IRAS Faint Source Catalog Format

The FSC was produced in "long" and "short" versions, the long version containing more detailed information about the processing history of each source. The "short" FSC, available here, is in FITS table format with the information about associations of FSC sources with sources in other astronomical catalogs in a separate file.

The catalog consists of the following:

• Two files - one for the non-association data (FSC_DATA) and one for the association data (FSC_ASSOC).
• There are two headers, each with 80-byte records, within each file.
• The first header in each file identifies that it is written in FITS format.
• The second header constitutes the FITS keyword file giving the format information for the rest of the file.
• The data part (i.e., non-header part) of FSC_DATA has 240-byte records.
• The data part of FSC_ASSOC has 64-byte records.

Specifically, the formats of the files are as follows:

• FILE 1:
  • Block 1 - short header file
  • Blocks 2-10 - FSC_DATA header
  • Blocks 11-END - FSC_DATA
• FILE 2:
  • Block 1 - short header file
  • Blocks 2-4 - FSC_ASSOC header
  • Blocks 5-END - FSC_ASSOC

    Table 1.  Format of FSC Data File for Short FSC File
Start
Byte  Name       Description                         Units              Format
   0  NAME *     Source Name                         _                    12A1
  12  RAHR       Right Ascension 1950                Hour of time           I2
  14  RAMIN      Right Ascension 1950                Minute of time         I2
  16  RASEC *    Right Ascension 1950                deci-Second of time    I3
  19  DECSGN *   Declination Sign                    ±                      A1
  20  DECDEG     Declination 1950                    Arc Degree             I2
  22  DECMIN     Declination 1950                    Arc Minute             I2
  24  DECSEC *   Declination 1950                    Arc Second             I2
  26  UNCMAJ *   Uncertainty ellipse major axis      Arc Second             I3
  29  UNCMIN *   Uncertainty ellipse minor axis      Arc Second             I3
  32  POSANG *   Uncertainty ellipse position angle  Degree (East           I3
                                                     of North)
  35  NOBS *+    Number of times observed in         _                     4I3
                 each band
  47  FNU *+     Flux densities (1 value per         Jansky (10^-26      4E9.3
                 band, non-color corrected)          W m-^2 Hz^-1)
  83  FQUAL *+   Flux density quality                _                     4I1
                 (1 value per band)
  87  RELUNC +   Percent relative flux density       _                     4I3
                 uncertainties (1 value/band)
  99  MINREL *   Minimum percent reliability         _                      I2
 101  MEDSNR *+  SNR in each band                    _                   4E7.1
 129  LOCSNR *+  Local SNR in each band              _                   4E7.1
 157  AREA *+    Number of pixels above              _                     4I3
                 threshold in each band
 169  CATNBR *   No. of nearby catalog sources       _                      I2
 171  EXTNBR *+  No. of nearby extractions           _                     4I2
                 in each band
 179  CIRRUS *   No. of nearby 100-micron-only       _                      I2
                 extractions
 181  CONFUSE *  Confusion flag (1 flag/band,        _                      A2
                 hex encoded)
 183  NOISCOR +  Noise correction factor             _                   4F5.2
                 (1 value per band)
 203  NID *      No. of positional associations      _                      I2
 205  IDTYPE     Type of object                      _                      I2
 207  NOISRAT    Ratio 85% to 68% of flux            _                   4F5.3
                 distribution (1 value per band)
 227  SPARE                                          -                    13A1

   * Quantities listed in microfiche version of catalog
   + In the FITS header, these quantities are suffixed by the wavelength.
     Example: FNU(4) is given as FNU_12, FNU_25, FNU_60, and FNU_100.


      Table 2.  Format of Association Data for FSC File
Start
Byte  Name       Description                         Units               Format
 0    NAME *     Source Name                         _                     12A1
12    RECNO *    Record No. in main data table       _                       I6
18    CATNO *    Catalog No.                         _                       I2
20    SOURCE *   Source ID                           _                     15A1
35    TYPE *     Source Type/Spectral Class          _                      5A1
40    RADIUS *   Radius vector from IRAS             Arc Second              I3
                 source to association
43    POS        Position angle from IRAS            Degree (East            I3
                 source to association               of North)
46    DSTMAJOR   Distance from IRAS source to        Arc Second              I3
                 association along the IRAS position
                 error major axis
49    DSTMINOR   Distance from IRAS source to        Arc Second              I3
                 association along the IRAS
                 position error minor axis
52    FIELD1 +   Object field #1                     Catalog                 I4
                 (magnitude/other)                   dependent
56    FIELD2 +   Object field #2                     Catalog                 I4
                 (magnitude/other)                   dependent
60    FIELD3     Object field #3                     Catalog                 I4
                 (magnitude/other)                   dependent

   * Quantities listed in printed version of catalog
   + FIELD1 is listed in microfiche version of catalog, except for catalogs 2
     and 19, where FIELD2 is listed.

Table V.C.2 of the Explanatory Supplement to the IRAS Faint Source Survey gives a sample of the header for the non-association data and Table V.C.3 gives a sample of the header for the association data. Tables 1 (FSC data) and 2 (association data) describe each entry in the short catalog file. All of this information is contained in the actual FITS headers, but is presented here in an easier-to-read format. Those columns that are also included in the microfiche version are marked. Each catalog entry requires 240 bytes of ASCII data for the non-association data and NID x 64 bytes of ASCII data for the association data. In these tables the column "Format" refers to the length and type of the (FORTRAN) character field used to read or write each entry.

In general, for quantities that have a value in each wavelength band, subscripts or array indices range from 1 to 4 and refer, respectively, to 12, 25, 60 and 100 microns. A number of the flags discussed below have values in each of the four wavelength bands. For compactness these are encoded into a single base-16 (Hex) digit (values 0-F) in the following manner. The four bits of the hex digit correspond to the four wavelength bands with bit 0 (Least Significant Bit) for 12 microns, bit 1 for 25 microns, bit 2 for 60 microns and bit 3 for 100 microns. The presence of a flag in a band is denoted by setting its bit to 1. Thus a flag set at 12 and 25 microns would have a value of 0011=3(Hex) while a flag set at 25, 60 and 100 microns would have a value of 1110=E (Hex). A flag encoded in this manner will be referred to as "hex-encoded by band".

The remainder of this section discusses individual entries in the catalog.

Source Name: NAME

The IRAS source name is derived from its position by combining the hours, minutes and tenths of minutes of right ascension and the sign, degrees and minutes of the declination. In obtaining the minutes of right ascension and declination for the name, the positions were truncated. This quantity is preceded by the letter 'F' to designate a FSC source, and the letter 'Z' to designate a Reject File source. The letters 'A', 'B', 'C', etc., are appended to names of sources so close together that they would otherwise have had identical names. Due to the large number of duplicate sources caused by plate overlap, we have changed the convention used in the PSC and eliminated the 'A' for the first such source with a duplicate name. Names were uniquely assigned to both catalog and reject file sources, including duplicate sources, with catalog sources named first. See Source Designations for IRAS Catalogs.

Position: HOURS, MINUTE, SECOND, DSIGN, DECDEG, DECMIN, DECSEC

Positions are given for the equinox 1950.0 and epoch 1983.5. Hours (HOURS) and minutes (MINUTE) of right ascension are given as integers while seconds (SECOND) are rounded to integer deciseconds. The declination is given as a character sign (DSIGN) followed by integer values of degrees (DECDEG), minutes (DECMIN) and seconds (DECSEC).

Position uncertainty: MAJOR, MINOR, POSANG

As discussed in Section II.F.3 of the Explanatory Supplement to the IRAS Faint Source Survey, the uncertainty in the position for a source depends primarily on its brightness in the various wavelength bands and the number of sightings. The final uncertainty is expressed as an ellipse whose semimajor (MAJOR) and semi-minor (MINOR) axes are the 1-sigma errors given in seconds of arc. The orientation (POSANG) of the ellipse on the sky is expressed in terms of the angle between the major axis of the ellipse and the local equatorial meridian. It is expressed in degrees east of north.

Number of sightings: NDET

The number of individual detector sightings is given.

Flux Density: FNU(4)

Each of the four wavelengths has a non-color-corrected flux density in units of Janskys (1 Jy = 10^-26 W m^-2 Hz^-1). The quality of each flux density is designated by FQUAL (see below).

The flux densities have been calculated assuming an intrinsic source energy distribution such that the flux density f_nu is proportional to nu^-1 . Corrections to other spectral shapes can be made.

Note that the flux density quoted for some sources could be zero if there were not enough data available to derive a good upper limit (see Sections II.F.4 and III.H of the Explanatory Supplement to the IRAS Faint Source Survey).

Flux Density Quality: FQUAL(4)

As described in Section II.F.4, a flux-density measurement can be either high quality (FQUAL=3), moderate quality (FQUAL=2) or an upper limit (FQUAL=1). Flux Density Uncertainties: RELUNC(4)

Each flux-density measurement other than an upper limit has an associated uncertainty expressed as a 1-sigma value in units of 100 x [delta(f_nu)/f_nu]. Uncertainties are discussed in Sections II.F.4 and III.F of the Explanatory Supplement to the IRAS Faint Source Survey.

Signal-to-Noise Ratios: MEDSNR(4), LOCSNR(4)

The SNR is calculated in two different ways. The median SNR (MEDSNR) is calculated through the use of a median noise calculated over an area of roughly 15 x 15 arcsec (and corrected by NCF), which is accurate to approximately 8% (see Section III.A.4 of the Explanatory Supplement to the IRAS Faint Source Survey). This median SNR is referred to simply as SNR elsewhere in this document. The local SNR (LOCSNR) is calculated using the noise for the pixel containing the peak flux density of the source (see Section II.C.4).

Area of source: AREA(4)

The area of each source is calculated as the number of contiguous pixels with flux density above three times the noise after possible rethresholding. Confusion: CONFUSE, CATNBR, EXTNBR(4)

As described in Section II.F.5 of the Explanatory Supplement to the IRAS Faint Source Survey, the bandmerger attempted to identify sources that were confused with neighboring sources in one or more bands. The CONFUSE flag is set in a given band if any instance of confusion was present in that band.

Other indicators of possible confusion are given by CATNBR and EXTNBR. CATNBR gives the number of nearby catalog sources within a radius of 6 arcmin. EXTNBR gives for each band the number of extractions within a radius of 6 arcmin.

Cirrus Indicator: CIRRUS

Over nearly the entire sky, portions of the FSS plates are affected by the infrared cirrus. Cirrus can seriously hamper efforts to extract point sources from the data and can also produce structure on a point source scale that can masquerade as true point sources. The CIRRUS flag gives the number of 100 microns-only sources in the extraction database within a radius of 30 arcmin. It is a fairly good discriminant that warns the user that cirrus which contains structure on a point source scale is present in a given region. Values above 2 usually indicate contamination.

Minimum reliability: MINREL

The minimum reliability for a source is the maximum of the reliability calculated individually for each band. See section III.D of the Explanatory Supplement to the IRAS Faint Source Survey for more information.

Positional Associations: NID, IDTYPE, CATNO, SOURCE, TYPE, RADIUS, DSTMINOR, DSTMAJOR, POS, FIELD1-3

Much of the utility of the FSC comes from the association of infrared objects with sources known to exist from other astronomical catalogs. As described in Section II.G of the Explanatory Supplement to the IRAS Faint Source Survey, a large number of catalogs have been searched for positional matches. The total number of matches found is given by NID. Each match results in a 64-character decription which is placed in a separate association file in order to conform to the FITS catalog format.

IDTYPE ranges from 1 to 15 and states whether an association was found in an extragalactic catalog (bit 0), a stellar catalog (bit 1), catalogs with other types of objects (bit 2) or in a catalog with mixed types (bit 3). Note that this differs from the convention used in previous IRAS data products where only the total of the association types was given. For example, if associations were found to both an extragalactic catalog and a stellar catalog the IDTYPE was 'multiple'. We are now preserving the information as to which type of catalogs were matched.

CATNO is the number of the catalog in wich the match was found (Table 3).

SOURCE is the name of the object in that catalog and TYPE its character or spectral type, if available. A vector is drawn from the IRAS position to the associated object. RADIUS is the length of that vector in arcseconds. POS is the angle between the vector and the local equatorial meridian expressed in degrees east of north. DSTMAJOR is the distance from the IRAS source to the associated object along the major axis of the positional uncertainty ellipse of the IRAS source and DSTMINOR is the similar distance along the minor axis.

Three fields (FIELD1-3) have values depending on the catalog in question (Table 2). Typically, FIELD1,2 are magnitudes (in decimag) and FIELD3 a size.

      Table 3.  Meaning of the Source Association Fields

Catalog             Field and Meaning
1 GCVS              Type              Blank
                    Field1            Code gives meaning for Fields 2-3
                    if Field1 = 1     Field2 and Field3 are B mag [decimag]
                                      at max, min
                    if Field1 = 2     Field2 and Field3 are V mag [decimag]
                                      at max, min
                    if Field1 = 3     Field2 and Field3 are photographic mag
                                      [decimag] at max,min
                    if Field1 = 4     Field2 and Field3 are estimated V mag
                                      [decimag] at max,min
                    if Field1 = 5     Field2 is -999 and Field3 is 0
2 Dearborn          Type              Blank
    Obs.
                    Field1            Code for Field2 (1,2)
                    Field2            if Field1 is 1, Field2 is red magnitude
                                      [decimag]
                                      if Field1 is 2, Field2 is -999
                    Field3            0
3 Revised           Type              Blank
    AFGL
                    Field1            Magnitude at 4.2 microns [decimag]
                    Field2            Magnitude at 11 microns [decimag]
                    Field3            Magnitude at 27 microns [decimag]
4 2-micron Sky      Type              Blank
    Survey
                    Field1            K magnitude [decimag]
                    Field2            I magnitude [decimag]
                    Field3            0
5 Globules          Type              Blank
    (Wesselius)
                    Field1            -999
                    Field2            Minimum diameter [arcsec]
                    Field3            Maximum diameter [arcsec]
6 R C 2             Type              Blank
                    Field1            Harvard V magnitude [decimag]
                    Field2            B_T  [decimag]
                    Field3            D_o [arcsec]
7 Stars with        Type              Blank
   em. lines
                    Field1            V magnitude [decimag]
                    Field2            -999
                    Field3            0
8 Equatorial        Type              Blank
   IR Cat.
                    Field1            Flux density [10^-16 W cm^-2 micron] at
                                      2.7 microns
                    Field2            -999
                    Field3            0
9 UGC               Type              Blank
                    Field1            Zwicky magnitude [decimag]
                    Field2            Minimum diameter [arcsec] in B
                    Field3            Maximum diameter [arcsec] in B
10 MCG              Type              Blank
                    Field1            -999
                    Field2            Minimum diameter [arcsec] in B
                    Field3            Maximum diameter [arcsec] in B
11 Strasbourg       Type              Blank
    Planetary
     Nebulae
                    Field1            V magnitude of Nebula [decimag]
                    Field2            B magnitude of Central Star [decimag]
                    Field3            Minimum diameter of Nebula [arcsec]
12 Zwicky           Type              Blank
                    Field1            Zwicky magnitude [decimag]
                    Field2            -999
                    Field3            0
13 SAO              Type              Spectral Type
                    Field1            V magnitude [decimag]
                    Field2            p_g magnitude [decimag]
                    Field3            0
14 ESO/             Type              First 3 characters of object type
     UPPSALA
                    Field1            B magnitude [decimag]
                    Field2            Maximum diameter [arcsec]
                    Field3            Minimum diameter [arcsec]
15 Bright           Type              Spectral Type
     Stars
                    Field1            V magnitude [decimag]
                    Field2            B-V [centimag]
                    Field3            U-B [centimag]
16 Suspected        Type              Spectral Information
     Var.
                    Field1            V magnitude at maximum [decimag]
                    Field2            -999
                    Field3            0
17 Carbon           Type              Spectral Type (may be truncated)
     Stars
                    Field1            p_g magnitude [decimag]
                    Field2            V magnitude [decimag]
                    Field3            I magnitude [decimag]
18 Gliese           Type              Spectral Type (may be truncated)
                    Field1            V magnitude [decimag]
                    Field2            B-V magnitude [millimag]
                    Field3            U-B magnitude [millimag]
19 S Stars          Type              Blank
                    Field1            p_g magnitude [decimag]
                    Field2            V magnitude [decimag]
                    Field3            I magnitude [decimag]
20 Parkes HII       Type             Blank
      Survey
                    Field1           -999
                    Field2           Minimum diameter [arcsec]
                    Field3           Maximum diameter [arcsec]
21 Bonn HII         Type             Blank
      Survey
                    Field1           Flux density at 4.875 GHz (Jy)
                    Field2           Diameter [arcsec]
                    Field3           0
22 Blitz            Type             Blank
                    Field1           Diameter [arcsec]
                    Field2           V_CO  [km/s]
                    Field3           Peak T_A  [Kelvin]
23 OSU              Type             Blank
                    Field1           -999
                    Field2           -999
                    Field3           Diameter [arcsec]
24 IRC              Type             C if 2.2-microns sources are possibly
     w/good pos.                     confused, blank otherwise
                    Field1           Right ascension difference (IRC-IRAS )
                                     [deciseconds of time]
                    Field2           Declination difference (IRC-IRAS )
                                     [arcsec]
                    Field3           0
25 DDO              Type             Blank
                    Field1           -999
                    Field2           -999
                    Field3           0
26 Arp              Type             Blank
                    Field1           -999
                    Field2           -999
                    Field3           0
27 Markarian        Type             Blank
                    Field1           -999
                    Field2           -999
                    Field3           0
28 Strong           Type             Object type (GAL or QSO)
     5 GHz
                    Field1           V magnitude [decimag]
                    Field2           5 GHz flux density [deciJy]
                    Field3           0
29 Veron-Cetty      Type             Object classification
     Veron
                    Field1           V magnitude [decimag]
                    Field2           Redshift x 1000
                    Field3           0
30 Zwicky           Type             Blank
     8 Lists
                    Field1           -999
                    Field2           -999
                    Field3           0
31 VV               Type             Blank
                    Field1           Special flag (see below)
                    Field2           -999
                    Field3           0
32 IRAS Small-      Type             Blank
     Scale Structure
                    Field1           Hex-coded bands
                    Field2           -999
                    Field3           -999
39* OSU             Type            Blank                                        *
     Radio                                                                       *
                    Field1          Frequency                                    *
                    Field2          Flux [deciJy]                                *
                    Field3          0                                            *
40 Michigan         Type            Class**                                      *
     Spectral                                                                    *
                    Field1          Mag [decimag]                                *
                    Field2          HD number [low byte]                         *
                    Field3          HD number [high byte]                        *
41 Serendipitous    Type            Blank                                        *
     Survey                                                                      *
                    Field1          Hex-coded SSC bands                          *
                    Field2          First SSC flux density [mJy]***              *
                    Field3          Second SSC flux density                      *
42 PSC II           Type            Blank                                        *
                    Field1          Hex-coded PSC bands                          *
                    Field2          First PSC flux density [mJy]***              *
                    Field3          Second PSC flux density [mJy]                *
48 Virgo            Type            Class**                                      *
     Cluster                                                                     *
     Catalog                                                                     *
                    Field1          B_T  magnitude [decimag]                     *
                    Field2          Heliocentric velocity [km/s]                 *
                                    (-999 when not available)                    *
                    Field3          Source size 100 x log D_est(.1 arcsec)
49 Dressel          Type            Blank
     and Condon
                    Field1          -999
                    Field2          -999
                    Field3          -999
50 Asteroids        Type            Blank

                    Field1          -999
                    Field2          First four digits of crossing time
                                    [seconds, since 1980.0]
                    Field3          Last four digits of crossing time


   * Catalog numbers 33-38 reserved for internal use.
  ** Source name and type fields were combined to hold spectral type and class
     (Catalog 40), or object classification (Catalog 48).
 *** Flux densities given are the first two measurements in order from 12 to
     100 microns.

                         VV Catalog Flags (Catalog 31)
     10       VV 10 has the same coordinates as VV29 in the VV Atlas.
              The UGC was used to confirm that the coordinate is correct
              for VV 29 and erronerous for VV 10. The UGC position for
              VV 10 = TGC 10814 was adopted.
     11       The VV position is substantially different (> 400 arcsec) from
              positions for the object in other catalogs. The VV position has
              been assumed to be in error because two or more other catalogs
              agree on a different position. The UGC position has been adopted.
     12       Same as for 11, but the OSU position has been adopted.
     13       The position in the VV Atlas, and the position listed for the
              VV object in the OSU are in disagreement. The true position has been
              established to be close to that of the OSU by the use of overlay
              transparencies on the POSS. The OSU position has been adopted.
     14       Same as for 13, but the OSU position is not very good either, so a new
              position has been measured (accurate to about 1 arcsec.)
   -999       No information given