VIII/59 the FIRST Survey, version 1999Jul (White+ 1999)
The FIRST Survey Catalog of 1.4GHz radio sources
White R.L., Becker R.H., Helfand D.J., Gregg M.D.
<Astrophys. J. 475, 479 (1998)>
=1997ApJ...475..479W 1997ApJ...475..479W
ADC_Keywords: Radio sources ; Surveys
Description:
The FIRST survey to produce Faint Images of the Radio Sky at Twenty
centimeters began in 1993. It uses the VLA (Very Large Array, a
facility of the National Radio Observatory (NRAO)) at a frequency of
1.4GHz, and it is slated to 10,000 deg2 to a sensitivity of about
1mJy with an angular resolution of about 5". The co-added images are
available on the Internet (see the FIRST home page at
http://sundog.stsci.edu/ for details). The source catalogue is derived
from the images.
This version of the FIRST Survey is derived from the 1993 through 1998
observations, and contains 549,707 sources covering the north and
south Galactic caps. The catalog covers about a total of 6060 square
degrees of sky (5450 square degrees in the north and 610 square
degrees in the south).
No new data were taken for the south Galactic cap from the preceding
version (catalog VIII/51), but the southern images south of about -2
degrees were reprocessed with an improved pipeline script that
substantially reduces the sidelobe levels in a small fraction of the
fields. Consequently, many of the southern sources have slight changes
in their positions, flux densities, and other properties; a small
number of sources from the previous catalog are missing from this
catalog.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
first.dat 124 549707 North & South Galactic Caps
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See also:
http://sundog.stsci.edu/ : home page of the VLA FIRST Survey
http://sundog.stsci.edu/first/catalogs/history.html : Version History
http://sundog.stsci.edu/cgi-bin/searchfirst : the FIRST search engine
http://third.llnl.gov/cgi-bin/firstcutout : the FIRST Cutout Server.
http://www.cv.nrao.edu/first/ : FIRST page at NRAO
Byte-by-byte Description of file: *.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- FIRST *FIRST Source designation
18- 19 I2 h RAh *Right Ascension J2000 (hours)
21- 22 I2 min RAm *Right Ascension J2000 (minutes)
24- 29 F6.3 s RAs *Right Ascension J2000 (seconds)
31 A1 --- DE- *Declination J2000 (sign)
32- 33 I2 deg DEd *Declination J2000 (degrees)
35- 36 I2 arcmin DEm *Declination J2000 (minutes)
38- 42 F5.2 arcsec DEs *Declination J2000 (seconds)
44 A1 --- Wflag *[W] Warning flag
46- 53 F8.2 mJy Fpeak *Peak flux density at 1.4GHz
56- 63 F8.2 mJy Fint *Integrated flux density at 1.4GHz
66- 71 F6.3 mJy Rms *Local noise estimate
73- 78 F6.2 arcsec MajAxis *Major axis (FWHM)
80- 85 F6.2 arcsec MinAxis *Minor axis (FWHM)
87- 91 F5.1 deg PA *Position angle
93- 98 F6.2 arcsec fMaj *Fitted MajAxis before deconvolution
100-105 F6.2 arcsec fMin *Fitted MinAxis before deconvolution
107-111 F5.1 deg fPA *Fitted PA before deconvolution
113-124 A12 --- Field *Name of the coadded image containing
the source
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Note on FIRST:
This column (not part of the original catalog) contains the source
name built from the rule registered at IAU ('J' followed by truncated
J2000-position)
Note on RAh, RAm, RAs, DE-, DEd, DEm, DEs:
Position (J2000) of the source. The accuracy of the position depends
on the brightness and size of the source and the noise in the map.
Point sources at the detection limit of the catalog have positions
accurate to better than 1 arcsec at 90% confidence; 2 mJy point
sources in typically noisy regions have positions good to 0.5 arcsec.
An empirical expression for the positional accuracy is
unc(90% confidence) = Size (1/SNR + 1/20) arcsec
where Size is either the major or minor axis fitted FWHM (fMaj or
fMin) as given in the catalog and SNR is the peak flux density
signal-to-noise ratio:
SNR = (Fpeak-0.25) / Rms
(The positional uncertainty is of course elliptical for elliptical
sources.) The best possible positional uncertainty is limited to about
0.1 arcsec by our ability to fit source positions in maps with 1.8
arcsec pixels and by various random calibration uncertainties.
Systematic errors in the positions are smaller than 0.05 arcsec.
Note on Wflag:
`W' is a warning flag indicating that the source may be a sidelobe of
a nearby bright source. In the catalog, 19,696 sources (3.6%) are
flagged as possible sidelobes. The flagging algorithm is an artificial
intelligence approach based on oblique decision trees. Our analysis of
the northern catalog indicates that < 10% of the objects flagged as
sidelobes are real sources and that less than 1% of the unflagged
sources in the catalog are sidelobes. Complete details of the sidelobe
identification procedure are included in our catalog paper.
IMPORTANT NOTE: The sidelobe flagging in the southern catalog is not
as reliable as in the north because the sidelobe characteristics
change with the zenith angle of the observation. We are working on
improvements to the sidelobe-flagging, but for now it is important to
be cautious in studies of individual southern sources. We recommend
checking the images using the FIRST Cutout Server if there is any
doubt about the reality of particular sources (in either the north or
the south). This is easily done when using the FIRST Search Engine to
search the catalog, since each source selected in the search has a
link to the Cutout Server.
Note on Fpeak, Fint:
Fpeak and Fint are the peak and integrated flux densities measured in
mJy. They are derived by fitting an elliptical Gaussian model to the
source. To correct for the ``CLEAN bias'' effect, 0.25 mJy has been
added to the peak flux density and the integrated flux density has
been multiplied by (1+0.25/Fpeak) (see our Astrophysical Journal paper
and our catalog paper for more details.)
The uncertainty in Fpeak is given by the rms noise at the source
position, while the uncertainty in Fint can be considerably greater
depending on the source size and morphology. For bright sources the
accuracies of Fpeak and Fint are limited to about 5% by systematic
effects. Note that for sources that are not well-described by an
elliptical Gaussian model, Fint is not an accurate measure of the
integrated flux density.
Note on Rms:
Rms is a local noise estimate at the source position measured in mJy.
Rms is computed by combining the measured noise from all grid pointing
images contributing to this coadded map position. Note that the
significance of detection for a source is (Fpeak-0.25)/Rms, not
Fpeak/Rms, because of the CLEAN bias correction to the peak flux
density. The catalog includes only sources brighter than 5 Rms.
FITS images giving the rms noise as a function of position on the sky
are available for the northern and the southern areas. These images
give the rms in mJy/beam tabulated on a ∼3 arcmin grid in RA and
Declination. If there is no source in the catalog at a given position,
the source peak flux density (before CLEAN bias correction) is less
than 5 times the coverage map rms value at that position.
Note on MajAxis, MinAxis, PA:
MajAxis, MinAxis, and PA give the major and minor axes (FWHM in
arcsec) and position angle (degrees east of north) derived from the
elliptical Gaussian model for the source. MajAxis and MinAxis have
been deconvolved to remove blurring by the elliptical Gaussian
point-spread function. (The fitted parameters before deconvolution are
given in the fMaj, fMin, and fPA columns.) In the north the beam is
circular 5.4 arcsec FWHM; in the south it is elliptical, 6.4x5.4
arcsec FWHM, with the major axis running north-south.
Noise can cause the fitted values of the major and minor axes (before
deconvolution) to be smaller than the beam. The corresponding
deconvolved size is given as zero in those cases.
The uncertainties in the deconvolved sizes depend on both the
brightness and the sizes. Objects at the catalog flux density limit
have uncertainties of about 2 arcsec in their sizes (so faint objects
with Maj < 2 arcsec are consistent with point sources.) An simple
empirical estimate of the uncertainty is
Sigma(Size) = 10 arcsec (1/SNR + 1/75)
where SNR is the signal-to-noise ratio defined above.
Note on fMaj, fMin, fPA:
fMaj, fMin, and fPA give the major and minor axes (FWHM in arcsec) and
position angle (degrees east of north) derived from the elliptical
Gaussian model for the source. These are the fitted sizes measured
directly from the image; the elliptical point-spread function has not
been deconvolved.
Note on Field:
The Field Name is the name of the coadded image containing the source.
Note that the field name encodes the position of the field center:
field hhmmm+ddmmm is centered at RA=hh mm.m, Dec=+dd mm.m. The images
are available from several archives and through the FIRST Cutout
Server.
All field names in the current catalog end with E, F, or G, indicating
that they were constructed using the imaging pipeline that applies a
small rotation and scale correction. See the catalog history page for
more information. The G fields are new in this catalog, while sources
extracted from the E and F fields are identical to those in the
previous version of the catalog. Some E fields from the last catalog
have been replaced by new F fields because there were additional grid
images available for the coadding. In addition, the southernmost south
Galactic cap images were reprocessed (producing 'G' images) with an
improved pipeline script that substantially reduces the sidelobe
levels in a small fraction of the fields.
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History:
The catalog file (originally catalog_99jul21.bin.gz) was copied from
http://sundog.stsci.edu/first/catalogs/readme.html
The preceding versions were numbered VIII/48 (1997 April 24) and
VIII/51 (1998 Feb 04) in CDS Archives.
See http://sundog.stsci.edu/first/catalogs/history.html
for the complete version history.
(End) Richard L. White, [StScI] 1999-Dec-24