IRAS Small-Scale Structure Catalog Format

The Catalog is contained in one file ordered by right ascension and by declination in case of conflict. For each source, data are shown only for those bands where detections have been found and have survived processing through "weeks confirmation". No upper limits are given where no detections were found; none are given where detections were dropped because of confusion ("cluster analysis"). Absence of data from this Catalog at a given position in a given band does not imply absence of emission, but simply lack of information (see Chapter VI of the reference for a discussion of completeness). On the other hand, flags NEARPS, SES1 and HD, which depend only on position in the sky, are always estimated for all four bands, at the mean position of each source.

In general, the data for each source are organized in three 80-byte records followed by a variable number of association records; Table 1 shows details of the organization. The first two records contain basic parameters and warning flags referring to the band-merged source, whereas the third record is made up of four 20-byte components containing position and quality flags for each of the bands. When present, the 12 micron data are inserted into the first 20 bytes of the record; the 25, 60, and 100 micron data go into the second, third, and last block of 20 bytes in the record, respectively. Quarters corresponding to bands absent from the source are filled with blanks. The fourth and subsequent records contain data on positional associations between the Catalog source and objects in a variety of astronomical catalogs listed in the description of the format of the Point Source Catalog. Each record holds data on two associations, in two 40-byte blocks. All data entries for each source are described below in their order of appearance in Table 1.

As distributed, the catalog is divided into two files. The first is a 240-byte record file containing the 16,740 sources. The second is a 58-byte record file containing the 5,178 associations. The file formats are described below in Tables 1 and 2.

Table 1.  Format of the Small-Scale Structure Sources File

 Start
 Byte      Name             Description                  Units       Format
 1       NAME          IRAS source name                  ---         10A1
 11      BMFLG         Number of bands in source         ---         A1 [1]
                       and "band merging" warning flag
 12      RAHR          Right ascension (1950)            Hours       I2
 14      RAMIN         Right ascension (1950)            Minutes     I2
 16      RASEC         Right ascension (1950)            Seconds     F4.1
 20      DSIGN         Declination (1950)                ---         A1
 21      DECDEG        Declination (1950)                Degrees     I2
 23      DECMIN        Declination (1950)                Arcminutes  I2
 25      DECSEC        Declination (1950)                Arcseconds  I2
 27      NH(4)         Number of hours-confirmed         ---         4A1
                       sightings
 31      FLUX(4)       Averaged, spatially               Jy          4E8.2 [1]
                       integrated flux density
                       (no color correction)
 63      XTALK(4)      Cross-talk flag                   ---         4A1 [1]
 67      NEARPS(4)     Number of near-by weeks-          ---         4A1
                       confirmed point sources
 71      SES1(4)       Number of near-by FISES entries   ---         4A1
 75      CIR           Number of hours-confirmed,        ---         I2
                       100 micron only point sources
 77      BLANK         Four spare bytes                  ---         4A1
 81      HD            High source density flag          ---         A1
                       (encoded, one bit per band)
 82      DBLPS         Possibility this is a double      ---         A1
                       point source (1 bit per band)
 83      PTSRC         Name of IRAS Point Source         ---         12A1
                       counterpart and conflict flag
 95      PSIZ(4)       Size estimate from comparison     Deciarc-    4I3
                       of FLUX with PTSRC flux           minutes
 107     NID           Number of associations            ---         I2
 109     IDTYPE        Type of objects associated        ---         I4
 113     BLANK         48 spare bytes                    ---         48A1
 161-180               (blank if no 12 micron component)
 161     FQLT          12 micron flux quality class      ---         A1 [1]
 162     FCAT          12 micron "final selection" flags ---         A1 [1]
 163     DRA           Right ascension offset from       Seconds     F6.1
                       mean position to 12 micron
 169     DDEC          Declination offset from mean      Arcseconds  I4
                       position to 12 micron
 173     UNC           95% confidence diameter for       Deciarc-    I3
                       position at 12 micron             minutes
 176     NS            Number of individual detections   ---         I3
                       in 12 micron component
 179     BLANK         Two spare bytes                   ---         2A1
 181-200               25 micron equivalent of bytes 161-180
                       (blank if no 25 micron component)
 201-220               60 micron equivalent of bytes 161-180
                       (blank if no 60 micron component)
 221-240               100 micron equivalent of bytes 161-180
                       (blank if no 100 micron component)

Table 2.  Format of the Small-Scale Structure Catalog Associations File
 Start
 Byte      Name             Description                  Units       Format
 1       NAME          IRAS Source Name                  ---         A10
 12      RECNO         Rec. Num. of source in data file  ---         I6
 19      CATNO         Catalog identifier                ---         I2
 21      SOURCE        Object ID in that catalog         ---         15A1
 36      TYPE          Object type in that catalog       ---         5A1
 41      RADIUS        Distance from IRAS source         Arcseconds  I3
                       to associated object
 44      POS           Position angle from IRAS          Degrees     I3 [1]
                       source to object                  E of N
 47      FIELD1        Object data field Nr 1            [2]         I4 [1]
                       (magnitude or other)
 51      FIELD2        Object data field Nr 2            [2]         I4 [1]
                       (magnitude or other)
 55      FIELD3        Object data field Nr 3            [2]         I4 [1]
                       (magnitude or other)

 [1] This quantity appears with a different format or representation, or is
     omitted in the printed version of the catalog.
 [2] The definition and units of the quantities in FIELD1-3 depend on the
     individual catalog in which the association is found.  See Table
     X.B.4 in the Explanatory Supplement.

Table 3.  Interpretation of Final Source Selection Flag

 Optical    N/M    Detection      Flux         Digital    Printed     Binary
  Cross-    Test     Count      Threshold      Version     FCAT      Notation
   Talk     [1]
                                             FQLT   FCAT    XEI        FCAT
 -----------------------------------------------------------------------------
    NO     MED*      PASS         PASS        B      0       00       00000
    NO     MED*      PASS         FAIL        F      1       01       00001
    NO     MED*      FAIL         PASS        F      2       02       00010
    NO     MED*      FAIL         FAIL        F      3       03       00011
    NO     LOW       PASS         PASS        F      4       10       00100
    NO     LOW       PASS         FAIL        F      5       11       00101
    NO     LOW       FAIL         PASS        F      6       12       00110
    NO     LOW       FAIL         FAIL        F      7       13       00111
    NO     HIGH      PASS         PASS        A      8       20       01000
    NO     HIGH      PASS         FAIL        B      9       21       01001
    NO     HIGH      FAIL         PASS        B      A       22       01010
    NO     HIGH      FAIL         FAIL        B      B       23       01011
    NO     2/2       PASS         PASS        B      C       30       01100
    NO     2/2       PASS         FAIL        F      D       31       01101
    NO     2/2       FAIL         PASS        F      E       32       01110
    NO     2/2       FAIL         FAIL        F      F       33       01111
    YES    MED*      PASS         PASS        F      G      X00       10000
    YES    MED*      PASS         FAIL        F      H      X01       10001
    YES    MED*      FAIL         PASS        F      I      X02       10010
    YES    MED*      FAIL         FAIL        F      J      X03       10011
    YES    LOW       PASS         PASS        F      K      X10       10100
    YES    LOW       PASS         FAIL        F      L      X11       10101
    YES    LOW       FAIL         PASS        F      M      X12       10110
    YES    LOW       FAIL         FAIL        F      N      X13       10111
    YES    HIGH      PASS         PASS        F      O      X20       11000
    YES    HIGH      PASS         FAIL        F      P      X21       11001
    YES    HIGH      FAIL         PASS        F      Q      X22       11010
    YES    HIGH      FAIL         FAIL        F      R      X23       11011
    YES    2/2       PASS         PASS        F      S      X30       11100
    YES    2/2       PASS         FAIL        F      T      X31       11101
    YES    2/2       FAIL         PASS        F      U      X32       11110
    YES    2/2       FAIL         FAIL        F      V      X33       11111

 [1]  In the repeatability test, "MED*" indicates that N/M is in the intermediate range (Table IV.B.2) excluding N/M = 2/2;  this latter case is indicated by "2/2".
Source Name: NAME
The letter "X" prefixes all names in this Catalog to distinguish them from Point Source Catalog names. The name is then derived from position by combining the hours and minutes from the right ascension with the sign, degrees, and decimal fraction of degrees from the declination. In case of name duplication the letters "A", "B", etc. are appended to the name in order of increasing right ascension, and of increasing declination in case of a tie. See Source Designations for IRAS Catalogs.
"Band Merging" Flag: BMFLG
The "band merging" flag gives the number of bands in which the source has an entry in the third record (regardless of flux quality in these bands), together with indications on the "band merging" processing history for that source. BMFLG=C or D indicates 3 or 4 mutually confirming (section V.E.6 in the Explanatory Supplement) components in the source. BMFLG=I, J, K or L indicates 1, 2, 3 or 4 components in a source having experienced "band merging" complications and having survived the modified "final selection" discussed in IV.B.3.a.
Position: RAHR, RAMIN, RASEC, DSIGN, DECDEG, DECMIN, DECSEC
Source position is the simple mean of the positions of all the individual band components in the source regardless of the individual flux quality flags. It is given in equatorial coordinates for the 1950 equinox. Positional accuracy is discussed in V.C.2 of the Explanatory Supplement.
Number of Sightings: NH(4)
The number of survey coverages contributing a sighting to the weeks-confirmed source is given for each band as a single character. The order for this and similar arrays in the Catalog is from 12 microns (subscript = 1) to 100 microns (subscript = 4).
Flux Density: FLUX(4)
An estimate of the spatially integrated flux density from the source is given in Jansky for each band, with three digit precision. This is transformed from the fluxes in the IRAS bands assuming a spectrum with Nu x f(Nu) = constant (see section VI.C of the Explanatory Supplement). To obtain the actual flux density at the nominal wavelength for each band, FLUX must be color corrected. The estimation algorithm is described in sections V.E.3, 4 and 5; calibration in II.C; accuracy is discussed in V.C.1.
Optical Cross Talk Flag: XTALK(4)
This quantity indicates the extent and kind of cross-talk for which each component in the source was flagged. As discussed in IV.B.2a of the Explanatory Supplement, the processor accumulated the number NXT of detections that might have been caused simply by optical cross-talk, along with the number NS of detections. If NXT=0 then XTALK=0 for that band. If NXT > 0, and either (i) (NS-NXT) > 2, or (ii) (NXT/NS) < 2/3 then XTALK=1 for moderate cross-talk. If NXT > 0, and neither (i) or (ii) is true then XTALK=2 for severe cross-talk. If in addition a band component was flagged for cross-talk at the level of "final source selection", then XTALK is increased by 4. Note that the component is given a low quality rating if XTALK > 0.
Near-by Point Sources: NEARPS(4)
This flag gives a count of all weeks-confirmed point sources (regardless of whether or not they are included in the Point Source Catalog) in each band within a 9' radius of the mean position of the SSS Catalog source. NEARPS=1 indicates most often that the emission has been detected as a point source as well as here. Larger values are a warning of possible confusion affecting the source, or an indication that this source combines emission from two or more barely-resolved point sources (see DBLPS below). NEARPS is given as a single character per band, and denoted by a letter when it exceeds 9 ('A' for 10, 'B' for 11, etc.).
Near-by Intermediate Small Extended Sources: SES1(4)
This flag gives a count in each band of all hours-confirmed entries in FISES (section V.E.3 of the Explanatory Supplement) within a 9' radius of the mean position of the source. In a clean sky, SES1 would be equal to NH; larger values implying a higher density of detections, either spurious or due to complex structure, point to areas where "cluster analysis" was active. Large values of SES1 for a band not represented in the source may indicate reliable extended detections discarded in subsequent processing. SES1 appears as a single character per band in the same notation as NEARPS.
Cirrus Indicator: CIR
This cirrus flag gives the number of point source detections at 100 microns only, hours confirmed but not necessarily weeks confirmed, within a 30' radius of the SSS source. Cirrus is rich in structure on all scales, and may supply a 100 micron or a 60 micron component to a source detected at other wavelengths, or may combine with the 100 micron or 60 micron emission from a source and severely compromise its flux measurement. Values of CIR above 6 invite caution in interpreting both 60 and 100 micron detections.
High Source Density Flag: HD
This single character flag denotes whether the source falls in a region of high source density in the sense of Point Source clean-up processing (section V.H.6 of the Explanatory Supplement). This flag was not used in SSS processing, but warns against confusion by pointing out areas where point source density may exceed the resolving capability of the instrument. When the flag is written out in binary notation the least significant bit refers to the 25 micron band, and so on. Values greater than 9 are denoted by letters as for NEARPS.
Double Point Source Flag: DBLPS
This flag indicates the possibility that this Catalog source is the combination of two point sources (see section IV.B.2.e of the Explanatory Supplement for definition). The indication is given for each band, encoded as for HD, with the bit set to 1 when the possibility exists for the corresponding band. This flag is estimated only for bands represented in the source, regardless of the flux quality in these bands.
Coincident Point Source: PTSRC
A small extended source will often trigger the point source detection processor, either because it is still small enough to produce an acceptable fit to the template (bright point sources with low correlation coefficients are examples of this), or because it actually contains one or more point-like components. When an entry from the Point Source Catalog lies within a distance D of the SSS Catalog source position it is identified as a point source counterpart to the SSS Catalog source, and its IRAS name is entered in this column. The distance D used is half the largest value of UNC (section V.C.3 of the Explanatory Supplement) among the bands in the source. When two or more counterpart candidates are found, the nearest one is chosen, and an asterisk precedes PTSRC as a warning. Clearly, the source descriptions here and in the Point Source Catalog are complementary, and neither can be ignored.
Inferred Source Size: PSIZ(4)
An estimate of the source size is given in deci-arcminutes for those bands where the point source counterpart is detected. The estimate is obtained from
PSIZ(i) = FWHM(i) x [SSSFLUX(i)/PTSRCFLUX(i)]^(1/2)
where FWHM(i) is the full width at half maximum of a point source in band i, namely 0.82', 0.84', 1.44', and 3.14', respectively, at 12, 25, 60, and 100 microns; SSSFLUX and PTSRCFLUX are the fluxes in the two catalogs. See V.B.1 and V.C.3 of the Explanatory Supplement for details and a discussion of accuracy.
Positional Associations: NID, IDTYPE
Positional associations between objects in the Small-Scale Structure Catalog and objects in astronomical catalogs were obtained following the same strategy used for point sources (see the format of the Point Source Catalog). The only difference is that the search radius around sources in this Catalog is never less than 120". NID is the total number of matches found. IDTYPE ranges from 1 to 4 to indicate an association found in an extraglactic catalog (1), a stellar catalog (2), other catalogs (3), or matches in multiple types of catalogs (4). Each record starting with the fourth data record for the source contains two of these blocks.
Flux Quality Flag: FQLT
This is the quality class assigned to the band component by the scheme described in Chapter IV and Table IV.A.1 of the Explanatory Supplement. High, intermediate and low qualities are denoted, respectively, by A, B, and F. Starting with this flag, data for each band appear separately in the corresponding quarter of the third data record for the source.
Final Catalog Selection Flags: FCAT
The results of the three main stages of "final selection" are presented in this flag which appears as a single character ranging from 0 to V (Table 3 and section VII.B of the Explanatory Supplement). When FCAT is written out in binary notation, the least significant bit indicates the result of the flux test (IV.B.2.d), and is set if failed; the next bit refers to the detection count test (IV.B.2.c), and is set if failed. The next two bits refer to the repeatability test (IV.B.2.b), and signal one of four possible outcomes: 00 for intermediate values of N/M except N/M = 2/2; 01 for low N/M; 10 for high N/M; and 11 for N/M = 2/2 (Table IV.B.3 of the Explanatory Supplement). The most significant bit in FCAT is set if the detection is flagged for cross-talk of any kind (IV.B.2.a). Table 3 shows all possible values of FCAT, their representation in the file, and their meaning.
Right Ascension Offset: DRA
This gives in seconds the amount to add to the mean right ascension of the source (shown in the first record) to obtain the right ascension measured for the emission in this band.
Declination Offset: DDEC
This gives in arcseconds the amount to add to the mean declination of the source (shown in the first record) to obtain the declination measured for the emission in this band.
Positional Uncertainty from Intensity Distribution: UNC
The second moments about major and minor axes of the roughly-modeled intensity distribution are estimated as explained in V.E.3 of the Explanatory Supplement. UNC is their geometric mean, given as a diameter in deci-arcminutes. Because of the coarse grid used in the definition, UNC is best interpreted as a positional uncertainty, namely the 95% confidence diameter for the position in this band.
Number of Detections: NS
This total number of seconds-confirmed detections in the source is accumulated through "cluster analysis" and "weeks confirmation", and includes detections that failed to seconds-confirm because of a failed detector.
Association File: CATNO,SOURCE,TYPE,RADIUS,POS,FIELD1,FIELD2,FIELD3
For each match in Table 5 in the discussion of the format of the Point Source Catalog, CATNO is the number identifying that catalog. SOURCE is the name of the object in that catalog, and TYPE its character or spectral or morphological type if available. RADIUS is the distance in arcseconds from the IRAS position to the associated object. POS is the position angle measured in degrees East of North of the direction from the IRAS source to the associated object. FIELD1-3 carry values depending on the catalog in question (Table X.B.4 of the Explanatory Supplement). Typically FIELD1 and FIELD2 contain magnitudes in decimag, and FIELD3 a size in arcseconds.

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