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

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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