Explanatory Supplement to the
IRAS SERENDIPITOUS SURVEY CATALOG
S. G. Kleinmann
University of Massachusetts
R. M. Cutri, E. T. Young, F. J. Low
University of Arizona
F. C. Gillett
National Optical Astronomy Observatories
Notes about this computer-readable documentation:
The following text is an excerpt from the original printed documentation,
including Section I "Introduction" and Chapter V "Formats of the IRAS
Serendipitous Survey Catalog", which was keyed-in at the Astronomical Data
Center (ADC). The text makes references to other chapters in the
printed documentation as well as to the IRAS Explanatory Supplement. These
references are in the form: XX.XX.XX, for chapter, section and subsection,
respectively. For example, IV.B.3a refers to chapter IV, section B,
subsection 3a, and Suppl.V.H.9 refers to chapter V, section H,
subsection 9 in the IRAS Explanatory Supplement. The reader is directed to
the printed versions of the IRAS documentation for these references.
Because of the restrictions of the ASCII character set, some notational
differences appear between the printed documentation and this text. When
appropriate, greek letters are written out as their english names, except the
word "micron" appears instead of the term "Mu-meters" or "um". In general,
subscripts appear in parentheses, for example, flux per unit frequency, "f
subscript Nu", appears as f(Nu) and can be pronounced, "f sub Nu". Terms
given in italics in the original text are enclosed in double quotation marks.
The description of the logical record format of the catalog (Chapter V) was
changed to reflect the version distributed by the ADC. At the end of this
computer-readable text is a discussion of the IRAS Serendipitous Survey
Catalog as it appears in FITS-table format. This was authored at the ADC,
and does not appear in the original documentation.
Please direct any reports of typographical errors to:
Lee Brotzman
Code 930.3
NASA/GSFC
Greenbelt, MD 20771
(301) 286-6953
BITNET: ZMLEB@SCFVM
SPAN: CHAMP::BROTZMAN
Internet: zmleb@scfvm.gsfc.nasa.gov
or zmleb%scfvm@dftsrv.gsfc.nasa.gov
I. INTRODUCTION
In 1983 the sky was surveyed by the Infrared Astronomical Satellite (IRAS)
in four broad spectral bands centered at 12, 25, 60, and 100 microns. During
the course of the 300 day mission, about two-thirds of the satellite time was
used to carry out an unbiased sky survey which led to the production of the
IRAS Point Source Catalog (IRAS/PSC) containing 245,889 sources. That
catalog, along with many other aspects of the mission, are described in the
IRAS Catalogs and Atlases Explanatory Supplement (1985) [Referenced below as
the "Main IRAS Supplement"]. A portion of the remaining mission was used to
conduct a series of Pointed Observations, usually directed at objects of
interest, and using the various capabilities of the satellite. Many of these
Pointed Observations were performed by repeatedly scanning the 0.5 deg. wide
survey detector array over the selected object. These scans were combined to
produce a sensitive map of area about one square degree around the selected
source (or in some cases, a targeted "blank" field). By spatially coadding
the repeated scans comprising each Pointed Observation a large data base was
generated. Young et al. (1985) have described these data, the Pointed
Observation program and the associated data processing at the Infrared
Processing and Analysis Center (IPAC).
This document describes the IRAS Serendipitous Survey Catalog (IRAS/SSC)
which has been constructed using the fortuitous observations at 12, 25, 60,
and 100 microns of 43,866 point-like sources that happened to lie in 1813 of
the individual fields included in the Pointed Observations program. Because
the Pointed Observations resulted in longer integration times for each source
in the field than was possible in the all-sky survey mode, the limiting
sensitivity for the IRAS/SSC is typically better than that of the IRAS/PSC by
a factor of about 4. Also, the photometric accuracy, especially for faint
sources, is improved relative to that of the IRAS/PSC. The amount of sky
sampled in the SSC is nearly 1400 square degrees but because of uneven
sensitivity across the Pointed Observation fields, the effective sky coverage
is 1108 square degrees. Excluding certain areas of great scientific interest
to the team who planned the observations (specifically, the Galactic plane and
the Magellanic Clouds), the Pointed Observations were widely distributed on
the sky. Positional association of the SSC sources with those in the PSC
reveal that 11,129 are in both IRAS catalogs; also, 5470 SSC sources have been
cataloged at other wavelengths.
The positional accuracy of the SSC is not as high as that of the PSC, in
part because of the restricted geometry of the observations themselves and
also because of compromises made in processing the data. In those cases where
more accurate positions are critically needed, it may be possible to obtain
better results through re-processing the observations at one of the IRAS data
centers.
The main objective of the Serendipitous Survey was to extend the detection
threshold for point sources over a finite but significant portion of the sky
so that those sources could be studied individually. No attempt has been made
to construct an unbiased sample by removing the targeted sources or by
correcting either for the non-random sky coverage or for the uneven depth of
the survey corresponding to different characteristics of the observing modes.
Because most of the SSC sources are faint and because it was inherently
difficult to achieve a uniform level of completeness in this program, it was
decided to emphasize high reliability and photometric accuracy. These
important SSC objectives were achieved by retaining only those sources which
were detected in each of two independent Pointed Observations and by averaging
the two measurements. No fields were included where only a single Pointed
Observation was available. This strategy also provided a means for rejecting
some asteroids and any other moving objects.
Frequently, independent observations were made of nearly the same region of
sky and were treated in this catalog as separate fields; this practice
resulted in partially redundant data sets. These overlapping fields are noted
in the SSC to facilitate their further study. Because each of the 1813 fields
in the SSC is unique in the sense that it may differ from other fields, both
according to the observing mode that was used and in the wide range of source
densities encountered, the catalog is presented field-by-field along with data
pertinent to each field. Enough information is retained to permit the user to
reorganize the source lists as needed. Those fields which overlap other
fields are cross referenced. The fields are ordered by the right ascensions
of their centers and the source list within each field is also ordered by
right ascension.
An important objective of this catalog is to present a list of reliable
sources found in all the selected Pointed Observation fields despite effects
of high source density and/or complex backgrounds. In those heavily crowded
fields where source confusion degrades the sensitivity, reliability,
completeness, positional accuracy or photometric accuracy, the guiding
philosophy has been as follows:
- (a) accept some degradation,
- (b) flag sources where confusion may have affected the confirmation
process,
- (c) indicate the local density of sources for each wavelength band,
- (d) provide information on the effects of confusion so that the user is
both warned and equipped to make scientific judgments.
This policy has the result that many of the SSC fields and sources must be
analyzed with particular care by the user, but it also means that many faint
sources are included in crowded regions of scientific interest.
In summary, the main features of the SSC relative to the PSC are as
follows:
- Enhanced sensitivity for point-like sources in all four wavelength bands
over a few percent of the sky.
- Excellent reliability in low surface density fields.
- Uneven sky coverage and completeness.
- Reduced positional accuracy.
- Improved photometric accuracy.
- Much greater depth in crowded fields at the expense of reliability and
accuracy.
The user of the SSC should be aware of the following considerations.
First, a set of high latitude, unique, less crowded fields, selected as
described in Section IV, was used in much of the current analysis. This
subset of the catalog is comprised of 450 fields containing 5100 sources.
Second, for these high latitude fields, reliability of order 99% was achieved
for sources brighter than the flux density limits of 100, 150, 120, and 400
mJy for the 12, 25, 60, and 100 micron bands, respectively; note that even
within these fields the completeness limits vary. Third, the measured
positional accuracy of SSC sources defines a search box with dimensions of 30"
x 180" at 12 microns, increasing to 60" x 140" at 60 microns. The orientation
of each search box is indicated by the grid orientation angle given in the
appropriate field header. Fourth, for the high latitude subset of fields, the
systematic photometric errors introduced by the absolute calibration are
nearly identical with those in the PSC. The remaining systematic errors and
uncertainties due to lack of reproducibility are less than +/- 10% on average
for sources detected with high signal-to-noise, and the effects of noise are
well quantified within each field. Fifth, for lower latitude fields, or
fields where the underlying background has a complex structure, the
reliability, completeness, positional, and photometric accuracies may be
substantially degraded and each field must be analyzed with caution. Sixth,
as in the PSC many sources are slightly extended and will have erroneous flux
densities; other IRAS data bases such as the INTN mode grids (Young et al.
1985) should be used for the study of small extended sources.
In the preparation and presentation of the SSC, an effort was made, so far
as possible, to conform to conventions established in the preparation and
presentation of the PSC. In the following sections only that information not
found in the Main IRAS Supplement (1985) or in the Users Guide to the IRAS
Pointed Observation Products (Young et al. 1985) and needed for the use of
this catalog will be given. This includes: information on the Pointed
Observation data base from which the point sources were extracted and the
extraction process (Section II), on the data processing used to develop the
SSC from the list of extracted sources (Section III), on the reliability and
sensitivity of the SSC, its positional and photometric accuracy and certain
statistical properties (Section IV), on the format and content of the SSC in
its machine-readable and printed forms (Section V), and warning to the user
concerning confusion effects, asteroid and redundant sources (Section VI), and
a table of overlapping fields (Appendix A).
V. THE FORMATS OF THE IRAS SERENDIPITOUS SURVEY CATALOG
A. Introduction
This chapter describes the formats of the IRAS Serendipitous Survey Catalog
in its printed and machine-readable forms. A brief description is given of
each entry in the catalog; tables describe each column of the catalog in more
detail and give, for the machine readable versions, the logical type of each
variable and its length in bytes. The catalog consists of three basic parts;
a) field headers, b) source listings, and c) redundant fields (a printed list
of the redundant fields is also available in Appendix A). The field header
includes information on the global properties of each pair of
reference/confirming grids. The source listing documents the properties of
the individual confirmed sources found in each field. The redundant field
listing indentifies those grid pairs with more than 5% overlapping coverage on
the sky.
Because the Serendipitous Survey fields are non-uniformly distributed on
the sky, the catalog has been arranged by fields, with the fields ordered by
the right ascension of the field center. The field header is located at the
beginning of the source listing for that field. Within a field the sources
are arranged in order of right ascension. The redundant field listing is
separate and follows the rest of the catalog. As much as possible, the
conventions established for the IRAS/PSC have been adopted for the
Serendipitous Survey.
B. The Machine-Readable Version
The machine-readable version of the SSC contains four files which are described
in the following tables. Tables V.B, V.B.1a, V.C, V.C.1a, and V.D describe the
formats of the field headers, source, association, and overlap field files
respectively.
Table V.B. Format of the Field Headers [1] (SSC M-R Version)
Start Name Description Units Format
Byte
00 FNAME [2] Field Name 13A1
13 RGRID [2] Reference Grid No. I5
18 RDATE [2] Obs. Date, Ref. Grid Days I3
JD 2445000 +
21 CGRID [2] Confirming Grid No. I5
26 CDATE [2] Obs. Date, Conf. Grid Days I3
JD 2445000 +
29 MACRO [2] Macro Type 1A1
30 GLON [2] Galactic Longitude Degree I3
33 GLAT [2] Galactic Latitude Degree I3
36 PDRAS Sign of R.A. Difference +/- 1A1
Between Grid Centers
37 PDRA Amplitude of R.A. Difference Arcsec I3
Between Grid Centers
40 PDDECS Sign of Dec. Difference +/- 1A1
Between Grid Centers
41 PDDEC Amplitude of Dec. Difference Arcsec I3
Between Grid Centers
44 RANGLE Reference Grid Scan Degree I4
Direction (E of N)
48 CANGLE Confirming Grid Scan Degree I4
Direction (E of N)
52 EFFAREA [2] 100x Effective Area Square Deg. I3
of Grid Overlap
55 RUNDF [2] No. of Fields with NN I2
Overlap > 5%
57 SPARES 23 spare bytes 23A1
80 RNOISE Median Noise of Ref. mJy 4I5
Grid (1 value per band)
100 CNOISE Median Noise of Conf. mJy 4I5
Grid (1 value per band)
120 NSOURC Number of Confirmed NNN 4I3
Sources (1 value per band)
132 NCONF [2] Number of Confused NNN 4I3
Confirmations (1 value per band)
144 CIRRUS [2] Number of 100 micron only NN I3
Confirmed Sources
147 NMERGE Number of Merged Sources NNN I3
150 SPARES 10 spare bytes 10A1
NOTES:
[1] Fields are listed in order of increasing Right Ascension of the Reference
Grid center. Field header records are located at the beginning of the
source listing for each field.
[2] This quantity is listed in the printed version of the catalog.
The following is a brief description of the individual entries in the Field
Headers of the machine-readable version of the SSC.
Field Name: FNAME
The IRAS/SSC field name is the position of the center of the reference
grid, given in the form hhmmssSddmmss.
Grid Number: RGRID, CGRID
The identifying number (see Section III.A) for the reference (R) and
confirming (C) grids for the field. The reference grid has the lower 60
micron median noise.
Observation Date: RDATE, CDATE
The observation dates for the reference (R) and confirming (C) grids in
Julian Days - 2445000.
Macro Type: MACRO
The macro identifying code is given in Table II.A.
Galactic Coordinates: GLON, GLAT
Galactic coordinates rounded to the nearest degree.
Positional Differences: PDRAS, PDRA, PDDECS, PDDEC
The sign and magnitude of the position difference between reference and
confirming grid centers, in the sense of (confirming - reference), in
right ascension and declination.
Grid Orientation: RANGLE, CANGLE
The orientation of the in-scan direction of the reference and confirming
grids on the sky, measured in degrees East of North.
Effective Area: EFFAREA
The effective area of the sky covered by both the reference and
confirming grids.
Redundant Fields: RUNDF
The number of additional grid pairs in the Serendipitous Survey, i.e.
with different OBSID's, which overlap this field by more than 5%.
Grid Noise: RNOISE, CNOISE
The median noise of the reference and confirming grids.
Confirmed Sources: NSOURC
The number of confirmed sources in this field, in the 12, 25, 60, and
100 micron bands, respectively.
Confused Sources: NCONF
The number of confused confirmations in this field, in the 12, 25, 60,
and 100 micron bands, respectively.
100 Micron Only Sources: CIRRUS
The number of 100 micron confirmed sources in the field that are not
band merged. The density of such sources is taken to be a measure of
the infrared "cirrus" in the field.
Band-Merged Sources: NMERGE
The number of band-merged sources in the field, i.e., the number of
source records following the field header.
Table V.C. Format of Source Listings [1] (SSC M-R Version)
Start Name Description Units Format
Byte
00 FNAME SSC field name 13A1
13 FLDRECNO Record number, SSC field file I6
20 NAME [2] Source Name 11A1
31 HOUR Right Ascension 1950 Hours I2
33 MINUTE Right ascension 1950 Minutes I2
35 SECOND Right ascension 1950 Deci-seconds I3
38 DSIGN Declination Sign +/- 1A1
39 DECDEG Declination 1950 Degree I2
41 DECMIN Declination 1950 Arcmin I2
43 DECSEC Declination 1950 Arcsec I2
45 SPARE 1 spare byte 1A1
46 ANGLE Position Angle of Degrees I3
Source Error Box (E of N)
49 SPARE 1 spare byte 1A1
50 FLUX [2] Averaged Non-color Jansky 4E9.3
Corrected Flux Densities (1.0E-26 W/m**2/Hz)
(1 value per band)
86 FQUAL [2] Flux Density Quality 4I1
(1 value per band)
90 RGRID Reference Grid Number I5
95 SPARE 9 spare bytes 5A1
100 RELUNC [2] Percent Relative Flux 4I3
Density Uncertainties
(1 value per band)
112 TLSNR 10x Local Signal-to-Noise 4I4
Ratio (1 value per band)
128 CC [2] Point Source Correlation 4A1
Coefficient (1 value per band)
132 TRFLUX 10x F(c)/F(r) 4I2
140 POSDRS12 Right Ascension Delta Sign +/- 1A1
141 POSDR12 Right Ascension Delta Arcsec I3
144 POSDDS12 Declination Delta Sign +/- 1A1
145 POSDD12 Declination Delta Arcsec I3
148 Repeat for 25 micron band +/- 1A1
-155 Arcsec I3
156 Repeat for 60 micron band +/- 1A1
-163 Arcsec I3
164 Repeat for 100 micron band +/- 1A1
-171 Arcsec I3
172 PNEARC [2] Number of Sources in 4I1
Confusion Window
(1 value per band)
176 NID [2] Number of Positional I2
Associations
178 IDTYPE [2] Type of Object I1
179 SPARE 1 spare byte 1A1
Table V.C.1a Format of Associations (SSC M-R Version)
Start Name Description Units Format
Byte
00 NAME SSC source name 11A1
11 RECNO Record number, source listings file I6
18 CATNO Catalog Number [4] I2
20 SOURCE Source ID 15A1
35 IDTYPE Source Type/Spectral Class [5] 5A1
40 RADIUS Radius Vector from Arcsec I3
SSC Position to Association
43 POS Position Angle from Degree I3
SSC Position to Association (E of N)
46 FIELD1 Object Field #1 Catalog I4
Dependent [6]
50 FIELD2 Object Field #2 Catalog I4
Dependent [7]
54 FIELD3 Object Field #3 Catalog I4
Dependent [8]
NOTES:
[1] Sources are listing in order of increasing Right Ascension within each
field.
[2] This quantity is listed in the printed version of the SSC.
[3] For associations with the IRAS/PSC, this value is 41.
[4] For associations with the IRAS/PSC, this value is 3.
[5] For associations with the IRAS/PSC, this field is left blank.
[6] For associations with the IRAS/PSC, this value is a flag indicating the
bands in which the source was detected with medium or high quality; it is
encoded as indicated in the PSC Supplement Table X.B.2.
[7] For associations with the IRAS/PSC, this value is the PSC 2.0 Flux
Density in the shortest (first) wavelength band in which it was detected.
Flux Densities higher than 10 Jy are encoded 9999.
[8] For associations with the IRAS/PSC, this value is the PSC 2.0 Flux
Density in the second wavelength band in which it was detected. Flux
Densities higher than 10 Jy are encoded 9999.
The following is a brief description of the individual entries in the
Source Listings of the tape version of the SSC.
Source Name: NAME, APPNAME
The IRAS/SSC source name is constructed as for the IRAS/PSC sources and
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 tuncated. If the sources
within a field have duplicate positional names, they are distinguished
by an appended letter (APPNAME), starting with the letter A.
Position: (HOUR, MINUTE, SECOND, DSIGN, DECDEG, DECMIN, DECSEC)
Positions are, as in the IRAS/PSC, given for the equinox 1950.0 and
epoch 1983.5. Hours (HOUR) 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 Angle: ANGLE
The position angle of the major axis of the SSC source error box
expressed in degrees East of North.
Flux Density: FLUX(4)
Each of the four wavelengths has a NON-COLOR-CORRECTED flux density in
units of Janksys, (1 Jy = 1.0E-26 W/m**2/Hz). The quoted value is the
noise weighted average as defined in Section III.C.2.
Flux Quality: FQUAL(4)
Each flux density measurement is designated high quality, moderate
quality, or upper limit (FQUAL = 3, 2, or 1, respectively) based on the
prescription discussed in Section III.C.
Flux Density Uncertainty: RELUNC(4), TLSNR(4)
Each flux density measurement other than an upper limit has an
associated uncertainty expressed in two ways; RELUNC is the uncertainty
expressed as a 1 sigma value in units of 100x/ (see Section
III.C.2), TLSNR is ten times the local signal to noise ratio (see
Section II.C) as determined from the reference or confirming grid,
whichever is least.
Point Source Correlation Coefficient: CC(4)
As discussed in Section III.B, SSC sources can have point source
correlation coefficients between 70-100%. These are encoded as
alphabetic characters with A=100, B=99... Z=75-70, one value per band.
The quoted correlation coefficients come from the reference or
confirming grids, whichever is higher, for high quality sources.
Flux Density Ratio: TRFLUX(4)
As discussed in Section III.B, SSC sources can have flux density ratios
0.5 < F(c)/F(r) < 2.0. TRFLUX is the flux density ratio for high
quality sources expressed as 10 x F(c)/F(r).
Position Differences: POSDRS12, POSDR12, POSDDS12, POSDD12 ...
The quoted positions of SSC sources are determined from a weighted
average of the positions of the confirmed sources in each band with a
high quality flux density (see Section III.C.1). POSDRS12, POSDR12,
POSDDS12 and POSDD12 give the sign and amplitude (in arcsec), of the
difference in right ascension and declination respectively, between the
final band-merged position and the 12 micron confirmed source position.
The following 12 entries repeat the above format for 25, 60, and 100
micron confirmed source components.
Confusion: PNEARC(4)
In regions of high source density, the Pointed Observation source
extraction process, as well as the Serendipitous Survey Confirmation and
Band Merging processing, can result in degraded positions and
incorrectly band merged sources. PNEARC is 1-(number of confirmed
sources in the confusion and band merge window). (See Section III.D.)
Any value greater than zero is indicative of potential confusion in the
processing and the resulting source information should be examined
carefully, e.g. by inspection of the grids in question.
Positional Assoc.: NID,IDTYPE,CATNO,SOURCE,TYPE,RADIUS,POS,FIELD1-3
The positional associations formats and definitions are done as per the
IRAS/PSC (Chapter X, Supplement), with the exceptions noted in Section
III.F.
Table V.D. Format of Overlapping Fields File (SSC M-R Version)
Start Name Description Units Format
Byte
00 GRID0 Prime Ref. Grid I5
05 SPARE 1 spare byte 1A1
06 GRID1 Overlapping Field #1, Ref. Grid I5
11 SPARE 1 spare byte 1A1
12 AOVLP1 Overlapping Sky Coverage Square Arcmin I4
16-26 Repeat of 5-15 for Overlapping
Field #2
27-37 Repeat of 5-15 for Overlapping
Field #3
.
.
60-70 Repeat of 5-15 for Overlapping
Field #5
71-79 SPARE 9 spare bytes 9A1
The quantities in the Overlapping Fields List are as follows:
Prime Reference Grid: GRID0
The grid against which other reference grids are checked for overlapping
sky coverage.
First Overlapping Grid: GRID1
The number of the first reference grid with more than 5% overlap with
GRID0.
Overlap Area: AOVLP1
Area of overlapping sky coverage between GRID0 and GRID1 in square
arcminutes.
The format is repeated for each additional grid overlapping with GRID0. If
more than 5 grids overlap with GRID0, they are listed in succeeding records,
as necessary; the first six bytes of each of these additional records begins
with a string of 6 spaces.
FITS-Table-Formatted Version of the IRAS SSC.
The IRAS Serendipitous Survey Catalog, as described above, does not
translate easily into FITS Tables format (see Harten, R.H., Grosbol, P.,
Greisen, E.W. and Wells, D.C. 1988, A. & A. Suppl. 73, 365). The format of
the field headers differs from the source data records following, and the
number of association fields varies for each object. This runs counter to the
stipulation that all records in a FITS table be of fixed length and have a
single uniform format.
In order to process the SSC Catalog into FITS format, the two 80-byte
records for the field headers were contatenated to form a single 160-byte
record with a uniform format (in other words, NAXIS1=160 for the field header
table). The records for each SSC source were split off into two separate
files -- one for the source data and one for the associations. The first two
records which always appear for each source were concatenated into bytes 21
through 180 of the source data table records. Each source record was prefixed
with (a) the field header name (bytes 1-13), and (b) the sequential record
number in the field header table where that field header could be found (bytes
14-19). Then, the associations were removed from the source data and written
in a uniform format to bytes 19-58 of the associations table. Each
association record was prefixed with (a) the SSC source name (bytes 1-11), and
(b) the record number in the source data table where that SSC source could be
found (bytes 12-17).
If the FITS table-formatted version of the catalog were loaded into some
database managagement system, a join operation should be applied to
"re-associate" the source data and associations records with the field
headers, either by using the field header/source name as a key, or the
sequential record numbers.
Please note that the separation of source data and association fields from
the main data file is only for the purposes of placing the catalog in FITS
tables format. The original, multi-record format is preserved in copies
of the catalog that are not FITS formatted.
-- Lee E. Brotzman, Astronomical Data Center