Explanatory Supplement for the Machine-Readable Version of the IRAS FAINT SOURCE CATALOG |b| > 10 Degrees Version 2.0 Moshir, M. et al. 1989 Notes to the CD-ROM Machine-Readable Version of the IRAS Faint Source Catalog The following text is an exerpt from the original "Explanatory Supplement to the IRAS Faint Source Survey, Version 1" (Moshir et al. 1989). Presented here is Chapter V: "The Formats of the FSS Products", section C.1, and Table II.G.6: "Meaning of the Source Association Fields". The text has been altered slightly to accommodate the changes in version 2.0. The formats and fields of the two machine-readable data files of the catalog are adequately described. For further explanation, the original published explanatory supplement should be consulted. The catalog was received from the Infrared Processing and Analysis Center (IPAC) as two FITS files. These were converted into two ASCII files with the following characteristics: 1. Data File Record Length: 240 Number of Records: 173,044 2. Associations File Rocord Length: 64 Number of Records: 235,935 The entire original published version of the explanatory supplement (version 1) was received as a set of TeX files. The mentioned sections were extracted and converted to ASCII in order to create this document. The TeX files as received were added to the CD-ROM so that one might have access to a complete copy of the published explanatory supplement. However, note that the data pertains to version 2.0, therefore, an addendum file was created in TeX to outline the changes in the 2.0 version. -- Susan Gessner, Space Data and Computing, NASA/GSFC C.1 The Short Machine-Readable Version of the Faint Source Catalog ------------------------------------------------------------------------------- The short FSC tape is presented as FITS table data. In order to do this, the associations block has been split out as a separate file. It takes one 6250-bpi tape to hold all of these files for |b| > 50 degrees. The FITS table format consists of the following: - The catalog is split into 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 the tape 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. - The tape is blocked to multiples of 2880 bytes. 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 The short header file is given in Table V.C.1. (in the published version of the explanatory supplement.) - 1 - TABLE V.C.4. Format of FSC Data File for Short FSC Tape _______________________________________________________________________________ Start Byte Name Description Units Format _______________________________________________________________________________ 000 NAME * Source Name _ 12A1 012 RAHR Right Ascension 1950 Hour of time I2 014 RAMIN Right Ascension 1950 Minute of time I2 016 RASEC * Right Ascension 1950 deci-Second of time I3 019 DECSGN * Declination Sign +/- A1 020 DECDEG Declination 1950 Arc Degree I2 022 DECMIN Declination 1950 Arc Minute I2 024 DECSEC * Declination 1950 Arc Second I2 026 UNCMAJ * Uncertainty ellipse major axis Arc Second I3 029 UNCMIN * Uncertainty ellipse minor axis Arc Second I3 032 POSANG * Uncertainty ellipse position angle Degree (East I3 of North) 035 NOBS *+ Number of times observed in _ 4I3 each band 047 FNU *+ Flux densities (1 value per Jansky (10^-26 4E9.3 band, non-color corrected) W m-^2 Hz^-1) 083 FQUAL *+ Flux density quality _ 4I1 (1 value per band) 087 RELUNC + Percent relative flux density _ 4I3 uncertainties (1 value/band) 099 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 _______________________________________________________________________________ - 2 - TABLE V.C.4 Format of FSC Data File for Short FSC Tape (continued) NOTES: * 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 V.C.5. Format of Association Data for FSC Tape _______________________________________________________________________________ Start Byte Name Description Units Format _______________________________________________________________________________ 00 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. - 3 - Table V.C.2 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 V.C.4 (FSC data) and V.C.5 (association data) describe each entry in the short catalog tape. 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 required 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. The tape is written with 240-character (ASCII) logical records for the non-association data and with 64-character records for the association data. The number of logical records per physical record is 36 for the headers, 12 for the non-association data, and 45 for the association data. These numbers apply to unblocked files only and should be multiplied by the blocking factors for blocked files. 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. An example of a reference to a FSC source is IRAS F12345-6789. Table V.C.6 collects all the various designations for sources in all IRAS catalogs. - 4 - TABLE V.C.6 Source Designations for all IRAS Catalogs __________________________________________________________ Catalog Source Name __________________________________________________________ Point Source Catalog IRAS 12345-6789 Point Source Reject File IRAS R12345-6789 Small Scale Structure Catalog IRAS X1234-678 Serendipitous Survey Catalog IRAS S12345-678 Faint Source Catalog IRAS F12345-6789 Faint Source Reject File IRAS Z12345-6789 __________________________________________________________ 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, 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 by consulting Section VI.C in the Main Supplement or by the table inside the back cover of this Supplement. 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). - 5 - 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. 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). 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, 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 arcminc. 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. - 6 - Minimum reliability: MINREL The minimum reliability for a source is the maximum of the reliability calculated individually for each band. See section III.D 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, 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. 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 V.C.5). Typically, FIELD1,2 are magnitudes (in decimag) and FIELD3 a size. - 7 - TABLE II.G.6 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] _______________________________________________________________________________ - 8 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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 _______________________________________________________________________________ - 9 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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] _______________________________________________________________________________ - 10 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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 [degrees K] 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 ______________________________________________________________________________ - 11 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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 ______________________________________________________________________________ - 12 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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) _______________________________________________________________________________ * 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. - 13 - TABLE II.G.6 Meaning of the Source Association Fields (cont.) _______________________________________________________________________________ Catalog Field and Meaning _______________________________________________________________________________ 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 _______________________________________________________________________________ VV Catalog Flags (Catalog 31) _______________________________________________________________________________ Field1 Explanation 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 _______________________________________________________________________________ - 14 -