Davy Kirkpatrick's analysis of 2MASS sources without optical counterparts in the Sloan Digital Sky Survey Early Release area shows that the majority of non-confirming sources that are not associated with asteroids or bright star bright star confusion in the optical data are H-band only sources in the PSC. Based on these findings, Mike Skrutskie examined the distribution and statistics of H-band sources on the sky. He found a significant number of H-only sources in the PSC with ph_qual values of 'UAU', which signifies otherwise the highest quality photometry. The majority of H-only sources are concentrated towards the Galactic plane, and are most likely associated with confusion. However, there are a significant number (~48,000) of H-only, high quality sources at high galactic latitude. Among these, many more are present in data taken from the southern 2MASS telescope than from the northern observatory.
H-band-only sources are difficult to explain astrophysically. Single-band J sources should be relatively common in the high latitude sky because of the intrinsic colors of stars and the relative sensitivity between the 2MASS bands. Single-band Ks sources can result from intrinsically very red objects, or those obscured by dust, such as extreme AGB and carbon stars, a number of which are found even at high latitude. The best way to produce an H-band "peaked" source would be by the presence of strong emission lines in the H-band. This might be possible with the Fe emission lines in some planetary nebulae, or perhaps in z~1.5 QSOs if the H-alpha line is redshifted into the H-band window. Either of these cases are expected to be rare, though, so we conclude that the majority of H-only detections at high latitude are spurious detections. The asymmetry between observatories also suggests that their occurrence is related to a characteristric of the instruments.
Table 1 contains some basic statistics for the high galactic latitude PSC. H-band-only sources constitute ~0.5% of the total number of high latitude, unconfused, high photometric quality sources that are believed to constitute the "Catalog-quality" part of the PSC. If the most of these are spurious, as suggest by the SDSS comparison, then this implies a source reliability below the 99.95% specified for the Level 1 Science Requirements.
Table 1 - Statistics for the High Latitude PSC
| Characteristic | Criteria | Number | |
|---|---|---|---|
| All high quality, unconfused sources | |b|>20o AND ph_qual MATCHES '*A*' AND prox>8 | 10322301 | |
| High quality, unconfused, H-band detections | ... AND rd_flg MATCHES '?[1-6]?' | 10302440 | |
| High quality, unconfused, H-band-only detections | ... AND rd_flg MATCHES '0[1-3]0' | 48279 | |
| North - old H array | 936 | ||
| North - new H array | 498 | ||
| South | 4685 |
Figure 1 shows the distribution of cross-scan positions (x_scan for the H-only sources in the high latitude, clean PSC. Sources from north and south observations are color-coded, as are those from the old and new north H-band arrays.
The H-only sources are strongly concentrated in a few narrow peaks in the x_scan distribution, and most (88%) are in the peak between 153 and 155 arcsec in the southern data. There are a total of nine identifiable peaks in the distribution, seven from the southern H array, and one each from the old and new northern H-arrays. The nine peaks contain 96.4% of all of the high latitude H-only sources. Table 2 gives the x_scan range of each peak, and the number of high latitdue, clean H-only sources contained in each peak.
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| Figure 1 |
Table 2 - Cross-scan position range of H-only Distribution Peaks
| H-array | x_scan Range | Number of H-only sources |
|---|---|---|
| North - old | +186 to +192 | 236 |
| North - new | -174 to -169 | 303 |
| South | -241 to -234 | 309 |
| -228 to -223 | 1102 | |
| -220 to -211 | 409 | |
| -207 to -201 | 575 | |
| -187 to -185 | 65 | |
| -163 to -161 | 851 | |
| +153 to +155 | 42,710 |
The distribution of the H-only sources indicates that they are detections of features in the arrays, rather than astrophysical sources. The isolated peaks are most likely caused by "hot" pixels in the arrays. The broader excess of H-only sources in the old northern H-array data is probably associated with the low coverage due to the growing number of dead pixels concentrated in the center of that array.
Noisy pixels were usually masked automatically by the DARKS and DFLAT subsystems in the pipeline. In those systems, pixels with consistently aberrantly large dispersions in the dark and sky-flat stacks were identifed and turned off in the processing. This was usually quite effective, and succeeded in supressing the large number of hot pixels in the northern Ks array. Gene Kopan has found the probable mechanism that allowed these spurious noise detections to slip by the masking procedure. These are most likely hot pixels situated next to masked, dead pixels.
Detections of spurious single-frame noise events should be characterized by large chi-squared values in the profile-fit photometry measurements. Figure 2 and Figure 3 show the chi-squared plotted against cross-can position for the high latitude northern and southern H-only samples, respectively. The H-only sources in each of the cross-scan peaks are clearly distinguished by having large chi-squared values, with the northern values. being generally higher than the southern sources.
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| Figure 2 | Figure |
The H-only sources outside of the cross-scan peaks, and with chi-squared values near unity have not been examined yet.
At least 97% of the high galactic latitude, high photometric quality H-band-only sources are associated with spurious detections of unmasked hot pixels. Most of these come from one pixel in the southern H-band array. These sources can be reliably identified using their cross-scan position and chi-squared values, so could be easily flagged in the PSC.
A more insidious consequence of the noisy pixels that has not been examine yet is their potential impact on photometry of real sources since they will occasionally coincide.
[Last Updated: 2002 November 6; by R. Cutri]