You can restrict your desired object sample by selecting from this catalogue, but it is not likely that you need to go beyond the content of this catalogue. Sub-selecting from here is trivial for you to do, asking for a remake/rematch would involve more work and should ideally not be necessary (if everything was done right here).
Details:
The columns appended fall into two categories:
- Columns describing up to three objects from 'the other list' which could possibly be related to the given source from 'this list', and holding positional differences and chi-square values for the match based on position and magnitude
- The full column set holding all information from 'the other list' for the most likely counterpart
The most likely counterpart is the one with the lowest chi-square, not necessarily the one with the shortest positional offset. About 10% of the sources have more than one possibly related object within the maximum search radius of 2 arcsec. In most cases, the second object is just a nearby, but unrelated source. However, a significant number of COMBO sources have two GEMS sources within 1 arcsec. In the COMBO image and photometry these objects are blended. If the two GEMS sources are at the same true redshift, then COMBO will see a mix of stellar populations and still get the redshift right. But if the two GEMS sources are physically at different redshifts, the COMBO photometry will see the sum of two SEDs which are redshifted with respect to each other. If one source is much brighter than the other, the COMBO photometry and interpretation is dominated by that one and should reflect its redshift and SED. If they have comparable brightness (maybe within a 1:3 ratio or so), COMBO will be confused and come out with the wrong redshift. We had such a case in CADIS, where a source at z=0.6 was blended with one at z=1.2, and the CADIS photo-z was 0.9, by chance the average. Depending on the application, it may be safer to eliminate all objects from a sample, where a blend could have occured.
The table is provided as a FITS table in order to
- remove the need for all future times to waste time on any sort of format description (just read the table with a FITS reader into an array of strctures, then use the object number as an array element index and the column header as a structure tag name, and you can access every entry regardless of its physical location in the table)
- to allow the table to change format at any time without any warning to any potential user and still ensure that not any of your software using it needs to be changed (e.g. when including further information as further columns, or changing the order of columns by random chance - rights for this to happen are reserved by the producer)
A description of the columns originating from the COMBO catalogue can be found here and one of those originating from the GEMS catalogue by Dan can be found just below. In order to conform with the FITS concept I have given column labels to the data entries from the proto_tot catalogue by Dan, which are:
Byte-by-byte Description of file: proto_tot.cat.dp
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Bytes Column Label # Description
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1- 24 GEMSname 1 GEMS sexagesimal coord.-based name
26- 35 r_a 2 Right ascension (2000)
38- 48 dec 3 Declination (2000)
51- 58 flux_best 4 Total flux (SExtractor FLUX_BEST)
60- 65 fluxerr_best 5 Total flux error (SExtr. FLUXERR_BEST)
68- 73 mag_best 6 Total magnitude (SExtr. MAG_BEST)
75- 80 magerr_best 7 Total magnitude error (SExtr. MAGERR_BEST)
83- 87 background 8 Background (SExtr. "local" BACKGROUND)
90- 95 isoarea_image 9 Isophotal area (SExtr. ISOAREA_IMAGE)
98-103 x_image 10 Image center x coordinate (SExtr. X_IMAGE)
105-110 y_image 11 Image center y coordinate (SExtr. Y_IMAGE)
112-116 theta_image 12 Position angle (SExtr. THETA_IMAGE)
119-123 ellipticity 13 Ellipticity (SExtr. ELLIPTICITY)
126-130 fwhm_image 14 Image FWHM (SExtr. FWHM_IMAGE)
133-134 flags 15 SExtractor FLAGS
137-140 class_star 16 Star/Galaxy class (SExtr. CLASS_STAR)
148-160 source_name 17 ACS source image name
162-175 mask_name 18 ACS mask image name
178-181 exptime 19 exptime (sec)
184-189 ABsys_zp 20 ABsys zeropoint
192-197 Kron_radius 21 Kron radius (SExtr. Kron apertures)
200-208 Cxx_ell 22 Cxx object ellipse parameter from SExtr.
210-218 Cyy_ell 23 Cyy object ellipse parameter from SExtr.
220-229 Cxy_ell 24 Cxy object ellipse parameter from SExtr.
252-253 nr_overlap 28 Number of overlapping sources
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Some column names have been changed to allow for the presence of similarly labelled GEMS and COMBO columns alongside in the same master table. In the GEMS master catalogue the following columns originating from COMBO are renamed:
r_a renamed into COMBOr_a
dec renamed into COMBOdec
mag_best renamed into COMBOmagR
mag_best_err renamed into COMBOmagRerr
mag_I_best renamed into COMBOmagI
class renamed into GEMSclass
class_val renamed into GEMSclassval
In the COMBO master catalogue the following columns from the GEMS table proto_tot.fits (as detailed above) are renamed:
r_a renamed into GEMSr_a
dec renamed into GEMSdec
mag_best renamed into GEMSmag
magerr_best renamed into GEMSmagerr
flags renamed into GEMSflags
class_star renamed into GEMSclass
Last update Oct 27, 2004, CW