VIII/92 The FIRST Survey Catalog, Version 2014Dec17 (Helfand+ 2015)
The last of FIRST: the final catalog and source identifications.
Helfand D.J., White R.L., Becker R.H.
<Astrophys. J. 801, 26 (2015)>
=2015ApJ...801...26H 2015ApJ...801...26H
=2015yCat.8092....0H 2015yCat.8092....0H
ADC_Keywords: Radio sources ; Surveys
Description:
The Faint Images of the Radio Sky at Twenty centimeters (FIRST) 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 of the North and South Galactic Caps, to a
sensitivity of about 1mJy with an angular resolution of about 5''. The
images produced by an automated mapping pipeline have pixels of
1.8'', a typical rms of 0.15mJy, and a resolution of 5''; the 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 catalog from the 1993 through 2011 observations contains 946,432
sources from the north and south Galactic caps. It covers a total of
10,575 square degrees of the sky (8444 square degrees in the north and
2131 square degrees in the south).
In this version of the catalog, images taken in the the new EVLA
configuration have been re-reduced using shallower CLEAN thresholds in
order to reduce the "CLEAN bias" in those images. Also, the EVLA
images are not co-added with older VLA images to avoid problems
resulting from the different frequencies and noise properties of the
configurations. That leads to small gaps in the sky coverage at
boundaries between the EVLA and VLA regions. As a result, the area
covered by this release of the catalog is about 60 square degrees
smaller than the earlier release of the catalog (13Jun05, also
available here as the "first13.dat" file), and the total number of
sources is reduced by nearly 25,000. The previous version of the
catalog does have sources in the overlap regions, but their flux
densities are considered unreliable due to calibration errors. The
flux densities should be more accurate in this catalog, biases are
smaller, and the incidence of spurious sources is also reduced.
Over most of the survey area, the detection limit is 1 mJy. A region
along the equatorial strip (RA=21.3 to 3.3hr, Dec=-1 to 1deg) has a
deeper detection threshold because two epochs of observation were
combined. The typical detection threshold in this region is 0.75mJy.
There are approximately 4,500 sources below the 1mJy threshold used
for most previous versions of the catalog.
The previous versions
http://sundog.stsci.edu/first/catalogs/
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
first14.dat 189 946432 The FIRST survey catalog, 14Dec17 Version
first13.dat 189 971268 The FIRST survey catalog, 13jun05 Version
(25,000 more sources, but with unreliable flux)
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See also:
http://sundog.stsci.edu/ : home page of the VLA FIRST Survey
http://sundog.stsci.edu/first/catalogs/readme_14mar04.html : this version
http://sundog.stsci.edu/first/catalogs/history.html : Version History
http://sundog.stsci.edu/cgi-bin/searchfirst : the FIRST search engine
http://third.ucllnl.org/cgi-bin/firstcutout : the FIRST Cutout Server.
Byte-by-byte Description of file: first14.dat, first13.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- FIRST FIRST Source designation (1)
18- 19 I2 h RAh Right Ascension J2000 (hours) (2)
21- 22 I2 min RAm Right Ascension J2000 (minutes) (2)
24- 29 F6.3 s RAs Right Ascension J2000 (seconds) (2)
31 A1 --- DE- Declination J2000 (sign) (2)
32- 33 I2 deg DEd Declination J2000 (degrees) (2)
35- 36 I2 arcmin DEm Declination J2000 (minutes) (2)
38- 42 F5.2 arcsec DEs Declination J2000 (seconds) (2)
44- 48 F5.3 --- p(S) [0,1] Probability of being a side lobe (3)
50- 57 F8.2 mJy Fpeak Peak flux density at 1.4GHz (4)
59- 67 F9.2 mJy Fint Integrated flux density at 1.4GHz (4)
70- 75 F6.3 mJy Rms Local noise estimate (5)
77- 82 F6.2 arcsec Maj Major axis (FWHM) (6)
84- 89 F6.2 arcsec Min Minor axis (FWHM) (6)
91- 95 F5.1 deg PA [0/180] Position angle (6)
97-102 F6.2 arcsec fMaj Fitted major axis before deconvolution (7)
104-109 F6.2 arcsec fMin Fitted minor axis before deconvolution (7)
111-115 F5.1 deg fPA [0/180] Fitted PA before deconvolution (7)
117-128 A12 --- Field Name of the coadded image containing
the source (8)
130-131 I2 --- N1 [0/10]?=-1 Number of SDSS-DR10 counterparts (9)
133-137 F5.2 arcsec r1 [0/8]?=99 Closest SDSS match (9)
139-143 F5.2 mag m1 ?=99 SDSS i magnitude of closest SDSS match (9)
145 A1 --- c1 [sg-] SDSS class: s=star, g=galaxy
147-148 I2 --- N2 Number of 2MASS counterparts (10)
150-154 F5.2 arcsec r2 [0/8]?=99 Closest 2MASS match (10)
156-160 F5.2 mag m2 ?=99 2MASS Ks mag. of closest 2MASS match (10)
162-169 F8.3 yr Ep Mean epoch of the FIRST flux density (11)
171-179 F9.1 d Ep(JD) Mean epoch in Julian Day (11)
182-189 F8.3 d s_Ep(JD) Epoch rms (11)
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Note (1):
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 (2):
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 (3):
p(S) indicates the probability that the source is spurious (most
commonly because it is a sidelobe of of a nearby bright source). Low
values mean the source is unlikely to be spurious.
Sidelobe probabilities for this version of the catalog have been
computed using an improved algorithm based on multiple voting oblique
decision tree classifiers. The classifiers were trained using deep VLA
fields that give reliable assessments of the reality of FIRST sources.
The algorithm will be described in detail in a future paper on the
final FIRST catalog; nevertheless, we still recommend checking the
images.
Note (4):
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 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 (5):
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 ∼3arcmin 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.
The sky area covered is displayed in the images at
http://sundog.stsci.edu/first/catalogs/
Note (6):
Maj, Min, 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 5.4arcsec FWHM;
south of +04d33'21" it is elliptical, 6.4x5.4arcsec FWHM, with the
major axis running north-south. In the southern Galactic cap (RA = 21h
to 3h), the elliptical beam size increases further to 6.8x5.4arcsec
south of declination -02d30'25"
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 this case.
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 2arcsec in their sizes (so faint objects
with Maj<2arcsec are consistent with point sources). A simple
empirical estimate of the uncertainty is
σ(Size) = 10arcsec (1/SNR + 1/75)
where SNR is the signal-to-noise ratio defined above.
Note (7):
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 (8):
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 a letter E through X,
depending on the date of the last catalog release in which the image
was modified. The W and X fields are new in this catalog, while
sources extracted from the E-T fields are essentially identical to
those in the previous version of the catalog. The W fields include
contributions from EVLA data taken in Spring 2011 and have both a
slightly different central frequency (1.335GHz instead of 1.400GHz)
and typically higher noise levels than the older images. The X fields
are images that neighbor the EVLA fields but differ from the previous
release in that they omit nearby EVLA observations from the co-adding
of overlapping grid images.
Note (9):
These columns give information on optical counterparts from the
SDSS-DR10 catalog (Cat. V/139). The matches were performed using the
SDSS-III CasJobs web interface.
There is a column giving the number of matches within a fiducial
radius of 8arcsec and, for the nearest match, the separation from the
FIRST position in arcsec and the magnitude. The morphological
classification is also given (s=stellar, g=nonstellar/galaxy). A count
of zero indicates there are no sources within this radius (which is
also indicated by a separation of 99.00 and a classification of '-');
a count of -1 indicates that the FIRST source falls outside the SDSS
DR10 survey area so that no SDSS data are available. The given
magnitudes corresponds to i-band; a magnitude of -1 indicates that the
magnitude in the DR10 catalog was given as -9999.
Note (10):
These columns give information on near-infrared counterparts from the
2MASS catalog (Cat. II/246): number of matches within a fiducial
radius of 8arcsec, the separation between the FIRST source and the
nearest match, and the Ks magnitude.
Note (11):
The Epoch Mean columns give the weighted mean of all the contributing
pointing epochs at the position of the source. It is given both in
decimal years and in JD for convenience. The s_Ep(JD) column gives the
weighted rms of the pointing epochs at the source position. It is a
measure of the spread in epochs that contribute to the measurement.
Many sources have small rms values of only a few minutes (dominated by
a single 3-minute pointings or by 2 adjacent pointings), but values of
days to weeks are also common (for sources observed in the overlap
between declination strips), and some objects have rms values of years
(for sources observed at the edges of regions in different observing
seasons or that were observed multiple times due to data problems).
The largest epoch rms in the survey is 6.8years.
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History:
The catalog file (originally catalog_12feb16.bin.gz) was copied from
http://sundog.stsci.edu/first/catalogs/ on 25-Sep-2012.
The preceding versions were numbered in CDS Archives:
VIII/71 (2003 April 11, 811117 sources)
VIII/59 (1999 July 21, 549707 sources)
VIII/51 (1998 Feb 04, 437429 sources)
VIII/48 (1997 April 24, 236040 sources)
See http://sundog.stsci.edu/first/catalogs/history.html
for the complete version history.
(End) Robert H. Becker [LLNL], Richard L. White [STScI] 17-May-2015