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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 14Dec17 version is derived from the 1993 through 2011 observations. The catalog covers a total of about 10,575 square degrees of sky (8,444 square degrees in the north Galactic cap and 2,131 square degrees in the south Galactic cap). See the coverage maps at http://sundog.stsci.edu/first/catalogs/readme_14dec17.html#coverage for more details of the area covered. Both the northern and southern areas were chosen to coincide approximately with the area covered by the Sloan Digital Sky Survey (SDSS). The catalog is identical to the previous version of the catalog (14Mar04) except that it has more accurate data on which sources are not covered by the SDSS DR10 catalog. Approximately 1000 sources that were indicated as covered by DR10 in the previous version are now correctly marked as not covered. The source list, radio fluxes, etc., are all the same as the 14Mar04 version.

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), 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 3.3 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 format of this catalog is the same as releases since 13Jun05 but differs from earlier versions of the catalog. It contains two parameters which give information on the epoch of observation for each source (called mean_epoch and rms_epoch in this HEASARC version) which are described below. The P(S) parameter (called sidelobe_prob herein), which indicates the probability that the source is a sidelobe, replaces the previous binary sidelobe flag column. The parameters sdss_matches, sdss_first_offset, sdss_imag, sdss_class, twomass_matches, twomass_first_offset and twomass_kmag give information on counterparts to the FIRST source in the SDSS DR10 catalog and the 2MASS catalog, respectively. Other catalog parameters are common with FIRST catalog releases extending back over the past decade.

The co-added images are available online: 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.

Catalog Bibcodes

2015ApJ...801...26H
1997ApJ...475..479W

References

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

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 May 2015 based on the file: http://sundog.stsci.edu/first/catalogs/catalog_14dec17.bin.gz which contains the 17 December 2014 version of the FIRST Source Catalog.

Latest Version

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"     54.5%
    SDSS        Counterpart < 1.5"              28.3%
    SDSS        Covered by DR10                 93.0%
    2MASS       1 or more counterparts < 8"     11.6%
    2MASS       Counterpart < 1.5"               7.5%

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 and registered by the authors with the IAU, viz. the prefix 'FIRST J' followed by the truncated (not rounded) J2000.0 positional coordinates.

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      76.3%
      0.05 < P(S) < 0.15       7.0%
      0.15 < P(S) < 0.25       4.4%
      0.25 < P(S) < 0.35       3.5%
      0.35 < P(S) < 0.45       2.1%
      0.45 < P(S) < 0.55       1.7%
      0.55 < P(S) < 0.65       1.6%
      0.65 < P(S) < 0.75       1.5%
      0.75 < P(S) < 0.85       1.1%
      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 is described in detail in Section 4 of the 2015 reference paper on the final FIRST catalog cited above; 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.

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 at http://sundog.stsci.edu/first/catalogs/coverage-north-3arcmin-14dec17.fits and http://sundog.stsci.edu/first/catalogs/coverage-south-3arcmin-14dec17.fits, respectively. 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. Small gaps can be seen around the edges of the area observed using the EVLA configuration in 2011, as noted in the Overview above.

Major_Axis
The major axis FWHM, in arcseconds. The parameters 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. The parameters 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. The parameters 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 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 (1.335 GHz instead of 1.400 GHz) 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.

SDSS_Matches
The number of potential optical counterparts to the radio source in the Sloan Digital Sky Survey (SDSS) DR10 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 '-'). A count of -1 indicates that the FIRST source falls outside the SDSS DR10 survey area so that no SDSS data are available. The SDSS catalog has multiple wavebands available; the given magnitude herein is for the i band. An SDSS i magnitude of -1 indicates that the magnitude in the DR10 catalog was given as -9999.

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 DR10 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 '-'). The 2MASS catalog has multiple wavebands available; the given magnitude herein is for the K band.

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 papers 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 6.8 years.

The epoch rms should be used as a guide to identify objects that do not have well-defined epochs. The table below gives an indication of the frequency of different epoch rms values in the catalog:

   Epoch rms range   Fraction

    < 5  minutes       5%
   5 min - 1 day      29%
     1 - 10 days      44%
    10 - 100 days     11%
   100 days - 1 yr     7%
     1 - 2 yrs         1%
     2 - 5 yrs         2%
    > 5 yrs          0.3%
  

Contact Person

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

Page Author: Browse Software Development Team
Last Modified: 19-May-2015