IRAS Explanatory Supplement
VII. Analysis of Processing
H. Meaning of Point Source Flags

Confusion Flags
Cirrus Flags

H.1 Confusion Flags

Confusion flags are essential in assessing the quality of a point source. Sources with all flags equal to zero can be accepted without question. Sources with non-zero flags may have some problems as discussed below.

The confusion flags include a count of neighbors around a given source that were found in either the point or small extended source WSDBs, as well as a flag which was set by the processors during seconds-confirmation, band-merging and hours-confirmations. This relatively large number of flags was required by the complexities of the processing and of the infrared sky.

H.1.a Point Source Neighbors

Point source neighbor flags warn of problems in the immediate vicinity of the source. The point source neighbors come from the cleaned-up WSDB. The number of neighbors is counted in a rectangle oriented along the largest position uncertainty direction. That rectangle usually reflects the original scan direction used to observe that source. The large dimension of the box is a half-width of 6' along the smallest position uncertainty axis, which corresponds to the maximum distance in which another source may have shadowed or stolen any detections of the source in question (Section V.C.7). The half-width is 4.5' along the orthogonal axis, corresponding to a full detector width. Neighboring sources within this distance produced at least one detection that was the sum of the two sources.

Weeks-confirmed and single hours-confirmed point source neighbors are counted separately in that box. Non-zero values for either count, but especially for PNEARW, the count of weeks-confirmed sources, should caution the user that one or more bands of the source may be reported incorrectly and to look in a small region around the source for confusing neighbors. Most sources with neighbors are found in regions of high source density.

H.1.b Small Extended Source Neighbors

Two types of small extended source counts are given. The SES1 flag counts separately in each band the total number of hours-confirmed small extended sources. Note that a weeks-confirmed small extended source containing three hours-confirmations would contribute three to this count, and a small extended source containing only one hour-confirmation would contribute one to this count. The SES2 flag counts separately in each band only the number of weeks-confirmed small extended sources. Both flags use the box described above for point source neighbor counting.

Both flags warn of the presence of structures larger than point sources, and hence cast doubt on whether the reported source is truly point-like or is only part of a larger complex. Two separate flags exist for1 the following reason. In cold regions containing only a single small extended source, SES1 would equal the number of hours-confirmations times SES2, and thus the flags would be redundant. However, in complex regions SES1 may be unrelated to SES2. That is, because of the effects of cluster analysis (Section V.E) there may have been no acceptable small extended source, even though many small extended source detections occurred. The distribution of sources with high values of SES1 and with non-zero values of SES2 is shown in Figs.VII.Ap.21 and Figs.VII.Ap.22.

H.1.c Confuse Flag

A final flag related to near neighbor confusion is derived from the repeated occurrence of confusion during the processing of a source. In a given band, if more than one candidate per scan could confirm with the source at either seconds-confirmation (excluding edge detections), band-merging, or hours-confirmation, the processor set a flag. If any such flag was set in two separate hours-confirmations of a source, then the confuse flag was also set in that band. Thus if the confuse flag is set, something other than an isolated point source was present at or near the position of the source in question, and the user should again be cautious. The distribution of sources with the confuse flag set correlates well with high source density areas.

H.1.d Summary

The user examining an (IRAS source can immediately rule out almost any problem if all of its neighbor flags are zero, and SESl is 0 or l. Sources that have all those flags set to zero are almost always completely clean point sources. There are about 63,000 such sources in the entire catalog, 25% of all sources. Above a Galactic latitude of 20°, the fraction is much higher, 30,000 out of 60,000 sources. Including sources with SES1 less than 2, which are still almost always clean sources, increases the percen- tage of clean sources to 38% in the entire sky, and to 70% in the sky above Galactic latitude 20°.

Sources with non-zero flags may be confused with point source neighbors or may only be part of an extended complex, and unfortunately must be examined in more detail.

H.2 Cirrus Flags

Cirrus flags are set to warn the user that a catalog source may be adversely affected by infrared cirrus or may even be no more than a bump in the infrared cirrus. These flags and the 2' sky brightness images are the best possible substitutes for examining the raw detector data. If all cirrus flags are smaller than the limits discussed below, then one can infer that the source is unlikely to have been affected by cirrus.

CIRR1
CIRR1 gives an estimate of the existence of cirrus on the point source scale. CIRRl is total number of sources detected only at 100 µm , including sources with single and multiple hours-confirmations, in a 1 sq. deg box centered on the source. The sky distribution of sources with CIRR1 greater than three is displayed in Fig. VII.Ap.23, and shows the areas where cirrus is a problem. This figure is similar to the map of sources detected only at 100 µm shown earlier (Fig. VII.Ap.10).

CIRR2
CIRR2 is a logarithmic function of the ratio of the 100 µm point source flux to an estimate of the 100 µm flux produced by cirrus, derived from the filtered 0.5' 100 µm data (Section V.H.4). Figure VII.Ap.24 shows that the distribution of sources with CIRR2 greater than 4 correlates well with other cirrus maps.

CIRR3
Another estimate of the importance of cirrus comes directly from the total 100 µm emission is a 0.5' beam data. CIRR3 is equal to the intensity of the 100 µm emission in MJy sr^-1.

SES1
The SES1 flag at 100 µm , discussed earlier, is a good cirrus indicator. See the earlier discussion.

Summary

No clear-cut prescription can be given to a user to guarantee that cirrus has not adversely affected a given source. If the source has a strong 100 µm flux, no missing bands, and all cirrus flags are well below the cutoff values discussed above, then it is likely that few problems exist. If any of the above conditions are violated a closer examination of nearby sources will usually indicate the importance of cirrus. However, in some cases, only an examination of images or, finally, the detector data will resolve all doubts.

IRAS Explanatory Supplement VII. Analysis of Processing H. Meaning of Point Source Flags