VIII.D.5 Completeness and Reliability in the Galactic Plane

IRAS Explanatory Supplement
VIII. Sky Coverage, Confusion, Completeness and Reliability
D. Point Source Catalog Reliability and Completeness

D.5 Completeness and Reliability in the Galactic Plane

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The completeness and reliability in regions of high source density, defined in Section V.H.6, can be estimated by methods similar to those discussed earlier in this section, but such estimates should be regarded with great caution. As an example, about 200 12 µm sources in a 4.5°2 area from the minisurvey near the Galactic plane were analyzed, The region was covered with seven HCONs and has an average source density of 39 per sq. deg. Table VIII.D.2 gives counts of cataloged 12 µm sources as a function of flux density and the number of HCONs in this area.

Table VIII.D.2 Number of HCONs in a 7 HCON High Source Density Region
Range in 12 µm
Flux Density
Numbers of HCONs in High
Source Density 7 HCON Area*
(Jy) N=1 2 3 4 5 6 7
1-2 49 9 1 6 13 17 30
2-4 22 2 1 3 5 17 26
4-8 6 1 1 0 1 10 14
8-16 0 0 0 0 1 3 6
16-32 0 0 0 0 0 0 2
32-64 0 0 0 0 0 4 0
64-128 0 0 0 0 0 1 3
* the area consisted of two 1.5° × 1.5° regions with the following Galactic coordinates:
334.5° < l < 336.0°; -2.0° < b < -0.5°
336.0° < l < 337.5°; -0.5° < b < +1.0°

If the parameters p^q^; and û are evaluated for the high density regions using Eqs. (VIII.D.14-.D.16), i.e., assuming that all single HCON sources are false and all two or more HCON objects are real, then p^ < 0.2 and q^ < 1.5 (sq. deg)-1 HCON-1 for fv   1 Jy. These values imply that for flux densities above 1 Jy the completeness of the survey in the two HCON high density sky exceeds 63% and in the three HCON high density sky exceeds 88%.

The reliability of a two or more HCON source can be estimated from the value of q^ and Eq. (VIII.D.7). for M equal to 2 or 3 and A equal to 25 beam areas, 0.02 sq. deg, the estimated reliability of a two HCON object brighter than 1 Jy at 12 µm exceeds 99.5%. The numbers in Table VIII.D.2 show that applying the formalism of Eqs. (VIII.D.14 -.D.16) to the high density region is not entirely valid. The fact that there is a minimum in the number of sources with a specified number of HCONs at a value of three HCONs suggests that sources with fewer than three HCONs are spurious. It should be noted, however, that including these false two and three HCON sources in the determination of q decreases the estimate of the net reliability of a source only slightly.

The existence of minisurvey sources (Table VIII.D.2) observed six or seven times in regions of high source density indicates that the completeness of bright sources is significant even in these regions. If the minisurvey region is representative of the entire Galactic plane region, then the completeness for sources with flux densities above 10 Jy will be about 86% for the part of the sky covered with two HCONs and about 98% for the sky covered with three HCONs. These results apply to sources observed at 12 and probably 25 µm where extended sources are few. At 60 and 100 µm the meaning of the reliability of a source is less clear because of the large population of extended objects seen against a very complex background. No attempt has been made to assess either the completeness or reliability of sources at these wavelengths.

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