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FIRST: Help

FIRST - Faint Images of the Radio Sky at Twenty cm (FIRST) Source Catalog


Overview

This catalog comprises the Faint Images of the Radio Sky at Twenty cm (FIRST) Survey. The FIRST survey began in 1993, and covers the north and south Galactic caps. Both the northern and southern areas were chosen to coincide with the area planned to be covered by the Sloan Digital Sky Survey. The FIRST Survey uses the VLA (Very Large Array, a facility of the US National Radio Astronomy Observatory, NRAO) at a frequency of 1.4 GHz, and is slated (when complete) to cover about 10,000 square degrees to a sensitivity of about 1 milliJansky (mJy) and 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 catalog presented here is derived from the images.

Catalog Bibcode

1997ApJ...475..479W

References

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

Provenance

This database was updated by the HEASARC in May 2003 based on the file: http://sundog.stsci.edu/first/catalogs/catalog_03apr11.bin.gz and contains the 11 April 2003 Version of the FIRST Source Catalog.

Latest Version

This version of the FIRST Survey is derived from the 1993 through 2002 observations, and contains 811,117 sources covering the north and south Galactic caps. The current catalog covers about a total of about 9033 square degrees of sky (8422 square degrees in the north Galactic cap and 611 square degrees in the south Galactic cap). No new sky area was covered in the south Galactic cap during the last observing season contained in this version, although the southern equatorial strip was reobserved. The results of the repeated observations will be released separately; the current catalog is identical to the previous catalog in the south.

All field names in the current catalog end with a letter E through J, depending on the date of the last catalog release in which the image was modified. The J fields are new in this version of the catalog, while sources extracted from the E, F, G, H and I fields are nearly identical to those in the previous version of the catalog.


Parameters

Name
The full FIRST Source Designation (not part of the original catalog as created by its authors): the source name constructed according to the rule recommended by the Dictionary of Astronomical Nomenclature, viz. '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 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, epsilon, is

  epsilon(90% confidence) = Size * (1/SNR + 1/20)  arcsec
  
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 arcsec by the 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.

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 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, epsilon, is

  epsilon(90% confidence) = Size * (1/SNR + 1/20)  arcsec
  
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 arcsec by the 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.

LII
The Galactic Longitude of the radio source.

BII
The Galactic Latitude of the radio source.

Warning_Flag
This is a flag that is set to `W' as a warning indicating that the source may be a sidelobe of a nearby bright source. The flagging algorithm is an artificial intelligence approach based on oblique decision trees. The authors' 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 their catalog paper (White et al. 1997, ApJ, 475, 479). It is important to note that 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. The authors are working on improvements to the sidelobe-flagging, but, for now, it is important to be cautious in studies of individual southern sources. They 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 (http://sundog.stsci.edu/cgi-bin/searchfirst) to search the catalog, since each source selected in the search has a link to the Cutout Server.

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


Contact Person

Questions regarding the FIRST database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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