ACDS V10.0 - Detailed Description of VIII/92

ACDS V10.0 - Tue Dec 17 23:09:43 CET 2024

| # Detailed Description of VIII/92 : The FIRST Survey Catalog, Version 2014Dec17 (Helfand+ 2015) |

Note : this is the description file of the original catalog. This HTML page corresponds well to the original description, but some minor changes in the format may have been introduced in the FITS output files in order to allow for arithmetic operations on quantities such as coordinates and times. h:m:s and d:m:s units are normally converted into degrees, and YYYY-MM-DD into julian days.
Similarly, some slight changes may have been introduced in the units; the value given in the HTML files supersedes the value indicated in the ReadMe file for this catalogue.

    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
       =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:

           FileName      Lrecl  Records   Explanations

    ×                   80        .   This file
    × 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)

    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

       Bytes Format Units   Label     Explanations

       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)

    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

            {sigma}(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.

    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