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

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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. The present 13Jun05 version is derived from the 1993 through 2011 observations. The catalog covers a total of about 10,635 square degrees of sky (8,444 square degrees in the north Galactic cap and 2,191 square degrees in the south Galactic cap). The area covered is almost identical to the previous version of the catalog; however, many images (particularly in the southern area) have been reprocessed with improved algorithms that have reduced noise and sidelobe levels. Consequently this catalog includes about 25,000 more sources than the previous version. See the coverage maps at http://sundog.stsci.edu/first/catalogs/readme.html#coverage for more details of the area covered. 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 JVLA (the Karl G. Jansky Very Large Array, a facility of the US National Radio Astronomy Observatory, NRAO) at a frequency of 1.4 GHz, and was designed to cover about 10,000 square degrees to a sensitivity of about 1 milliJansky (mJy) and with an angular resolution of about 5". Indeed, over most of the survey area, the detection limit is 1 mJy. A region along the equatorial strip (RA = 21.3 to 3.3 hrs, Dec = -1 to 1 deg) has a deeper detection threshold 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 most previous versions of the catalog.

The co-added images are available on the Internet (see the FIRST page at http://sundog.stsci.edu/first/images.html for details). The source catalog presented here is derived from the images.

Data for the FIRST survey were collected in all VLA B-configurations from Spring 1993 through Spring 2004. For all data collected for the FIRST project, the raw u-v visibility data are placed in the VLA public archive on the day they are taken, and are available for use without restriction. Additional data in the southern Galactic cap were acquired in Spring 2009 and Spring 2011. The VLA was in a hybrid condition in 2009, with some new EVLA receivers and some old VLA receivers. The characteristics of those images are slightly different from the older data, but for most purposes they should be equivalent. In 2011 the EVLA receivers were available with an early version of the new EVLA data system, so there are a number of differences from the old data:

Date	        Frequencies	Bandpass	         Integration

Before 2011	1365, 1435 MHz	2x7 3-MHz channels	 180 seconds
2011	        1335, 1730 MHz	2x64 2-MHz channels	 60 seconds
Note particularly the frequency difference between the new and older data. The new data are in co-added fields with names ending with 'S' (and later letters in the alphabet) and are found entirely in the south Galactic cap.

In the areas observed using the new configuration, there is currently a small flux calibration error that affects the catalog flux density measurements for some sources in the south Galactic cap. Affected sources have observation epochs later than 2011. The authors estimate that for most sources the calibration error is smaller than 3%, although a few percent of the sources have flux density errors as large as 8%. The authors plan to release a new version of the catalog that corrects this problem.


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 (1997)>
    =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 table was last updated by the HEASARC in July 2013 based on the file: http://sundog.stsci.edu/first/catalogs/catalog_13jun05.bin.gz which contains the 05 June 2013 version of the FIRST Source Catalog.

Latest Version

This version of the FIRST Survey is derived from the 1993 through 2011 observations, and contains 971,268 sources covering the north and south Galactic caps. The current catalog covers about a total of about 10,635 square degrees of sky (8444 square degrees in the north Galactic cap and 2191 square degrees in the south Galactic cap).

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 05 June 2013 version of the FIRST Catalog differs from that in the previous version (16 February 2012) of the catalog. It contains 2 new parameters which give information on the mean epoch of observation (called mean_epoch herein) and the rms range of the observations (called rms_epoch herein) for each source. Two other changes were new to the 16 February 2012 version of the FIRST Catalog: (1) the sidelobe probability parameter (P(S), called sidelobe_prob herein), which indicates the probability that the source is a sidelobe, replaced the previous binary sidelobe flag parameter; (ii) parameters which gave information on counterparts to the FIRST sources in the SDSS DR8 catalog and the 2MASS catalog were added. These parameters are described in detail below. All of these catalogs have multiple wavebands available; the given magnitudes in the present catalog are in the i (SDSS) and K (2MASS) bands.

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"     54.2%
    SDSS        Counterpart < 1.5"              27.9%
    SDSS        Covered by DR8                  93.1%
    2MASS       1 or more counterparts < 8"     12.5%
    2MASS       Counterpart < 1.5"               7.4%

Parameters

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 (flux_20_cm in this table), 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      74.9%
      0.05 < P(S) < 0.15       7.0%
      0.15 < P(S) < 0.25       4.5%
      0.25 < P(S) < 0.35       3.8%
      0.35 < P(S) < 0.45       2.3%
      0.45 < P(S) < 0.55       1.9%
      0.55 < P(S) < 0.65       1.7%
      0.65 < P(S) < 0.75       1.6%
      0.75 < P(S) < 0.85       1.3%
      0.85 < P(S) < 0.90       0.9%
  
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 per beam (mJy/beam). 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 per beam (mJy/beam), 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 co-added 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 times the 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. The coverage in the southern Galactic cap is ragged (and discontiguous) at the northern edge because poor weather and equipment failures prevented the planned survey area from being filled in there.

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 arcseconds. 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 arcseconds. 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 co-added 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 '-').

Warning: The sky coverage for SDSS DR9 relies on the fInFootprintEq SQL function, which currently gives the wrong answer in some sky areas because it includes observations that were not actually included in the DR9 catalog. The FIRST Catalog authors have attempted to adjust the coverage empirically, but unfortunately the information in this catalog on the SDSS sky coverage is somewhat unreliable in this version of the FIRST catalog. The errors affect a few percent of the FIRST sources. The authors will release an updated catalog when this problem with the DR9 database has been corrected.

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.

Mean_Epoch
The mean epoch of the FIRST flux density measurement of the source. The FIRST survey consists of 3-minute integrations at a grid of points on the sky. But the catalogs are derived from co-added images that use a weighted sum of the individual pointing maps to achieve nearly uniform sensitivity. (See the FIRST survey paper for more details.) That means that each measurement is a weighted sum of multiple observations. The mean_epoch give the weighted mean of all the contributing pointing epochs at the position of the source.

RMS_Epoch
The RMS of the FIRST flux density measurement of the source, in days. This parameter 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 7.7 years.


Contact Person

Questions regarding the FIRST database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 18-Jul-2013