The GOODS Team is releasing version r2.0z 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 v2.0 of the reduced, calibrated, stacked and mosaiced images acquired with HST and ACS as part of the GOODS ACS Treasury program and of the PANS program of search for Type Ia supernovae at high redshift.
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.
The r2.0z release of the multi-band source catalogs comprises: eight (8) primary ASCII files (the updated catalogs); one (1) updated file required to configure SExtractor; and this "README" file.
The eight primary files are named:
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 the survey identifier.
The first letter of the following two-letter group indicates which GOODS field is cataloged (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 "r2.0z" specifies that this is the release 2.0 of the catalogs, and that detections are based on the z band.
The following group "_cat" specifies the nature of the data product (source catalog).
The extension ".txt" specifies that the catalogs consist of text ASCII files.
The updated SExtractor configuration files are:
These file specify the list of columns output by SExtractor in its photometry runs, the size of the convolution kernel, and the setting of the neural network algorithm for image deblending, respectively. They are the same as the files released as part of the previous r1.1 GOODS ACS catalog version.
All other SExtractor configuration files are also the same as the previous r1.1 release (listed in the r1.0z release notes), except that they contain updated photometric zero points and exposure time (see also the documentation of the v2.0 image release). These files configure SExtractor to perform source detection in the F850LP (z) band (the _detect_ files) and to carry out photometry of the detected sources in each of the four passbands (the _phot_ files). They are:
h_goods_sb_r2.0z_phot_sex.txt
h_goods_sv_r2.0z_phot_sex.txt
h_goods_si_r2.0z_phot_sex.txt
h_goods_sz_r2.0z_phot_sex.txt
h_goods_sz_r2.0z_detect_sex.txt
3.0 Changes from previous GOODS ACS data releases
The r2.0z catalog release is based on the v2.0 images, which, as described above, have significantly longer total exposure times in the z850 bandpass, and somewhat longer exposure times in the i775 and V606 bands as well.
The only significant difference in the catalogs, other than that of being based on deeper data, is that we have applied a small astrometric offset to the GOODS-North images (only), as explained in the v2.0 documentation. This offset is a simple translational change to the world coordinates for the GOODS-N data, based on comparisons to catalogs from the Sloan Digital Sky Survey, 2MASS, and to new, deep VLA 20cm data (Morrison et al. 2010, ApJ, 188, 178). This correction is:
RA(v2.0) - RA(v1.0) = 0.000 arcsec = 0.0000000 degrees
Dec(v2.0) - Dec(v1.0) = -0.320 arcsec = -0.0000889 degrees
This correction was applied to the images by updating the CRVAL2 header keyword in each image:
CRVAL2: 62.238661 -> 62.238572 (62:14:19.18 -> 62:14:18.86)
Coordinates in the v2.0 source catalogs were also updated accordingly.
We note that as of this writing (14 May 2008), this change in GOODS-N astrometry has not yet been applied to other GOODS-North public data sets, such as the Spitzer IRAC and MIPS data. This will be corrected in future releases of data from those other facilities.
4.0 How the catalogs were prepared
With the scale of the images set at 0.03 arcsec per pixel (to improve the 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.
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 section borders. After each section was SExtracted independently, objects 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 ID_MOSAIC identifiers were assigned. Some duplicated objects were removed from the catalog by hand, and as a result the ID_MOSAIC values do not include every ordinal number. 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_MOSAIC numbers.
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 v2.0 images. We also use this information to create a flag map, with various bits set to represent the exposure time of the image at each pixel position. The algorithm for creating 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:
The thresholds are set roughly in to catch the typical weight (w) of pixels, calculated as median in a 7x7 pixel box, with a given fraction of the median weight value of the stack (m_w). The bit values have been assigned as follows:
| Number epochs | Bit set |
|---|---|
| 0 weight (off the edge) | 32 |
| within 33 pixels of edge (zero weight regions) | 16 |
| w <= (1.2/5)*m_w | 8 |
| w <= (2.2/5)*m_w | 4 |
| w <= (3.2/5)*m_w | 2 |
| w <= (4.2/5)*m_w | 1 |
| w > (4.2/5)*m_w | 0 |
The median weight ("m_w") in each band for both GOODS fields are:
| FIELD | F435W | F606W | F775W | L850LP |
|---|---|---|---|---|
| GOODS-N | 438511 | 119113 | 158074 | 423859 |
| GOODS-5 | 477099 | 121406 | 146677 | 407598 |
The flag maps were was passed to SExtractor using the FLAG_IMAGE configuration parameter. The catalog output parameter IMAFLAGS_ISO gives the logical 'OR' of the flag map bit values for pixels within the isophotal "segmentation" area of the object. This information can be used to identify objects that have less than the typical exposure time. As a rule-of-thumb: Pixels with flag values > 8 are generally suspect due to poor cosmic-ray rejection.
Source detection was carried out on the z-band images, and photometry was then measured
in the other bandpasses using SExtractor "dual image mode", i.e., through fixed apertures
defined from the z-band detection catalog.
Because the exposure times and local noise levels vary over the images, the weight map images
provided with the v2.0 data release were fed to SExtractor to ensure that source detection
occurred at a constant S/N level based on the local sky noise. Specifically, the weight maps
were constructed according to a noise model, as described in the GOODS ACS v2.0 image product
documentation, and were normalized so that their values are approximately equal to the inverse
variance of the background noise. As described in the image data release documentation,
the RMS computed from the weight maps takes into account account the small scale correlation
between pixels that is introduced by the drizzling process, so that noise over larger areas
(such as photometry apertures) should be approximately correct. Before cataloging, we verified
the noise scaling by measuring the amplitude and autocorrelation of the background noise in
the drizzled science images, and using this information to rescale the weight maps ("w") by
a weight scale factor ("s") before converting to RMS maps ("rms"), rms = 1/sqrt(s*w).
The scaling factors that were used used are:
In most cases, these factors change the scaling of the RMS maps by less than 5%.
When SExtractor is provided with RMS maps (WEIGHT_TYPE = MAP_RMS), it assumes that they
are absolutely normalized and does not attempt to rescale them further. Flux and magnitude
errors in the catalogs are computed as described in the SExtractor documentation, using
the normalized RMS maps to estimate the contribution from the background shot noise.
The primary parameters that affect the detection process, namely the minimum connected area,
the isophotal threshold (in unit of the background fluctuation RMS) and the convolution
kernel have been set with the help of numerical simulations to maximize
sensitivity to faint and relatively compact sources while reducing the number
of spurious sources to a minimum. In the simulations we inserted a number of
artificial galaxies in the images and retrieved them with the same
procedures adopted for real sources. The morphology of these galaxies
was, with equal probability, either an exponential disk or a de Vaucouleur
spheroid. Apparent magnitudes and half-light radii were extracted from
uniform distribution functions, covering the range
20 <= z_850 <= 28 (AB) and PSF <= r_1/2 <= 2.0 arcsec, respectively.
The simulated disk sample (viewed from arbitrary directions) is drawn from
a population of oblate optically-thin spheroids with a Gaussian
distribution of intrinsic axial ratios with mean b/a = 0.05 and
sigma = 0.01. The spheroids were drawn from a population of oblate
spheroids with intrinsic axial ratios uniformly populating the range
0.3 < b/a < 0.9. Position angles were randomly distributed.
We fine-tuned the SExtractor parameters to maximize the number of detected
sources, while keeping the number of spurious sources, estimated with the
method of the "negative images", essentially negligible.
One should keep in mind, however, that there is no one single catalog that is
perfect for all scientific problems and applications. While we believe that
the released catalogs are good for most investigations on faint galaxies,
users with specific problems (e.g. low-surface brightness galaxies) are
strongly urged to experiment with different settings of the SExtraction
detection parameters.
SExtractor measures photometry through a variety of apertures,
which are explained in more detail the SExtractor documentation.
The ones of primary interest in the GOODS catalogs are the
isophotal aperture (ISO), which the "AUTO" aperture which uses
an elliptical aperture whose size and orientation are determined
by the galaxy light profile in the 2D images, and a series of
circular apertures (APER) with fixed radii. The radii of the
11 circular apertures used for photometry in the GOODS catalogs
are spaced by factors of sqrt(2), and are:
or equivalently:
6.0 Modifications to SExtractor
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.
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 gap size
between the inner edge of the square annulus and the "end" of the source
equivalent to 1 arcsec. For the GOODS drizzled images, 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.
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 noncontiguous 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 noncontiguous objects with
little or no impact on photometry.
7.0 Column description for the catalogs
A listing of the individual columns of the catalogs, reproduced from
the catalog file headers, is provided below. We have added brief
descriptions of the output quantities here, but in many cases the user
should refer to the SExtractor documentation for more detailed explanations.
Most of the output parameters are as defined by SExtractor, but we have added
several other parameters. Most of these additional parameters end with _MOSAIC,
to indicate global pixel positions pertaining to a virtual mosaic of the image
sections. This virtual mosaic has coordinate value (0,0) at the central reference
point of the world coordinate system (CRVAL1,2), and spans coordinate ranges
from -19000 to +19000.
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.
The SECT_REFNUM indicates the _SECT.FITS file from the v2.0 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 few parameters occur in groups, particularly, the FLUX_RADIUS values (3),
and the APER magnitudes, fluxes, and errors (11 of each).
A description of the GOODS observations and data products is given
in "The Great Observatories Origins Deep Survey: Initial Results from
Optical and Near-Infrared Imaging" by M. Giavalisco and the GOODS Team
[2004, ApJ, 600, L93]. Another paper describing in more details
the GOODS HST/ACS data and source catalogs is in preparation and, for
the time being, should be referenced as "M. Giavalisco and the GOODS
Team, 2008, in preparation".
Bandpass s (North) s (South) F435W 0.9052 0.8487 F606W 0.9386 0.8987 F775W 0.9094 0.9199 F850LP 0.9307 0.9295
2.93, 4.17, 5.87, 8.33, 11.77, 16.67, 23.57, 33.33, 47.13, 66.67, 94.27 pixels
0.088, 0.125, 0.176, 0.250, 0.353, 0.500, 0.707, 1.000, 1.414, 2.000, 2.828 arcsec
# 1 ID_IAU IAU-format coordinate-based name for object
# 2 ALPHA_J2000 J2000 Right Ascension (degrees)
# 3 DELTA_J2000 J2000 Declination (degrees)
# 4 SECT_REFNUM section number from GOODS v2.0 imaging data
# 5 X_SECT centroid pixel x coordinate within section
# 6 Y_SECT centroid pixel y coordinate within section
# 7 X_MOSAIC centroid pixel x coordinate within a virtual mosaic of all sections
# 8 Y_MOSAIC centroid pixel y coordinate within a virtual mosaic of all sections
# 9 XPEAK_MOSAIC peak pixel x coordinate within virtual mosaic
# 10 YPEAK_MOSAIC peak pixel y coordinate within virtual mosaic
# 11 XPEAK_WORLD peak RA coordinate within virtual mosaic
# 12 YPEAK_WORLD peak Dec coordinate within virtual mosaic
# 13 XMIN_MOSAIC minimum x coordinate of object segmentation map within virtual mosaic
# 14 YMIN_MOSAIC minimum y coordinate of object segmentation map within virtual mosaic
# 15 XMAX_MOSAIC maximum x coordinate of object segmentation map within virtual mosaic
# 16 YMAX_MOSAIC maximum y coordinate of object segmentation map within virtual mosaic
# 17 ISOAREAF_IMAGE filtered isophotal area (pixels)
# 18 ISOAREA_IMAGE isophotal area (pixels)
# 19 THETA_IMAGE position angle (degrees)
# 20 ELLIPTICITY ellipticity
# 21 ELONGATION elongation
# 22 ERRTHETA_IMAGE positional angle uncertainty (degrees)
# 23 KRON_RADIUS "Kron radius" from light profile (pixels)
# 24 FLUX_RADIUS(1) 20% enclosed flux radius (pixels)
# 25 FLUX_RADIUS(2) 50% enclosed flux radius (pixels)
# 26 FLUX_RADIUS(3) 80% enclosed flux radius (pixels)
# 27 FWHM_IMAGE full width at half maximum (pixels)
# 28 CLASS_STAR star-galaxy classification parameter
# 29 FLAGS SExtractor internal flags
# 30 IMAFLAGS_ISO external flags from flag map, logical OR combination over segmentation area
# 31 NIMAFLAGS_ISO number of pixels with non-zero IMAFLAGS_ISO
# 32 BACKGROUND local sky background value (DN/s)
# 33 FLUX_MAX peak counts (DN/s)
# 34 MAG_ISO isophotal AB magnitude
# 35 MAGERR_ISO isophotal AB magnitude uncertainty
# 36 FLUX_ISO isophotal flux (DN/s)
# 37 FLUXERR_ISO isophotal flux uncertainty (DN/s)
# 38 MAG_ISOCOR corrected isophotal AB magnitude
# 39 MAGERR_ISOCOR corrected isophotal AB magnitude uncertainty
# 40 FLUX_ISOCOR corrected isophotal flux (DN/s)
# 41 FLUXERR_ISOCOR corrected isophotal flux uncertainty (DN/s)
# 42 MAG_AUTO AB magnitude in SExtractor "AUTO" aperture
# 43 MAGERR_AUTO AB magnitude uncertainty in SExtractor "AUTO" aperture
# 44 FLUX_AUTO flux in SExtractor "AUTO" aperture (DN/S)
# 45 FLUXERR_AUTO flux uncertainty in SExtractor "AUTO" aperture (DN/s)
# 46 MAG_BEST SExtractor "BEST" AB magnitude (not recommended!)
# 47 MAGERR_BEST SExtractor "BEST" AB magnitude uncertainty (not recommended!)
# 48 FLUX_BEST SExtractor "BEST" flux (DN/S) (not recommended!)
# 49 FLUXERR_BEST SExtractor "BEST" flux uncertainty (DN/S) (not recommended!)
# 50-60 MAG_APER circular aperture AB magnitudes (see documentation for aperture sizes)
# 61-71 MAGERR_APER circular aperture AB magnitude uncertainties (see documentation for aperture sizes)
# 72-82 FLUX_APER circular aperture fluxes (DN/s) (see documentation for aperture sizes)
# 83-93 FLUXERR_APER circular aperture flux uncertainties (DN/s) (see documentation for aperture sizes)
# 94 X2_IMAGE second order x moment of the light distribution (pixels)
# 95 Y2_IMAGE second order x moment of the light distribution (pixels)
# 96 XY_IMAGE second order xy moment of the light distribution (pixels)
# 97 ERRX2_IMAGE second order x moment uncertainty (pixels)
# 98 ERRY2_IMAGE second order x moment uncertainty (pixels)
# 99 ERRXY_IMAGE second order x moment uncertainty (pixels)
# 100 A_IMAGE semi-major axis length (pixels)
# 101 B_IMAGE semi-minor axis length (pixels)
# 102 ERRA_IMAGE semi-major axis uncertainty (pixels)
# 103 ERRB_IMAGE semi-minor axis uncertainty (pixels)
# 104 ID_MOSAIC object identifier number from virtual mosaic
8.0 Citations
Back to ftp site.