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Chandra Deep Field South: The 1Msec Catalog

In this Paper we present the source catalog obtained from a 942 ks exposure of the Chandra Deep Field South (CDFS), using ACIS-I on the Chandra X-ray Observatory. Catalog generation proceeded simultaneously using two different methods; a method of our own design using a modified version of the SExtractor algorithm, and a wavelet transform technique developed specifically for Chandra observations. The detection threshold has been set in order to have less than 10 spurious sources, as assessed by extensive simulations. We subdivided the catalog into four sections. The primary list consists of objects common to the two detection methods. Two secondary lists contain sources which were detected by: 1) the SExtractor algorithm alone and 2) the wavelet technique alone. The fourth list consists of possible diffuse or extended sources. The flux limits at the aimpoint for the soft (0.5--2 keV) and hard (2--10 keV) bands are 5.5E-17 erg/s/cm^2 and 4.5E-16 erg/s/cm^2 respectively. The total number of sources is 346; out of them, 307 were detected in the 0.5--2 keV band, and 251 in the 2--10 keV band. We also present optical identifications for the catalogued sources. Our primary optical data is R band imaging to a depth of R~26.5 (Vega). We found that the R-band/Chandra offsets are small, ~1 arcsec. Coordinate cross-correlation finds 85% of the Chandra sources covered in R to have counterparts within the 3-sigma error box (>~1.5 arcsec depending on off-axis angle and signal-to-noise). The unidentified fraction of sources, approximately ~10--15 %, is close to the limit expected from the observed X-ray flux to R-band ratio distribution for the identified sample.


Introduction
The X-ray background (hereafter XRB) was first identified by Giacconi et al. (1962) and its resolution into discrete sources has since been a major goal of X-ray astronomy. Deep surveys by the major X-ray facilities UHURU (Matilsky et al. 1973), HEAO-1 A2 (Piccinotti et al. 1982), Einstein (Giacconi et al. 1979), ROSAT (Hasinger et al. 1998), ASCA (Ueda et al. 1999) and BeppoSAX (Giommi et al. 2000) have resolved an increasing fraction of the XRB in the 0.5-2 keV and in the 2-7 keV band and produced catalogs of sources which were then used for a wide variety of other astrophysical investigations. These surveys were primarily limited by effective area and source confusion at the faintest achievable fluxes. Unlike these previous missions, Chandra (Weisskopf et al. 2000) provides arcsecond resolution over a majority of the detector, in particular the Advanced CCD Imaging Spectrometer (ACIS-I, Garmire et al. 1992;Bautz et al. 1998). Superior spatial resolution coupled with a substantial gain in light-gathering power over ROSAT has allowed our 942 ks exposure (hereafter 1Msec) of the CDFS (and the analogous exposure of the Hubble Deep Field North) to become the deepest X-ray exposure(s) ever taken, improving by factors of ∼ 20 and ∼ 200 the deepest ROSAT and ASCA surveys, respectively.
Along with the Chandra Deep Field North (CDFN) (Hornschemeier et al. 2000(Hornschemeier et al. , 2001Brandt et al. 2001a,b), the CDFS provides a unique dataset in which to investigate both statistical and source-by-source properties of active galaxies over a large range of redshift and parameter space. Already, the CDFS has discovered both single interesting objects (Norman et al. 2001) and statistical correlations (Giacconi et al. 2001;Tozzi et al. 2001, hereafter Paper I and II).
Here we present the catalog of the sources in the CDFS field found in the 1Msec exposure along with fluxes (or upper limits) in R, for their presumed optical counterparts. The first scientific results derived from the 1Msec observation are presented in Rosati et al. 2001 (Paper III).
The Paper is structured as follows: in Section 2 we discuss the data, in Section 3 we discuss extraction for point and extended sources and their photometry; in Section 4 the optical identifications are presented. We conclude with a discussion of this dataset. The common catalog is given in Table 2. In Tables 3, 4 and 5 are listed sources detected only by SExtractor and wavelet techniques, and extended properties respectively.

Diary of Observations
The final X-ray image used to generate the catalog is a combination of 11 individual Chandra ACIS-I pointings summarised in Table 1. The maximum total exposure time is 942 ks, but varies across the detector(s), with a minimum of 25 ksec at the edge of the field where we obtained only single exposures due to rotation of the field-of-view. ACIS consists of ten CCDs, distributed in a 2x2 array (ACIS-I) and a 1x6 array (ACIS-S, see the Chandra Observatory Guide at http://asc.harvard.edu/udocs/docs/docs.html). All four ACIS-I chips and the ACIS-S3 chip were used for the CDFS observations. The telescope aimpoint was centered on the ACIS-I3 chip for each exposure. Effective area and spatial resolution of the telescope vary inversely as off-axis angle. Unfortunately the S3 CCD is far enough off-axis to decrease its effective area by a factor of 0.7 (at 4.5 keV) and its spatial resolution by a factor of 0.1 or less relative to the aimpoint. We have therefore chosen to ignore any data in the S3 CCD for the purpose of this work.
Our first two observations in 1999 (1431-0, 1431-1, see Table 1) were taken with the ACIS-I at -110 C. All other observations were taken at -120 C. The CTI-induced QE loss due to grade redistribution (see Townsley et al. 2001) was mitigated by the ten degree change, especially in the hard band. As an example, the effective area improved by 5% at 4.5 keV. The improvement is higher at larger energies, where, however, the effective area is much smaller. The 7 observations taken in December 2000 were processed by the Chandra X-ray Center using the most recent calibration files which were introduced in versions R4CU5UPD11 (and later) of the processing software. Those observations taken in 1999 and the summer of 2000 were also re-processed by the Chandra X-ray Center using the new calibration files.

Processing Standards
CIAO 2.0.1 (see http://asc.harvard.edu/ciao) was used to reduce all of the data sets. While calculating exposure maps, the program asp apply sim was used to fix the shifts between exposure maps and X-ray images. The data were filtered to include only the standard ASCA event grades 0, 2, 3, 4 and 6. All hot pixels and columns were removed. We also removed the flickering pixels (caused mainly by cosmic rays afterglows), defined as the pixels with more than two contiguous events within 3.3 sec. Time intervals with background rates larger than 3σ over the average global value were removed. Due to the high background, about 8800 sec were removed from the level 2 files; most of them were removed from the first exposure, which contained the only long-lasting high background flare. The final exposure time of 942 ks also reflects corrections for CCD read-out time and time lost due to bad satellite aspect.
There are small shifts (between 20" and 1") between the aimpoints of each exposure. We calculated these relative shifts by registering the brightest sources and combining the eleven observations. The combined data has the same coordinate system as Obs. ID 2406. We extracted three images from the total 1Msec data: a soft image (0.5-2.0 keV), a hard image (2.0-7.0 keV) and the total image (0.5-7.0 keV). The images were binned 2 × 2, which gives an image scale of 0.982 ′′ /pixel. The hard and total bands were cut at 7 keV since above this energy the effective area of Chandra is decreasing, while the instrumental background is rising, giving a very inefficient detection of sky and source photons. Figures 1 and 2 shows the soft and hard images. Exposure maps for the soft and hard bands were calculated for each observation, and then combined using the determined shifts and weighting for the individual exposure times. The soft exposure map is shown in Figure 3. A single ACIS-I pointing covers a field of about 0.08 deg 2 . Due to the different roll angles of the individual pointings, the final image covers a total of 0.109 deg 2 , decreasing rapidly near the flux limit.
The solid angle as a function of the effective exposure time is shown in Figure 4. The sky coverage (see Figure 5) is defined as the solid angle within which a source with a given X-ray flux can be detected at S/N> 2.1. MARX simulations verify that our model for the sky coverage is accurate within a few percent. The computation of the sky coverage and the simulations are described in Tozzi et al. (2001).

Sextractor
To detect sources in the Chandra Deep Field South we ran a modified version of the SExtractor algorithm (Bertin & Arnouts 1996) on the 0.5-7 keV image. This modified detection algorithm is several orders of magnitude faster than the wavelet algorithm of Rosati et al. (1995) or WAVDETECT in the CIAO software (Dobrzycki et al. 1999;Freeman et al. 2001). Detection parameters (e.g., threshold, characteristic object size, and ultimately the signal-to-noise) were chosen to find very faint sources, while limiting the number of spurious sources to 10, or approximately ∼ 3% of the total sample. The number of fake sources as a function of the algorithms' parameters was determined via extensive simulations as described in Tozzi et al. (2001).
SExtractor was first run on the 0.5-7 keV image to produce an initial list of candidate sources. SExtractor detection parameters were chosen as a result of simulations and we adopted a detection threshold of 2.4, with a Gaussian filter with 1.5 ′′ FWHM and a minimum area of 5 connected pixels. SExtractor parameters were chosen to detect sources to the faintest limits, thus including a sizeable number of spurious sources. Object photometry for each source was then performed separately in the soft and hard bands and its significance was determined by measuring the signal-to-noise for each source. The definition of S/N was that adopted by Tozzi et al. (2001). Source counts are measured in a circle of radius R s , where R s is in units of pixels and is determined by a function R s = 2.4 ′′ × FWHM (with a minimum of 5 pixels). The FWHM was modeled as a function of the off-axis angle to reproduce the broadening of the PSF. The parabolic fit we adopted has the coefficients (a 0 , a 1 , a 2 ) = (0.6779, −0.0405, 0.0535). The radius R s reproduces the 95% encircled-energy radius as included in the WAVDETECT algorithm. Note that the fit that we used for photometry is different from the fit of the average FWHM of the observed sources that we used later to select extended sources.
The local background was calculated for each source in an annulus with outer radius of R s + 12 pixels (11.8 ′′ ) and inner radius R s + 2 pixels (1.9 ′′ ), after masking out nearby sources. We considered only sources with S/N ≥ 2.1 in either the soft or hard band. This procedure removed 47 sources from the raw catalog obtained with SExtractor. Detailed simulations have shown that a S/N>2.1 threshold results in less than 10 spurious sources in the whole sample. Our simulations have also shown that aperture photometry leads to an underestimate of the source count rate by approximately 4% (see panel c of Figure 1 of Paper II). We have corrected this photometric bias before converting count-rates into energy fluxes. We also notice that our aperture photometry is based on apertures larger than both the hard and soft PSFs, so that we do not suffer any effect from the energy dependent nature of the PSF.
Incidentally, we remark that the same procedure run separately in both the soft and hard bands did not add any new detections. Conversely, the combined catalog obtained from the detections in the two separate bands, was missing several bona fide sources found in the 0.5-7 keV image. This is expected since we are far from being background limited: the soft background is (2.6 ± 0.4) × 10 −7 counts/arcsec 2 /s, while the hard (2-7 keV) background is (4.2 ± 0.5) × 10 −7 counts/arcsec 2 /s. The higher background in the total image is therefore more than compensated for by the higher signal-to-noise of the sources.
We checked the detections by eye, removing a spurious double detection far off-axis, and splitting two blended sources. We checked also that point sources included in the brightest extended sources (see §3.4) are correctly identified. Thus, we have 294 sources with S/N ≥ 2.1 from the soft image, and 247 sources from the hard image. The total number of SExtractor sources is 332. There are 85 only detected in the soft band, and 38 only detected in the hard band.

Wavelet Transform
To independently check the sources detected with SExtractor, we ran WAVDETECT (Dobrzycki et al. 1999;Freeman et al. 2001) on the 0.5-7 keV image. A probability threshold of 1 × 10 −6 , and scales of 1, 1.414, 2.0, 2.828, 4.0, 5.656, 8.0 pixels were used for source identification. The combined image of 1 Msec has ∼ 1.4 × 10 6 pixels; therefore we expect ∼ 1.4 false detections per image for a uniform, static background. However, this is not the case for our data, where the background is not static and varies across the field, especially at the edges, between individual observations. Instead of defining another completeness criterium for the sources detected with WAVDE-TECT, and thus a separate catalog, we use this sample as a complement to the SExtractor catalog. Therefore we apply the same S/N>2.1 threshold on the list of candidates obtained with WAVDETECT, removing 23 of them. Again, the measure of the signal-to-noise ratio for each source candidate has been done independently in both the soft and hard images, using the same extraction regions described in the previous Section. Finally, we have 288 sources with S/N > 2.1 from the soft image, and 240 sources from the hard image. The total number of sources is 318. There are 78 detected in the soft band only, and 30 detected in the hard band only.

Joint Data Set
We compared the two catalogs from SExtractor and WAVDETECT, using a matching radius of 2 ′′ . We found 304 sources were detected by both methods. These jointly detected sources constitute the first section of the catalog, given in Table 2. We also have 28 sources which are detected by SExtractor only, which we list Table 3, and 14 which are detected by WAVDETECT only, which we list in Table 4. In Figs 6 and 7 we compare the photon counts measured with the SExtractor method with the counts measured with WAVDETECT for each detected source. The comparison has been done both in the soft and hard band. We note an excellent agreement between the independent photometric measurements. It should also be noted that all quoted counts in the catalog (Tables 2-4) were determined using the aperture method described in Section 3.1 above. The adopted cut at S/N>2.1 corresponds to ∼ 11(13) counts for the faintest sources in the soft(hard) band at the aimpoint. The net count rates were obtained by dividing the net counts by the effective exposure time, which includes the effect of vignetting.
The energy fluxes were computed separately in the soft and hard bands. We prefer to quote the energy flux in the 2-10 keV band as extrapolated from the counts in the 2-7 keV bands, in order to make a direct comparison with previous results from the literature. To derive the energy flux from the observed count rate for the soft and hard bands, we have assumed a conversion factor appropriate for the measured average spectrum of all the sources Γ = 1.375 ± 0.015 (found for all the 1 Msec sources). After fixing the local absorption to the Galactic value of 8 × 10 19 cm −2 , we have a conversion factor of (4.6 ± 0.1) × 10 −12 from count rate to erg s −1 cm −2 in the soft band and (3.0 ± 0.3) × 10 −11 in the hard band. The uncertainties correspond to Γ = 1.4 ± 0.3. The error on the fluxes in the table include only the Poissonian error resulting from the source and background counts. With the adopted conversion factors, the minimum fluxes achieved in the soft and hard band are 5.5 × 10 −17 erg s −1 cm −2 and 4.5 × 10 −16 erg s −1 cm −2 , respectively.

Extended Sources
The CDFS 1Msec source catalog contains a sizeable fraction of sources which are resolved by Chandra. A search for extended sources in the Msec exposure (Paper III) yielded 18 diffuse sources. For relatively rare objects like groups and clusters of galaxies, which are known to contribute up to ∼ 10% of the soft background at 10 −14 erg s −1 cm −2 (Rosati et al. 1995), the CDFS probes the very faint end of their X-ray luminosity function. In order to search for extended sources associated with hot halos of galaxies, groups and clusters, we used the soft image and characterized the source extent with the FWHM of the best fit Gaussian profile. A higher S/N cut, S/N > 3, is required for a robust determination of the source extent. The likelihood that a source is extended can be estimated as a function of the off-axis angle by comparing the measured FWHM with the local PSF width. The latter was empirically derived with the same profile fitting procedure by using a sample of 346 sources which were drawn from the combination of the CDFS with two additional deep fields in the Chandra Archive, MS1137.5+6625 (sequence number 800044, 120 ks) and CL0848.6+4453 (Lynx field, 800103, 190 ks). This control sample was constructed including only sources with S/N > 3 and excluding outliers in the extent distribution with a 3σ clipping procedure. The best fit of the PSF FWHM as a function of the off-axis angle is shown in Figure 10 (top panel) as a solid line. The coefficients of the parabolic fit are: (a 0 , a 1 , a 2 ) = (1.152, 0.193, 0.025). This fit was subtracted from each data point to obtain the FWHM residuals ( Figure 10, bottom panel). We then fitted a Gaussian distribution to these residuals in four different off-axis angle bins (0-3, 3-6, 6-9, 9-12 arcmin) and derived the 3σ upper values whose parabolic fit defines our cut off line for extended sources (dashed line). A catalog of the 18 extended sources selected in this way is given in Table 5. We have visually inspected these sources to make sure that strong variations in the exposure map do not affect source characterization. This procedure will generally not work well for very extended low surface brightness sources for which a simple Gaussian fit is not appropriate, or which might have been missed by both our standard detection algorithms. To search for very diffuse sources we used a complementary approach. We ran WAVDETECT with a small characteristic scale to preferentially select point-like sources. These objects are removed from the soft image by replacing the source region with a simulated background. We then rebinned the resulting image to a larger pixel size (4 arcsec) and used WAVDETECT to search for significant sources (probability threshold of 1 × 10 −6 ) with characteristic scales of 1.0, 1.414, 2.0, 2.828, 4.0, 5.656, 8.0 pixels. We remind the reader that running WAVDETECT on the original image with large characteristic scales would produce poor results due to well-known cross talk between the small and large scales in the wavelet analysis. One diffuse source is clearly detected with this method (#645 detected by WAVDETECT only). This is a low surface brightness feature also apparent by visual inspection of the field (Figure 1, lower right).
We have added this source to our extended source catalog. The K-band finding chart ( Figure 11) shows an asymmetric distribution of X-ray emission, which is likely associated with an intermediate redshift poor cluster.
Visual inspection and optical colors indicate that some of them are groups or isolated early type galaxies at z < 1. In several cases however, the X-ray emission is dominated by a central, hard component which is likely due to low-level nuclear activity. Of all the 18 diffuse sources, one has been identified (#594; see Figure 12) with a poor cluster at z = 0.72. The X-ray luminosity is L X (0.5−2keV) = 4×10 42 erg s −1 , and the temperature kT = 1.2 +0.6 −0.3 keV (1 sigma errors). The stacked spectrum of the three brightest extended sources (F X = 1 − 3 erg s −1 cm −2 in the soft band) clearly identified as groups, is well fitted by a Raymond-Smith model with kT = 1.7 +0.6 −0.4 keV, assuming metallicity 0.3 solar and redshift equal to the spectroscopic or photometric one. The mean surface brightness of these diffuse sources, computed as the ratio between the flux and a circular area of 2*FWHM radius, is as low as 10 −16 erg s −1 cm −2 arcmin −2 , i.e. between 10 and 20 times fainter than the faintest extended sources discovered by ROSAT.

Optical Identifications
Our primary optical imaging was obtained using the FORS1 camera on the ANTU (UT-1 at VLT) telescope. The R band mosaics from this data cover 13.6 ′ × 13.6 ′ to depths between 26 and 26.7 (Vega magnitudes). This data does not cover the full CDFS area and must be supplemented with other observations. The ESO Imaging Survey (EIS) has covered this field to moderate depths in several bands, of which we present R (< 26.1 Vega) here (Arnouts et al. 2001;Vandame et al. 2001). The EIS data has been obtained using the Wide Field Imager (WFI) on the ESO-MPG 2.2 meter telescope at La Silla.
The Chandra data itself has a pointing accuracy of roughly 1 ′′ . Identification of optical counterparts then depends on registering the X-ray and optical data to the highest possible accuracy. To do this we required a stable and fixed astrometric frame. We have chosen to tie all image astrometry to the FORS1 frame as described below.

Positional Accuracy
The registration of the X-ray to optical coordinates was done by first cross-correlating the optical and X-ray source catalogs assuming that any residual offsets were smaller than the search box size (10 ′′ ). After correcting for the bulk offsets, the correlation was re-run with a smaller box size. These catalogs were then used to generate an input file for the IRAF task CCMAP, which generates an astrometric solution from the correlated image and sky coordinates. We found shifts in (RA R -RA X , Dec R -Dec X ) of (1.1, -0.8) arcseconds from the FORS1 R band to X-ray imaging (see Figures 8 and 9). The measured positional r.m.s. is ∼ 0. ′′ 5. We have adopted 1. ′′ 5 as our 3σ error box. Due to the strong effect of off-axis angle on the X-ray PSF and centroid, we scaled the error radius δ r with off-axis angle (θ) and signal-to-noise: δ r = c 0 * (FW HM ′ (θ)/(S/N)) c 1 , where c 0 = 0.77 and c 1 = 0.68 and FWHM ′ is the same quadratic as above but is re-normalized such that a source at zero off-axis angle would have the 1. ′′ 5 3σ error box. We are likely over-estimating the number of possible counterparts for some sources. The resulting optical counterpart candidates presented in Tables 2, 3 and 4 are sorted by separation from the X-ray centroid (the optical ids are also marked with boxes in Figures 15, 16 and 17).
After applying the shifts and error estimates we find that 85% of the sources have a counterpart within the 3σ error circle. This percentage is 78% when calculated with the shallower WFI R band data.

Optically Undetected Sources
About 15% of the sources in our sample have no optical counterpart within the assumed error circle down to the limiting R magnitude. Some of those sources might have unusually high f x /f R ratios, extending the known parameter space of X-ray sources, see figure 13 (here f R is the energy flux in the R-band). We have therefore compared the distribution of the f x /f R ratios of our X-ray sources without optical counterparts -which we assumed have the limiting R magnitude -with the analogous distribution for our X-ray sources with optical counterparts. As shown in Figure 14, the distribution of the f x /f R lower limits of optically undetected sources (dashed line) is consistent with the high f x /f R tail of the distribution of optically detected sources (solid line). For a concise discussion of the nature of these sources see §5.

Discussion and Conclusions
We briefly summarize the detection criteria adopted in this Paper. There are three basic steps: 1) a detection algorithm (either SExtractor or WAVDETECT) has been run on the full 0.5-7 keV image to generate a list of source candidates; 2) aperture photometry has been performed independently in the 0.5-2 keV image and 2-7 keV image at the position of each source candidate; 3) all those sources with S/N>2.1 either in the soft or in the hard image are considered as real sources and appear in our catalogs. This procedure may not reach the deepest sensitivity achievable with our observations, however, it is a good compromise achieving low limiting fluxes without introducing numerous fake sources. As assessed by extensive simulations described in Tozzi et al. (2001), the fraction of fake sources to be expected in our sample is less than 10 (or ∼ 3%) with this method. The faintest sources in our samples have approximately 10 net counts. Given the exposure time and the adopted conversion factors the limiting fluxes achieved in the 0.5-2 keV and in the 2-10 keV band are 5.5 × 10 −17 erg s −1 cm −2 and 4.5 × 10 −16 erg s −1 cm −2 , respectively.
These limiting fluxes are about 20 and 200 times lower than that previous X-ray missions in the soft and hard bands respectively. The catalog of sources presented in this Paper allows us to investigate the nature of X-ray faint sources with unprecedented sensitivity, together with their optical properties. Most of the sources in our sample fall within the X-ray to optical flux ratio range typical of AGN as determined from the EMSS sources (Stocke et al. 1991). In addition a significant population of X-ray faint/optically bright sources is observed, with f x /f R ≤ 0.1, lower than those typical of AGNs. Most of these sources are detected in the soft band only (circles in Figure 13). This population of sources has been already partially identified as nearby, bright normal galaxies by Tozzi et al. (2001);Hornschemeier et al. (2001); Barger et al. (2001). Only a few sources seem to have f x /f R values higher than usual, which may be obscured AGN at very high redshift. This population of optically faint sources (R > 25) is consistent with a mix of obscured AGN at z = 1 − 3 and evolved, high-z galaxies (see Alexander et al. 2001;Cowie et al. 2001). The nature of these sources will be investigated in greater detail in following papers.
The data presented here constitute (along with the Chandra Deep Field North) the deepest X-ray exposure ever taken. As such, it is a unique dataset for current and future research. A proposal to re-observe this field with Chandra will await detailed analysis of the current dataset. However, a SIRTF Legacy program will be observing this field with both MIPS and IRAC during their own deep survey. Also, a 500ks observation with XMM has been planned in the CDFS this year. The high XMM throughput, combined with the arcsec Chandra resolution, will allow high quality X-ray spectra and secure optical identifications to be obtained for most of the X-ray sources in the CDFS. The intensive optical and infrared coverage is planned to continue and to be vigorously pursued at ESO. Deep VLA radio observations are being analyzed.
We thank Dr. Harvey Tananbaum for making available 500 kiloseconds of Director Discretionary Time which doubled our original guaranteed time on this field, thus permitting us to achieve the quoted sensitivity. We thank the entire Chandra Team for the high degree of support we have received in carrying out our observing program. In particular, we wish to thank Antonella Fruscione for her constant help in the use of the CXC software. Finally, we thank the anonymous referee for a detailed report that considerably improved the presentation of the results. R. Giacconi and C. Norman gratefully acknowledge support under NASA grant NAG-8-1527 and NAG-8-1133.             The distribution of Log f X /f R for the current sample. The solid line histogram is for the CDFS data, the dotted line is the ROSAT Lockman Hole data  and the dashed line is upper limits in the CDFS/FORS1 dataset for the optically undetected counterparts (arrows in Figure 13).   The iso-intensity contours are at 2, 5, 10 and 20 sigma above the background. The box indicates an optical counterpart candidate. The positional error is indicated by the circle in