VIII/48 the FIRST Survey (White+ 1997)
The FIRST Survey Catalog
White R.L., Becker R.H., Helfand D.J., Gregg M.D.
<Astrophys. J. 475, 479 (1997)>
=1997ApJ...475..479W 1997ApJ...475..479W
ADC_Keywords: Radio sources ; Surveys
Description:
This version of the FIRST Survey is derived from the 1993 through 1996
observations, and contains 236,177 sources for the north Galactic cap,
and 31,870 sources for the south Galactic cap. The northern catalog
covers about 2575 square degrees of sky, including most of the area:
7h20m < RA(2000) < 17h20m 22.2° < Dec < 42.5°
The southern catalog covers about 350 square degrees of sky, including
most of the area:
21h20m < RA(2000) < 3h20m -2.5° < Dec < 1.6°
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
north.dat 124 236177 North Galactic Cap
(7h20<RA<17h20, 22.2deg<Dec<42.5deg)
south.dat 124 31870 South Galactic Cap
(21h20<RA<3h20, -2.5deg<Dec<+1.6deg)
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See also:
http://sundog.stsci.edu/top.html : main page of the VLA FIRST Survey
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
56- 63 F8.2 mJy Fint *Integrated flux density
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 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, 8,779 sources (3.7%) are
flagged as possible sidelobes; in the southern catalog, 1,331 sources
(4.2%) are flagged as possible sidelobes. The flagging algorithm is an
artificial intelligence approach based on oblique decision trees. Our
analysis indicates that <10% of the objects flagged as sidelobes are
real sources and that considerably 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.
NOTE: The sidelobe flagging in the southern catalog may not be 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.
A FITS image giving the rms noise as a function of position on the sky
is available. This is a 2200 x 479 pixel image (4.2 Mbytes) with 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. Maj and Min 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, 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.
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History:
The files (originally catalognorth97apr24.bin and
catalogsouth97apr24.bin) were copied from
http://sundog.stsci.edu/first/catalogs/readme.html
(End) Richard L. White, [StScI] 1997-Apr-24