4045.39</td><td>22 32 53.18</td><td>-60 31 55.0</td></tr><tr><td>855</td><td>26.41</td><td>0.03</td><td>2.47</td><td>2582.79</td><td>4120.85</td><td>22 32 53.12</td><td>-60 31 52.0</td></tr><tr><td>868</td><td>26.94</td><td>-0.01</td><td>1.96</td><td>2606.04</td><td>1104.37</td><td>22 32 52.85</td><td>-60 33 52.2</td></tr><tr><td>870</td><td>26.33</td><td>0.15</td><td>1.82</td><td>2606.41</td><td>1228.31</td><td>22 32 52.86</td><td>-60 33 47.3</td></tr><tr><td>871</td><td>24.89</td><td>0.74</td><td>3.19</td><td>2606.45</td><td>1869.42</td><td>22 32 52.89</td><td>-60 33 21.7</td></tr><tr><td>913</td><td>26.92</td><td>0.13</td><td>1.92</td><td>2676.51</td><td>2941.20</td><td>22 32 52.56</td><td>-60 32 39.0</td></tr><tr><td>914</td><td>25.62</td><td>0.56</td><td>2.58</td><td>2681.97</td><td>2405.91</td><td>22 32 52.51</td><td>-60 33 00.3</td></tr><tr><td>949</td><td>25.26</td><td>0.49</td><td>2.15</td><td>2740.41</td><td>2984.83</td><td>22 32 52.21</td><td>-60 32 37.2</td></tr><tr><td>957</td><td>24.66</td><td>1.05</td><td>2.96</td><td>2759.70</td><td>1344.73</td><td>22 32 52.03</td><td>-60 33 42.6</td></tr><tr><td>960</td><td>26.62</td><td>-0.02</td><td>2.00</td><td>2765.76</td><td>2729.73</td><td>22 32 52.07</td><td>-60 32 47.4</td></tr><tr><td>968</td><td>26.68</td><td>0.23</td><td>1.96</td><td>2778.16</td><td>4205.43</td><td>22 32 52.08</td><td>-60 31 48.6</td></tr><tr><td>991</td><td>26.69</td><td>0.54</td><td>1.76</td><td>2808.66</td><td>3222.52</td><td>22 32 51.86</td><td>-60 32 27.7</td></tr><tr><td>992</td><td>24.48</td><td>1.12</td><td>2.76</td><td>2808.81</td><td>3768.18</td><td>22 32 51.89</td><td>-60 32 06.0</td></tr><tr><td>1007</td><td>26.54</td><td>0.53</td><td>1.94</td><td>2834.69</td><td>3331.53</td><td>22 32 51.72</td><td>-60 32 23.4</td></tr><tr><td>1025</td><td>25.06</td><td>0.42</td><td>2.93</td><td>2874.27</td><td>2037.31</td><td>22 32 51.45</td><td>-60 33 14.9</td></tr><tr><td>1068</td><td>26.94</td><td>0.23</td><td>1.50</td><td>2938.20</td><td>3806.90</td><td>22 32 51.19</td><td>-60 32 04.4</td></tr><tr><td>1081</td><td>26.99</td><td>0.38</td><td>1.64</td><td>2964.56</td><td>3705.72</td><td>22 32 51.04</td><td>-60 32 08.4</td></tr><tr><td>1100</td><td>23.32</td><td>0.75</td><td>1.97</td><td>3012.83</td><td>1694.70</td><td>22 32 50.68</td><td>-60 33 28.6</td></tr><tr><td>1123</td><td>26.43</td><td>0.19</td><td>1.87</td><td>3052.65</td><td>3719.02</td><td>22 32 50.57</td><td>-60 32 07.9</td></tr><tr><td>1149</td><td>25.91</td><td>0.50</td><td>2.50</td><td>3091.54</td><td>2122.05</td><td>22 32 50.28</td><td>-60 33 11.5</td></tr><tr><td>1167</td><td>25.93</td><td>0.30</td><td>2.66</td><td>3121.38</td><td>1434.79</td><td>22 32 50.08</td><td>-60 33 38.9</td></tr><tr><td>1174</td><td>24.14</td><td>0.81</td><td>2.85</td><td>3125.97</td><td>985.42</td><td>22 32 50.04</td><td>-60 33 56.8</td></tr><tr><td>1179</td><td>25.84</td><td>0.27</td><td>1.85</td><td>3137.28</td><td>3750.66</td><td>22 32 50.11</td><td>-60 32 06.6</td></tr><tr><td>1196</td><td>26.90</td><td>0.19</td><td>1.84</td><td>3175.93</td><td>1894.17</td><td>22 32 49.81</td><td>-60 33 20.5</td></tr></tbody></table>
<table><caption>TABLE 5—Continued</caption><thead><tr><th>ID</th><th>I<sub>AB</sub></th><th>(B-I)<sub>AB</sub></th><th>(U-B)<sub>AB</sub></th><th>X</th><th>Y</th><th>R.A.</th><th>Decl.</th></tr></thead><tbody><tr><td>1204......</td><td>26.42</td><td>-0.09</td><td>1.96</td><td>3197.90</td><td>1817.71</td><td>22 32 49.69</td><td>-60 33 23.6</td></tr><tr><td>1212......</td><td>25.04</td><td>0.12</td><td>1.93</td><td>3211.36</td><td>2333.12</td><td>22 32 49.64</td><td>-60 33 03.0</td></tr><tr><td>1225......</td><td>25.54</td><td>0.67</td><td>2.66</td><td>3234.36</td><td>826.16</td><td>22 32 49.45</td><td>-60 34 03.1</td></tr><tr><td>1232......</td><td>25.69</td><td>0.37</td><td>2.33</td><td>3256.86</td><td>3861.56</td><td>22 32 49.47</td><td>-60 32 02.1</td></tr><tr><td>1242......</td><td>22.92</td><td>0.66</td><td>3.57</td><td>3265.10</td><td>3254.94</td><td>22 32 49.40</td><td>-60 32 26.3</td></tr><tr><td>1249......</td><td>26.24</td><td>0.27</td><td>2.43</td><td>3271.88</td><td>1251.97</td><td>22 32 49.26</td><td>-60 33 46.1</td></tr><tr><td>1261......</td><td>25.90</td><td>0.59</td><td>2.45</td><td>3288.07</td><td>2844.31</td><td>22 32 49.26</td><td>-60 32 42.6</td></tr><tr><td>1277......</td><td>23.28</td><td>0.69</td><td>3.76</td><td>3305.11</td><td>3261.82</td><td>22 32 49.18</td><td>-60 32 26.0</td></tr><tr><td>1281......</td><td>26.86</td><td>0.00</td><td>1.48</td><td>3316.19</td><td>2780.19</td><td>22 32 49.09</td><td>-60 32 45.2</td></tr><tr><td>1303......</td><td>25.13</td><td>0.43</td><td>2.72</td><td>3350.99</td><td>3022.72</td><td>22 32 48.92</td><td>-60 32 35.5</td></tr><tr><td>1322......</td><td>25.52</td><td>0.16</td><td>1.62</td><td>3401.43</td><td>2983.13</td><td>22 32 48.65</td><td>-60 32 37.0</td></tr><tr><td>1372......</td><td>25.79</td><td>0.92</td><td>2.26</td><td>3459.33</td><td>3083.83</td><td>22 32 48.34</td><td>-60 32 33.0</td></tr><tr><td>1374......</td><td>26.44</td><td>0.45</td><td>1.73</td><td>3469.88</td><td>3413.81</td><td>22 32 48.30</td><td>-60 32 19.9</td></tr><tr><td>1397......</td><td>24.86</td><td>0.83</td><td>2.32</td><td>3508.06</td><td>2171.68</td><td>22 32 48.03</td><td>-60 33 09.4</td></tr><tr><td>1411......</td><td>26.56</td><td>0.28</td><td>2.07</td><td>3529.57</td><td>1963.53</td><td>22 32 47.90</td><td>-60 33 17.6</td></tr><tr><td>1414......</td><td>26.69</td><td>0.21</td><td>1.64</td><td>3533.03</td><td>3549.59</td><td>22 32 47.97</td><td>-60 32 14.4</td></tr><tr><td>1432......</td><td>24.22</td><td>0.78</td><td>2.49</td><td>3562.83</td><td>3420.07</td><td>22 32 47.80</td><td>-60 32 19.6</td></tr><tr><td>1452......</td><td>25.94</td><td>0.65</td><td>2.32</td><td>3589.04</td><td>3993.43</td><td>22 32 47.68</td><td>-60 31 56.7</td></tr><tr><td>1457......</td><td>26.02</td><td>0.26</td><td>2.62</td><td>3599.39</td><td>1368.64</td><td>22 32 47.50</td><td>-60 33 41.3</td></tr><tr><td>1466......</td><td>25.86</td><td>0.36</td><td>2.53</td><td>3618.27</td><td>2690.07</td><td>22 32 47.46</td><td>-60 32 48.6</td></tr><tr><td>1523......</td><td>25.51</td><td>0.33</td><td>2.43</td><td>3721.70</td><td>2488.36</td><td>22 32 46.89</td><td>-60 32 56.7</td></tr><tr><td>1524......</td><td>26.09</td><td>0.08</td><td>1.89</td><td>3725.12</td><td>2092.66</td><td>22 32 46.85</td><td>-60 33 12.4</td></tr><tr><td>1532......</td><td>26.38</td><td>0.08</td><td>2.37</td><td>3740.41</td><td>2895.03</td><td>22 32 46.81</td><td>-60 32 40.4</td></tr><tr><td>1572......</td><td>26.92</td><td>0.29</td><td>1.56</td><td>3830.48</td><td>1300.51</td><td>22 32 46.25</td><td>-60 33 44.0</td></tr><tr><td>1588......</td><td>24.95</td><td>0.41</td><td>1.98</td><td>3883.97</td><td>1288.26</td><td>22 32 45.95</td><td>-60 33 44.4</td></tr></tbody></table>

NOTES.—List of U-band dropout candidates. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.

of 2.7 arcmin⁻². Ten of these EROs lie in the area covered by our multicolor catalog and six of them were selected with the above-described (I-K)ₐB > 2.7 threshold. In the catalog of Saracco et al. (2001) there is only one object detected with a S/N > 5 in the Ks band that is not present in our catalog.

This object is clearly seen in the Ks image, while no optical counterpart is present in the I band. It is marked in Figure 10 with an upward-pointing arrow as a 5 σ lower limit at IₐB ~ 28 and listed in Table 7 with the identification

9999. Adding this object to our sample the EROs surface density turns out to be (3.2 ± 0.9) arcmin⁻² (2.5 ± 0.8) arcmin⁻² without the three upper limits in the Ks band.

The EROs surface density measured in the present work is similar to the result obtained by Saracco et al. (2001) in the same HDF-S field (but over a larger area). On the other hand, the surface density observed by Saracco et al. (2001) in the Chandra Deep Field (CDF, Giacconi et al. 2001) is significantly different (1.2 ± 0.5 arcmin⁻²). The discrepancy between the two fields is probably due to the clustering

<table><caption>TABLE 6
HDF-S B DROPOUTS</caption><thead><tr><th>ID</th><th>I<sub>AB</sub></th><th>(V-I)<sub>AB</sub></th><th>(B-V)<sub>AB</sub></th><th>X</th><th>Y</th><th>R.A.</th><th>Decl.</th></tr></thead><tbody><tr><td>36</td><td>26.79</td><td>0.20</td><td>1.71</td><td>397.39</td><td>1030.43</td><td>22 33 04.79</td><td>-60 33 55.9</td></tr><tr><td>209</td><td>26.38</td><td>0.84</td><td>2.25</td><td>1119.71</td><td>815.73</td><td>22 33 00.88</td><td>-60 34 04.2</td></tr><tr><td>320</td><td>26.65</td><td>0.53</td><td>1.67</td><td>1425.84</td><td>1247.15</td><td>22 32 59.24</td><td>-60 33 46.9</td></tr><tr><td>344</td><td>26.47</td><td>0.19</td><td>1.84</td><td>1479.11</td><td>917.34</td><td>22 32 58.94</td><td>-60 34 00.1</td></tr><tr><td>369</td><td>26.08</td><td>0.74</td><td>2.59</td><td>1539.58</td><td>692.30</td><td>22 32 58.60</td><td>-60 34 09.0</td></tr><tr><td>442</td><td>26.31</td><td>0.20</td><td>1.82</td><td>1745.15</td><td>1205.75</td><td>22 32 57.51</td><td>-60 33 48.5</td></tr><tr><td>627</td><td>26.97</td><td>0.23</td><td>1.68</td><td>2196.19</td><td>2326.58</td><td>22 32 55.13</td><td>-60 33 03.6</td></tr><tr><td>721</td><td>26.89</td><td>0.66</td><td>2.36</td><td>2358.17</td><td>2817.65</td><td>22 32 54.27</td><td>-60 32 44.0</td></tr><tr><td>892</td><td>25.77</td><td>0.82</td><td>3.12</td><td>2653.70</td><td>2543.59</td><td>22 32 52.67</td><td>-60 32 54.8</td></tr><tr><td>897</td><td>27.00</td><td>0.55</td><td>2.39</td><td>2659.16</td><td>3657.16</td><td>22 32 52.69</td><td>-60 32 10.5</td></tr><tr><td>1004</td><td>26.22</td><td>0.73</td><td>2.29</td><td>2831.33</td><td>2109.59</td><td>22 32 51.69</td><td>-60 33 12.1</td></tr><tr><td>1144</td><td>26.43</td><td>0.93</td><td>2.50</td><td>3084.84</td><td>2152.57</td><td>22 32 50.32</td><td>-60 33 10.3</td></tr><tr><td>1151</td><td>26.79</td><td>0.60</td><td>2.43</td><td>3093.98</td><td>2959.69</td><td>22 32 50.30</td><td>-60 32 38.1</td></tr><tr><td>1155</td><td>26.23</td><td>0.14</td><td>1.69</td><td>3099.80</td><td>2050.02</td><td>22 32 50.23</td><td>-60 33 14.4</td></tr><tr><td>1400</td><td>26.53</td><td>0.77</td><td>2.26</td><td>3516.67</td><td>3877.74</td><td>22 32 48.07</td><td>-60 32 01.4</td></tr><tr><td>1461</td><td>26.08</td><td>0.46</td><td>2.03</td><td>3606.20</td><td>3451.91</td><td>22 32 47.56</td><td>-60 32 18.3</td></tr><tr><td>1494</td><td>26.38</td><td>0.76</td><td>2.72</td><td>3670.06</td><td>3641.84</td><td>22 32 47.23</td><td>-60 32 10.7</td></tr></tbody></table>

NOTES.—List of B-band dropout candidates. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.
<caption>TABLE 7</caption>

EROs

<table><thead><tr><th>ID<br>(1)</th><th>R.A.<br>(2)</th><th>Decl.<br>(3)</th><th>K<sub>AB</sub><br>(4)</th><th>I<sub>AB</sub><br>(5)</th><th>(I-K)<sub>AB</sub><br>(6)</th><th>Morphology<br>(7)</th></tr></thead><tbody><tr><td>4</td><td>22 33 05.65</td><td>-60 33 12.90</td><td>23.63</td><td>28.08</td><td>4.44*</td><td>S</td></tr><tr><td>26</td><td>22 33 04.96</td><td>-60 33 22.20</td><td>22.89</td><td>26.15</td><td>3.26</td><td>S</td></tr><tr><td>373</td><td>22 32 58.60</td><td>-60 33 46.58</td><td>22.31</td><td>25.75</td><td>3.45</td><td>E (radio gal.)</td></tr><tr><td>822</td><td>22 32 53.39</td><td>-60 31 54.33</td><td>23.39</td><td>28.11</td><td>4.72</td><td>S</td></tr><tr><td>828</td><td>22 32 53.34</td><td>-60 31 46.58</td><td>23.09</td><td>26.56</td><td>3.47*</td><td>S</td></tr><tr><td>850</td><td>22 32 53.01</td><td>-60 33 57.03</td><td>23.67</td><td>27.58</td><td>3.90</td><td>S</td></tr><tr><td>879</td><td>22 32 52.91</td><td>-60 32 15.72</td><td>23.71</td><td>26.81</td><td>3.10</td><td>S</td></tr><tr><td>1011</td><td>22 32 51.54</td><td>-60 33 58.40</td><td>23.35</td><td>26.21</td><td>2.87</td><td>S</td></tr><tr><td>1130</td><td>22 32 50.49</td><td>-60 32 22.64</td><td>23.23</td><td>26.89</td><td>3.66</td><td>S</td></tr><tr><td>1235</td><td>22 32 49.46</td><td>-60 31 57.88</td><td>23.45</td><td>26.21</td><td>2.76</td><td>E</td></tr><tr><td>1239</td><td>22 32 49.46</td><td>-60 31 46.58</td><td>22.70</td><td>26.25</td><td>3.55</td><td>E</td></tr><tr><td>1269</td><td>22 32 49.25</td><td>-60 32 11.62</td><td>22.93</td><td>27.06</td><td>4.13</td><td>S</td></tr><tr><td>1392</td><td>22 32 48.15</td><td>-60 32 17.47</td><td>23.52</td><td>26.35</td><td>2.83*</td><td>L</td></tr><tr><td>1534</td><td>22 32 46.79</td><td>-60 32 33.74</td><td>23.05</td><td>26.92</td><td>3.87</td><td>S</td></tr><tr><td>9999</td><td>22 32 55.92</td><td>-60 32 50.28</td><td>23.95</td><td>...</td><td>>4</td><td>S</td></tr></tbody></table>

NOTES.—A list of EROs in the HDF-S with $(I-K)_{AB} > 2.7$. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. The colors marked with an asterisk (*) are upper limits in the infrared bands. The morphological classification is listed in col. (7) as “E”: early type, “L”: late type, “S”: too faint for a reliable classification. The one object detected with a S/N > 5 in the Ks band by Saracco et al. (2001) that is not present in our multicolor catalog is listed with the ID 9999.

properties of EROs associated to the relatively small field of CDF and HDF-S. Daddi et al. (2000) detect a strong clustering signal of EROs, selected on the basis of the R-K color from a $K_{\text{Vega}} \sim 19$ limited sample in a wide area of $\sim 25$ arcmin$^2$. We select EROs on a sample 3 mag fainter and with different color criteria. Thus, the results of Daddi et al. (2000) cannot be directly applied to our data. Nevertheless, the distribution of EROs in the HDF-S field is not uniform: there are 10 EROs (two of them are upper limits in the Ks band) inside the upper WFPC2 chip.

In the EROs sample there is an interesting radio source identified and discussed by Norris et al. (1999) which has an $(I-K)_{AB} = 3.45$. This object is unusual in that its radio-optical ratio is about 1000 times higher than that of any galaxy known in the local universe.

We adopted the photometric criterion of Pozzetti & Mannucci (2000) to distinguish between the two EROs populations of ellipticals and starbursts. In doing so we assume that our EROs sample belongs to the redshift range of applicability of the criterion, $1 < z < 2$. While an old stellar population is characterized by sharp breaks (especially around 4000 Å), young dusty galaxies show shallower SEDs in the rest-frame optical part of the spectrum because dust extinction does not produce sharp breaks. It is possible to measure the steepness of the SED for objects with $1 < z < 2$ using the observed J-K color. In the bottom panel of Figure 10 the I-K versus J-K plane for the ERO sample is shown. The dashed line separating the elliptical from the starburst galaxies corresponds to $J_{AB} - K_{AB} = 0.36(I_{AB} - K_{AB}) + 0.045$.

The morphological selection of early-type galaxies is usually carried out by fitting a generalized de Vaucouleurs law ($\mu_r - \mu_0 \propto (r/r_e)^{1/n}$ to the luminosity profile of the objects, where $\mu_r$ is the surface brightness at the radius $r$, $r_e$ is the half light radius, and $n$ is the Sersic index (Sersic 1968). For elliptical galaxies $n > 2$ (see, for example, Rodighiero, Franceschini, & Fasano 2001). In the present case, the EROs have typically $I_{AB} > 26$ and except for the few bright-

er objects it is impossible to compute a reliable fit, even on the $V+I$ image. For the fainter EROs a rough morphological classification was carried out comparing the central intensity versus the area of the images. A visual inspection of all the EROs was also carried out to check the results. In Table 7 the EROs are listed, according to their morphology, as early type (“E,” three objects), late type (“L,” one object), and objects too faint for a reliable classification (“S,” 11 objects). In the bottom panel of Figure 10, the three ellipticals are marked with a cross inside the symbol, and the late-type galaxy is shown with a filled symbol. The other points belong to the S category. The classification obtained with the morphological method agrees with the photometric classification for the three ellipticals. The object morphologically classified as “late-type” corresponds to an IR upper limit, and it is not clear whether it can be considered an ERO.

In the top panel of Figure 10, the elliptical galaxies (according to the photometric classification) are marked with a star. About half of the EROs in the sample seem to belong to the “starburst” category and, in general, show an I-K color redder than the corresponding I-K of the “ellipticals.”

In Figures 11 and 12 the $V+I$ and Ks images of the ERO sample are shown.

It is worth noting that the above-mentioned radio galaxy (marked with a triangle in Fig. 10) belongs to the “early-type” class according to the color-color separation technique. Rodighiero et al. (2001) also put it in a sample of “early-type” objects, estimating for this object a Sersic index $n \sim 2.2$ (G. Fasano 2001, private communication), i.e., at the border of the early- versus late-type discrimination.

We are greatly indebted to F. Poli for valuable suggestions about the FFT method to measure colors from images of different PSF and to Pamela Bristow for her careful reading of the manuscript. This work was partially sup-
ported by the ASI grants under contracts ARS-98-226 and ARS-96-176, by the research contract of the University of Padua, The High redshift Universe: From HST and VLT to NGST, and by the Research Training Network, The Physics of the Intergalactic Medium, set up by the Euro-

pean Community under the contract HPRN-CT2000-00126 RG29185. E. V. acknowledges the European Southern Observatory, the Space Telescope European Coordinating Facility, and the Merate Observatory for their hospitality during the visits when this work was completed.

REFERENCES

Bertin, E., & Arnouts, S. 1996, A&AS, 117, 393

Casertano, S., et al. 2000, AJ, 120, 2747

Cohen, J. G., Hogg, D. W., Blandford, R., Cowie, L. L., Hu, E., Songaila, A., Shopbell, P., Richberg, K. 2000, ApJ, 538, 29

Cristiani, S., et al. 2000, A&A, 359, 489

Cuby, J. G., Lidman, C., & Moutou, C. 2001, ISAAC User Manual

Daddi, E., Pozzetti, L., Hoekstra, H., Röttgering, H. J. A., Renzini, A., Zamorani, G., & Mannucci, F. 2000, A&A, 361, 535

Djorgovski, S., et al. 1995, ApJ, 438, L13

Fernández-Soto, A., Lanzetta, K. M., & Yahil, A. 1999, ApJ, 513, 34

Fruchter, A., et al. 2001, in preparation

Gardner, J., et al. 2000, AJ, 119, 486

Giacconi R., et al. 2001, ApJ, 551, 624

Madau, P., Ferguson, H. C., Dickinson, M., Giavalisco, M., Steidel, C. C., & Fruchter, A. S. 1996, MNRAS, 283, 1388

Madau, P., & Pozzetti, L. 2000, MNRAS, 312, L9

Moorwood, A. F. M., et al. 1999, Messenger, 95, 1

Norris, R. P., et al. 1999 (astro-ph/9910437)

Oke, J. B. 1974, ApJS, 27, 21

Persson, E., Murphy, D. C., Krzeminski, W., Roth, M., & Rieke, M. J. 1998, AJ, 116, 2475

Pozzetti, L., & Mannucci, F. 2000, MNRAS, 317, L17

Rodighiero, G., Franceschini, A., & Fasano, G. 2001, MNRAS, 324, 491

Saracco, P., Giallongo, E., Cristiani, S., D'Odorico, S., Fontana, A., Iovino, A., Poli, F., Vanzella, E. 2001 (astro-ph/0104284)

Savaglio, S., et al. 1999, ApJ, 515, L5

Sersic J. L. 1968, Atlas de galaxias australes (Cordoba: Obs. Astron.)

Szalay, A. S., Connolly, A. J., & Szokoly, G. P. 1999, AJ, 117, 68

Volonteri, M., Saracco, P., Chincarini, G. & Bolzonella, M. 2000, A&AS, 362, 487

Williams, R. E., et al. 1996, AJ, 112, 1335

———. 2000, AJ, 120, 2735