- HEASARC Director: Dr. Alan P. Smale
- HEASARC Associate Director: Dr. Roger Brissenden
- NASA Official: Phil Newman
- Web Curator: Meredith Gibb
- Last Modified: 2-Apr-2012
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FIRST - Faint Images of the Radio Sky at Twenty cm (FIRST) |
HEASARC Archive |
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
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.ucllnl.org/cgi-bin/firstcutout : the FIRST cutout server
http://www.cv.nrao.edu/first/ : FIRST page at NRAO
This version of the catalog has a region along the equatorial strip (RA = 21.3 to 3.3 hrs, Dec = -1 to 1 deg) where the detection threshold is deeper than 1 mJy because two epochs of observation were combined. The typical detection threshold in this region is 0.75 mJy. There are approximately 4,500 sources below the 1-mJy threshold used for some previous versions of the catalog.
The format of this version of the FIRST Catalog differs from that in some previous versions of the catalog. The sidelobe probability parameter (P(S), called sidelobe_prob herein), which indicates the probability that the source is a sidelobe, replaces the previous binary sidelobe flag parameter. Also present in this version are parameters which give information on counterparts to the FIRST sources in the SDSS DR8 catalog and the 2MASS catalog. These parameters are described in detail below. All of these catalogs have multiple wavebands available; the given magnitudes in the present catalog are i (SDSS) and K (2MASS).
Some information summarizing the frequency of counterparts in the various catalogs is given in the table below:
Survey Characteristic Percentage
SDSS 1 or more counterparts < 8" 58.3%
SDSS Counterpart < 1.5" 29.9%
SDSS Covered by DR8 94.0%
2MASS 1 or more counterparts < 8" 11.6%
2MASS Counterpart < 1.5" 7.5%
Name
The full FIRST Source Designation (not part of the original catalog as
created by its authors): the source name was constructed by the HEASARC
according to the rule recommended by the CDS Dictionary of Nomenclature of
Celestial Objects, viz. the prefix 'FIRST J' followed by the truncated J2000
position.
RA
The Right Ascension of the radio source in the selected equinox: this
was given in J2000 equatorial coordinates with a precision of 0.001 seconds
of time in the original catalog. 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 arcsecond at 90% confidence; 2 mJy point sources in typically
noisy regions have positions good to 0.5 arcseconds.
An empirical expression for the positional accuracy, epsilon, is
epsilon(90% confidence) = Size * (1/SNR + 1/20) arcseconds
where Size is either the major or minor axis fitted FWHM (fit_major_axis or
fit_minor_axis) as given in the catalog and SNR is the peak flux density,
Fpeak, signal-to-noise ratio given by 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 arcseconds by
the ability to fit source positions in maps with 1.8 arcsecond pixels and by
various random calibration uncertainties. Systematic errors in the positions
are smaller than 0.05 arcseconds.
Dec
The Declination of the radio source in the selected equinox: this
was given in J2000 equatorial coordinates with a precision of 0.01 arcseconds
in the original catalog. 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 arcsecond at 90% confidence; 2 mJy point sources in typically
noisy regions have positions good to 0.5 arcseconds.
An empirical expression for the positional accuracy, epsilon, is
epsilon(90% confidence) = Size * (1/SNR + 1/20) arcseconds
where Size is either the major or minor axis fitted FWHM (fit_major_axis or
fit_minor_axis) as given in the catalog and SNR is the peak flux density,
Fpeak, signal-to-noise ratio given by 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 arcsecond by the
ability to fit source positions in maps with 1.8 arcsecond pixels and by
various random calibration uncertainties. Systematic errors in the positions
are smaller than 0.05 arcseconds.
LII
The Galactic Longitude of the radio source.
BII
The Galactic Latitude of the radio source.
Sidelobe_Prob
The probability P(S) that the radio source is spurious,
most commonly because it is a sidelobe of a nearby bright source.
Low values for this parameter mean the source is unlikely to be spurious.
Here is the distribution of P(S) values in this version of the catalog:
Probability Range Fraction of sources
0.00 < P(S) < 0.05 76.5%
0.05 < P(S) < 0.15 7.1%
0.15 < P(S) < 0.25 4.5%
0.25 < P(S) < 0.35 3.5%
0.35 < P(S) < 0.45 2.0%
0.45 < P(S) < 0.55 1.6%
0.55 < P(S) < 0.65 1.5%
0.65 < P(S) < 0.75 1.4%
0.75 < P(S) < 0.85 1.0%
0.85 < P(S) < 0.90 0.8%
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; the authors
believe it is a substantial improvement over the previous sidelobe flagging
approach. Nevertheless, they still recommend checking the images using the
FIRST Cutout Server at http://third.ucllnl.org/cgi-bin/firstcutout if there
is any doubt about the reality of particular sources.
Flux_20_cm
The peak flux density at 1.4 GHz, in milliJanskies (mJy).
The peak and integrated flux densities 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 the published papers
for more details).
The uncertainty in the peak flux density is given by the rms noise at the source position, while the uncertainty in the integrated flux density can be considerably greater, depending on the source size and morphology. For bright sources the accuracies of the peak and integrated flux densities are limited to about 5% by systematic effects. Note that, for sources that are not well-described by an elliptical Gaussian model, the integrated flux density calculated as above is not an accurate measure of the actual integrated flux density.
Int_Flux_20_cm
The integrated flux density at 1.4 GHz, in milliJanskies
(mJy). The peak and integrated flux densities 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 the published papers
for more details).
The uncertainty in the peak flux density is given by the rms noise at the source position, while the uncertainty in the integrated flux density can be considerably greater, depending on the source size and morphology. For bright sources the accuracies of the peak and integrated flux densities are limited to about 5% by systematic effects. Note that, for sources that are not well-described by an elliptical Gaussian model, the integrated flux density calculated as above is not an accurate measure of the actual integrated flux density.
Flux_20_cm_Error
The estimated error in the flux density, in milliJanskies
(mJy), is a local rms noise estimate at the source position. The 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, Fpeak. 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.
Major_Axis
The Major Axis FWHM, in arcseconds. major_axis, minor_axis,
and position_angle (measured in degrees, east of north) are derived from the
elliptical Gaussian model for the source. The major_axis and minor_axis
are the values after deconvolution to remove blurring by the elliptical
Gaussian point-spread function. (The fitted parameters before deconvolution
are given in the fit_major_axis, fit_minor_axis, and fit_position_angle
parameter). In the north, the beam is circular, with a 5.4 arcsec FWHM, south
of declination +4 degrees 33' 21", the beam is elliptical, with a 6.4" x 5.4"
FWHM, with the major axis running north-south, while in the southern Galactic
cap (RA = 21 hrs to 3 hrs), the elliptical beam size increases further to
6.8" x 5.4" south of declination -2 degrees 30' 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 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 major_axis <2 arcsec are consistent with point sources). A simple empirical estimate of the uncertainty is Sigma(Size) = 10 arcsec * (1/SNR + 1/75), where SNR is the signal-to-noise ratio defined as SNR = (Fpeak-0.25) / rms.
Minor_Axis
The Minor Axis FWHM, in arcseconds. major_axis, minor_axis,
and position_angle (measured in degrees, east of north) are derived from the
elliptical Gaussian model for the source. The major_axis and minor_axis
are the values after deconvolution to remove blurring by the elliptical
Gaussian point-spread function. (The fitted parameters before deconvolution
are given in the fit_major_axis, fit_minor_axis, and fit_position_angle
parameter). In the north, the beam is circular, with a 5.4 arcsec FWHM, south
of declination +4 degrees 33' 21", the beam is elliptical, with a 6.4" x 5.4"
FWHM, with the major axis running north-south, while in the southern Galactic
cap (RA = 21 hrs to 3 hrs), the elliptical beam size increases further to
6.8" x 5.4" south of declination -2 degrees 30' 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 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 major_axis <2 arcsec are consistent with point sources). A simple empirical estimate of the uncertainty is Sigma(Size) = 10 arcsec * (1/SNR + 1/75), where SNR is the signal-to-noise ratio defined as SNR = (Fpeak-0.25) / rms.
Position_Angle
The Position Angle of the Major Axis, in degrees,
measured in an eastwards direction from north. major_axis, minor_axis,
and position_angle (measured in degrees, east of north) are derived from the
elliptical Gaussian model for the source. The major_axis and minor_axis
are the values after deconvolution to remove blurring by the elliptical
Gaussian point-spread function. (The fitted parameters before deconvolution
are given in the fit_major_axis, fit_minor_axis, and fit_position_angle
parameter). In the north, the beam is circular, with a 5.4 arcsec FWHM, south
of declination +4 degrees 33' 21", the beam is elliptical, with a 6.4" x 5.4"
FWHM, with the major axis running north-south, while in the southern Galactic
cap (RA = 21 hrs to 3 hrs), the elliptical beam size increases further to
6.8" x 5.4" south of declination -2 degrees 30' 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 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 major_axis <2 arcsec are consistent with point sources). A simple empirical estimate of the uncertainty is Sigma(Size) = 10 arcsec * (1/SNR + 1/75), where SNR is the signal-to-noise ratio defined as SNR = (Fpeak-0.25) / rms.
Fit_Major_Axis
The Fitted Major Axis before Deconvolution, in arcsecs.
fit_major_axis, fit_minor_axis, and fit_position_angle give the major and
minor axes (FWHM in arcsec) and position angle (in degrees, measured eastwards
from the 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.
Fit_Minor_Axis
The Fitted Minor Axis before Deconvolution, in arcsecs.
fit_major_axis, fit_minor_axis, and fit_position_angle give the major and
minor axes (FWHM in arcsec) and position angle (in degrees, measured eastwards
from the 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.
Fit_Position_Angle
The Position Angle of the Fitted Major Axis, in degrees
measured eastwards from the north. fit_major_axis, fit_minor_axis, and
fit_position_angle give the major and minor axes (FWHM in arcsec) and position
angle 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.
Field_Name
This is the name of the coadded image containing the source.
Note that the field name encodes the position of the field center: thus,
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 at
http://third.ucllnl.org/cgi-bin/firstcutout .
All field names in the current catalog end with a letter E through S, depending
on the date of the last catalog release in which the image was modified. The Q,
R and S fields are new in this catalog, while sources extracted from the E-N
fields are essentially identical to those in the previous version of the
catalog. The Q fields include contributions from data taken in Spring 2009;
the R fields include remade grid images that were found to have instrument or
processing problems; and the S fields include data taken in Spring 2011.
SDSS_Matches
The number of potential optical counterparts to the radio
source in the Sloan Digital Sky Survey (SDSS) DR8 within a fiducial radius of
8 arcseconds. The SDSS matches were found using the SDSS-III CasJobs web
interface at http://skyservice.pha.jhu.edu/casjobs/ .
A count of zero indicates there are no sources within this radius (which is
also indicated by a null value for the corresponding offset and a
classification of '-').
SDSS_FIRST_Offset
The angular offset or separation of the nearest matching
SDSS source from the FIRST radio source, in arcseconds.
SDSS_Imag
The i magnitude of the nearest matching SDSS source.
A magnitude of -1 indicates that the magnitude in the DR8 catalog was given
as -9999.
SDSS_Class
The morphological classification of the nearest matching SDSS
source, where 's' stands for stellar and 'g' for non-stellar/galaxy.
TwoMASS_Matches
The number of potential infrared counterparts to the radio
source in the 2MASS Catalog within a fiducial radius of
8 arcseconds. The 2MASS matches were found using the SDSS-III CasJobs web
interface at http://skyservice.pha.jhu.edu/casjobs/ .
A count of zero indicates there are no sources within this radius (which is
also indicated by a null value for the corresponding offset and a
classification of '-')
TwoMASS_FIRST_Offset
The angular offset or separation of the nearest
matching 2MASS source from the FIRST radio source, in arcseconds.
TwoMASS_Kmag
The K magnitude of the nearest matching 2MASS source.