GOODS Source Catalogs release r1.0z based on v1.0 data

• 1.0 General information

• 2.0 What is being released

• 3.0 How the catalogs have been made

• 4.0 Source detection

• 5.0 Modifications to SExtractor
  • 5.1 Local sky background
  • 5.2 Splitting/merging

• 6.0 Tests and comparisons with simulations
  • 6.1 Completeness
  • 6.2 Photometric precision
  • 6.3 Comparison to the Hubble Deep Field North WFPC-2 photometry
  • 6.4 Photometric accuracy

• 7.0 Column listing

• 8.0 Citations

README

1.0 General Information

On Monday, December 22 2003, at 5pm EST, the GOODS Teams released the version r1.0 of the ACS multi-band source catalogs.

These catalogs, prepared using the SExtractor package (Bertin & Arnouts 1996, A&A 117, 393), are based on the version v1.0 of the reduced, calibrated, stacked and mosaiced images acquired with HST and ACS as part of the GOODS ACS Treasury program.

The catalogs are z-band based, that is, source detection has been made using the z-band images. A variety of photometric apertures defined during the detection process have then been used as "fixed apertures" in the i, v and b-band images to derive the multi-band photometry.

2.0 What is being released

The r1.0 release of the multi-band source catalogs consists of eight (8) primary ASCII files (the catalogs); of fourteen (14) files required to configure SExtractor; of fourteen (14) ancillary postscript files (the figures), which illustrate relevant features of the catalogs; and of this "README" file.

The eight primary files are named:

1 h_goods_nb_r1.0z_cat.txt
2 h_goods_ni_r1.0z_cat.txt
3 h_goods_nv_r1.0z_cat.txt
4 h_goods_nz_r1.0z_cat.txt
5 h_goods_sb_r1.0z_cat.txt
6 h_goods_si_r1.0z_cat.txt
7 h_goods_sv_r1.0z_cat.txt
8 h_goods_sz_r1.0z_cat.txt

Groups of letters, numbers or punctuation symbols separated by underscore ("_") are used to specify the main parameter of the catalogs.

The prefix "h" is used in the STScI multimission archive to indicate HST data products.

The second group is obvious.

The first letter of the following two-letter group indicates which GOODS field the catalog is relative to (values are either "s" for GOODS-S, or "n" for GOODS-N), while the second letter indicates the photometric passband (values are "b", "v", "i" and "z").

The group "r1.0" specifies that this is the release 1.0z of the catalogs and that detections are based on the z band.

The following group "_cat" specifies the nature of the data product.

The extension ".txt" specifies that the catalogs consist of text ASCII files.

The fourteen SExtractor configuration files are

1 h_goods_r1.0z_detect_param.txt
2 h_goods_r1.0z_phot_param.txt
3 h_goods_sb_r1.0z_phot_sex.txt
4 h_goods_sz_r1.0z_phot_sex.txt
5 h_goods_si_r1.0z_phot_sex.txt
6 h_goods_sv_r1.0z_phot_sex.txt
7 h_goods_sz_r1.0z_detect_sex.txt
8 h_goods_nz_r1.0z_detect_sex.txt
9 h_goods_r1.0z_detect_nnw.txt
10 h_goods_r1.0z_detect_conv.txt
11 h_goods_nz_r1.0z_phot_sex.txt
12 h_goods_nv_r1.0z_phot_sex.txt
13 h_goods_ni_r1.0z_phot_sex.txt
14 h_goods_nb_r1.0z_phot_sex.txt

The *detect_sex.txt files contain the parameters used for the extraction on the z band detection images. The *phot_sex.txt files contain the parameters used for the photometric analysis in dual-image mode. The *detect_nnw.txt and *detect_conv.txt are the default neural net and convolution filters used by SExtractor. The *detect_param.txt and *phot_param.txt contain the list of columns output by SExtractor in its detection and photometry runs respectively.

We refer the reader to the original documentation provided with the SExtractor package for the meaning and content of these files.

The 14 figure files are

1 h_completeness_all_r1.0_plt.ps fig1
2 h_completeness_disks_r1.0_plt.ps fig2
3 h_completeness_spheroids_r1.0_plt.ps fig3
4 h_star_s_r1.0_plt.ps fig4
5 h_star_n_r1.0_plt.ps fig5
6 h_goods_vs_hdf_iso_r1.0_plt.ps fig6
7 h_goods_vs_hdf_auto_r1.0_plt.ps fig7
8 h_simul_z_r1.0_plt.eps fig8
9 h_simul_z_disk_r1.0_plt.eps fig9
10 h_simul_z_spher_r1.0_plt.eps fig10
11 h_simul_z_err_r1.0_plt.eps fig11
12 h_simul_z_dmag_all_r1.0_plt.eps fig12
13 h_simul_z_dmag_disk_r1.0_plt.eps fig13
14 h_simul_z_dmag_spher_r1.0_plt.eps fig14

These figures are discussed below.

3.0 How the catalogs have been made

With the scale of the images set at 0.03 arcsec per pixel (to maximize sampling of the PSF), each GOODS field would have resulted in images too large in size for practical purposes (40,000x40,000 pixels for the HDF-N and 32,000x40,000 pixels for the CDF-S). As a result, each field has been divided into contiguous sections, each 8,192 x 8,192 pixels in size. A total of 17 sections cover the HDF-N, and a total of 18 sections cover the CDF-S.

Because the images are derived from overlapping ACS frames, taken at different orientations, the exposure time is not uniform across the images, and there is a complex pattern to the exposure-time variations. This is recorded in the weight maps released with the v1.0 images. We use this information to set IMAFLAGS_ISO in the catalogs to highlight areas that have less than typical exposure time. As a rule-of-thumb: Pixels with flag values > 8 are generally pretty suspect due to poor cosmic-ray rejection.

The algorithm for the flag maps does two levels of smoothing prior to setting a flag. (Even so, there is still some blotchiness in the flags). The algorithm for setting a flag is as follows:

(1) median smooth the weight map with a 7x7 kernel
(2) Flag pixels below a certain threshold
(3) boxcar smooth this 1,0 flag map with a 7x7 kernel
(4) Set pixels with values > 0 to the desired flag value

The thresholds are set roughly in to catch pixels with less than 1,2,3, and 4 epochs of exposure time.

Number epochs	Bit set
0 epochs (off the edge)	32
within 2" of edge	16
t <= 1.2 epochs	8
t <= 2.2 epochs	4
t <= 3.2 epochs	2
t <= 4.2 epochs	1

For source detection we prepared special overlapping sections, still 8192x8192 pixels in size but with a 600 pixel overlap at the borders to ensure that no objects were split across file borders. After each section was SExtracted independently, object in the overlapping areas were matched by RA and Dec, and duplicates were eliminated by selecting the objects furthest from the section edge. The resulting catalog was sorted by RA and Dec, and unique sequential IDs were assigned. Object detection and merging were performed on the z band images only, and the b, v and i photometry was obtained processing these images in dual-image mode. The merged b, v and i catalogs were obtained by selecting the appropriate object record based on the z-band object ID numbers.

Most of the columns in the catalog are as defined by SExtractor, but we have added several additional columns. The column numbers and names are listed in the file headers in the usual SExtractor output file style. The additional, non SExtractor-created columns all end with _MOSAIC, to indicate that these are global values pertaining to the virtual, north-up mosaic.

The ID_MOSAIC column contains the unique sequential id assigned to the objects. There are a few small gaps in this sequence as a result of objects being removed at a later quality assessment stage of the process. This should not be a cause of concerns.

The X_MOSAIC, Y_MOSAIC, XMIN_MOSAIC, YMIN_MOSAIC, XMAX_MOSAIC, YMAX_MOSAIC are the various x and y pixel positions in the reference frame of the full (virtual) mosaic of the GOODS field, with (0,0) at the central reference point of the mosaic. Thus, these columns will contain values that may range over {-19000,19000}.

The SECT_REFNUM indicates the _SECT.FITS file from the v1.0 image release which contains the barycenter of the object, at pixel coordinates (X_SECT, Y_SECT).

Finally, we have also assigned object names based on their position in the sky, in accordance with the IAU naming conventions. This is recorded in first column of each catalog, named ID_IAU.

A listing of the individual columns of the catalogs is provided at the end of this document. Please note that these are logical columns, not in fixed format.

In the process of building the catalogs, we realized that two small modifications to the SExtractor software (version 2.2.2) resulted in somewhat better catalogs. These modifications affect the splitting/merging behavior and the computation of the local sky background, and are described below.

• 5.1 Local sky background

  Tests of early versions of the catalogs indicated that the photometry of faint galaxies was being somewhat underestimated and that this problem was, in part, due to the local sky background being overestimated when the wings of faint galaxies were incorrectly included in the sky measurement.

  To compute the local sky, SExtractor selects pixels from a rectangular annulus around the object. The inner "radius" of this annulus is located G pixels away from the edge of the object (as defined by its min/max extent in x and y), where G = (object extent in x or y)/4. Since we used a minimum detection threshold of 16 pixels, this formula produced an inner radius abutting the detected object edge for most faint, compact objects.

  To remedy this problem, we modified the code to enforce a minimum gapsize between the inner edge of the square annulus and the "end" of the source equivalent to 1 arcsec. For the GOODS v1.0 release data with scale 0.03 arcsec/pixel, this produced a minimum gap of 33 pixels around the detected object extent before selecting pixels for the sky calculation. For very large objects, where (extent/4 pixels) exceeds 1 arcsec, the original formula is used.

  Comparisons between this modified version of the code and the original SExtractor run showed no difference in the sky background for large objects, and a systematic brightening of the faintest objects, as expected. Comparison of the new sky values against sky values measured with the IRAF task PHOT for faint, compact sources, run as an independent check, shows no discrepancies and, in fact, shows excellent agreement between the background estimates of the two packages.

• 5.2 Splitting/merging

  As documented in the SExtractor manual, after the splitting is performed, a CLEANing algorithm is run to attempt to determine false splits. Each split child is analyzed to determine whether it would have been detected had its parent object not been present. If not, the pixels from the child object are merged back into the parent object.

  As has been discussed on the SExtractor mail list, this procedure sometimes results in dramatically discontiguous objects, with pixels assigned to the same object that are clearly (to the human eye) unrelated.

  To resolve this problem, we modified the code so that, instead of merging such false detections back in with the parent objects, they are simply discarded. Tests indicate that this eliminates nearly all discontiguous objects with little or no impact on photometry.

The GOODS Team conducted a number of tests of the completeness of the catalogs and of the accuracy of the measures that they include throughout their development. Here we show relevant plots that should be of help to investigators in assessing the completeness and photometric accuracy and precision of the catalogs for most (but certainly not all!) applications.

• 6.1 Completeness
  Figures 1, 2 and 3 show completeness curves as a function of apparent magnitude for exponential disks, de Vaucouleur spheroids and for a mix, obtained from numerical simulation. The completeness was determined from the recovery rate of artificial galaxies inserted in the images and retrieved with identical procedurs as those used for real sources. Apparent magnitudes and galaxy size (half-light radii) were extracted at random from uniform distribution functions, covering the range 20 <= z_{850} <= 28. (AB) and PSF <= r_1/2 <= 2.0 arcsec, respectiely. The plots have been made for the z-band only, since this the band used for source detection. They show that for compact sources the 50% completeness curves extend to very faint fluxes. They also show how the completeness decay as a function of the galaxy size.

• 6.2 Photometric Precision
  Figures 4 and 5 show comparisons between synthetic stellar colors derived from the Gunn-Stryker Atlas of stellar spectral libraries and unresolved sources from the GOODS fields. This comparison aims at checking the precision of the absolute photometric calibration of the ACS images, as well as at the presence of systematics in the photometry introduced by the measure. While there are some scatterers, which might very well be due to point-like sources (or nearly so) of no stellar nature (e.g. QSO), the overall agreement between the observations and the loci of stellar color is excellent.

• 6.2 Comparison with the HDF Wide-Field Planetary Camera data
  Figure 6 and 7 show a comparison between the F606W isophotal and MAG_AUTO magnitudes from GOODS and from the WFPC2 HDF-N observations. The HDF-N images were taken in 1995. The catalog was generated in 1998, and is posted at ftp://archive.stsci.edu/pub/hdf_south/version2/wfpc_hdfn_v2catalog/. Black symbols show positional matches (in world coordinates) between the two surveys better than 0.25 arcsec (after applying a uniform 0.3 arcsec shift in declination to the HDF-N coordinates). Blue symbols show positional matches in the range 0.25 < r <= 0.5 arcsec. The figures show very good agreement between the photometry of the two surveys.

  The table below shows the mean F606W magnitude offsets (GOODS-HDF), along with the number of sources matched within 0.25 arcsec, and the standard deviation and standard error of the magnitude difference. GOODS Isophotal magnitudes are systematically brighter at the faint end than HDF isophotal magnitudes (reflecting the fact that the GOODS images are shallower). There is much less bias for MAG_AUTO.

  MAG_ISO
  Mag range   N    mean     stdev    stderr
  --------- ---    -----    -----    ------ 
  21    22    4    0.094    0.077    0.0192
  22    23    6    0.057    0.089    0.0149
  23    24    9    0.114    0.098    0.0109
  24    25   22    0.156    0.138    0.0062
  25    26   52    0.223    0.221    0.0042
  26    27  116    0.485    0.435    0.0037
  27    28   82    0.627    0.463    0.0056
  
  MAG_AUTO
  Mag range   N    mean     stdev    stderr
  --------- ---    -----    -----    ------ 
  21    22    4    0.059    0.075    0.0189
  22    23    6    0.006    0.055    0.0092
  23    24   10   -0.001    0.026    0.0026
  24    25   22    0.007    0.095    0.0043
  25    26   55    0.047    0.230    0.0041
  26    27  129    0.038    0.181    0.0014
  27    28   78    0.071    0.277    0.0035
  

• 6.2 Photometric Accuracy
  Figures 8, 9 and 10 show the comparison between the apparent magnitude of artificial galaxies used in the numnerical simulations and the magnitudes measured by SExtractor. This test aims at checking for systematics in the magnitudes introduced by the measure process as a function of galaxy morphology and apparent magnitude. The red error bars in Figures 9 and 10 show the mean photometric errors (MAG_AUTO) in 1-mag bins measured by SExtractor, while the red ones show the rms of the difference between input magnitudes and recovered magnitudes (MAG_AUTO) in the same bins (observed scatter).
  

  The difference between the observed scatter and the SExtractor error is large, because the observed scatter is driven by outliers (due to crowding) and the distribution is non-gaussian. A fairer comparison, plotted in Figure 11, is between the SExtractor errrors and the semi 2-quartile width (ie. the half width of the distribution once the smallest and largest quartiles have been clipped), which indeed shows a much better agreement. At bright magnitudes SExtractor errors are underestimates because they do not account for variations in aperture corrections with galaxy shape, crowding, and other non-shot-noise effects. A reasonable "floor" to use for the errors is 0.07 magnitudes. Uncertainties in colors will be somewhat better because crowding and aperture-correction variations contribute less scatter in a differential comparison between two bands.

  Figures 12, 13 and 14 show the distribution of the observed scatter as a function of morphological type and observed magnitude (MAG_AUTO). Note that the with of the distributions are indeed narrower than the standard deviations plotted in Figure 2 and 3.

# 1    ID_IAU
# 2    ALPHA_J2000
# 3    DELTA_J2000
# 4    SECT_REFNUM
# 5    X_SECT
# 6    Y_SECT
# 7    X_MOSAIC
# 8    Y_MOSAIC
# 9    XPEAK_MOSAIC
# 10   YPEAK_MOSAIC
# 11   XPEAK_WORLD
# 12   YPEAK_WORLD
# 13   XMIN_MOSAIC
# 14   YMIN_MOSAIC
# 15   XMAX_MOSAIC
# 16   YMAX_MOSAIC
# 17   ISOAREA_IMAGE
# 18   THETA_IMAGE
# 19   ELLIPTICITY
# 20   ELONGATION
# 21   ERRTHETA_IMAGE
# 22   KRON_RADIUS
# 23   FLUX_RADIUS
# 26   FWHM_IMAGE
# 27   CLASS_STAR
# 28   FLAGS
# 29   IMAFLAGS_ISO
# 30   NIMAFLAGS_ISO
# 31   BACKGROUND
# 32   FLUX_MAX
# 33   MAG_ISO
# 34   MAGERR_ISO
# 35   FLUX_ISO
# 36   FLUXERR_ISO
# 37   MAG_ISOCOR
# 38   MAGERR_ISOCOR
# 39   FLUX_ISOCOR
# 40   FLUXERR_ISOCOR
# 41   MAG_AUTO
# 42   MAGERR_AUTO
# 43   FLUX_AUTO
# 44   FLUXERR_AUTO
# 45   MAG_BEST
# 46   MAGERR_BEST
# 47   FLUX_BEST
# 48   FLUXERR_BEST
# 49   MAG_APER
# 60   MAGERR_APER
# 71   FLUX_APER
# 82   FLUXERR_APER
# 93   X2_IMAGE
# 94   Y2_IMAGE
# 95   XY_IMAGE
# 96   ERRX2_IMAGE
# 97   ERRY2_IMAGE
# 98   ERRXY_IMAGE
# 99   A_IMAGE
# 100  B_IMAGE
# 101  ERRA_IMAGE
# 102  ERRB_IMAGE
# 103  ID_MOSAIC

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