The limiting magnitude of the objects in the Preliminary Release Source Catalog is regulated by the net exposure time accumulated by WISE and characteristics of the sky where source extraction takes place. Among the factors that dictate the achieved limiting depth are:
The bulk variation in depth of the Source Catalog around the sky is illustrated in Figures 1-4 that show maps of the peak of the Catalog source count histograms in each band. The impact on the effective depth of the Catalog by survey depth-of-coverage is manifested as the increasing depth from the ecliptic plane towards the poles. The loss of depth due to confusion in the Galactic Plane is most prominent in W1 and W2 where the source density is highest.
|Figure 1 - W1||Figure 2 - W2|
|Figure 3 - W3||Figure 4 - W4|
|False-color maps showing the spatial distribution and variation of the peak of the differential source count histograms computed in 0.2x0.2 deg spatial bins. The color scale in the images gives the peak magnitude color encoding for each band. The Maps are Hammer projections in ecliptic coordinates. The centers correspond to ecliptic coordinate 180°,0°. Ecliptic north is towards the top, and ecliptic longitude increases towards the right.|
To emphasize reliability of the Preliminary Release Catalog, a conservative source detection threshold was used during first-pass Multiframe processing. Sources were required to have SNR>5 on the multiband sigma images (IV.5.b for detection and characterization to construct the Working Database (WDB) of extractions. Furthermore, a WDB entry was required to have a SNR>7 in at least one detected band to be included in the Catalog, as described in V.3.b.
You will very likely be able to see faint sources on
Atlas Images that do not have counterparts in the Preliminary Release
Source Catalog as a result.
Very bright sources effectively mask the sky in their vicinity. Detections of fainter sources are suppressed because the local background levels are elevated from the scattered light halos, and because of confusion with diffractions spikes, latent images, and optical ghost images from the bright sources. Furthermore, even when fainter sources are detected near very bright objects, they may be flagged as contaminated and inadvertently filtered from the Catalog. The impact of masking by bright sources is discussed in VI.2.c.ii.
As discussed below, Sources that are significantly
larger than the WISE beam may be artificially split into multiple
extractions. Hence, very large galaxies and complex nebulosity in the
Galactic Plane may produce clusters of entries in the Source Catalog.
The duplicate source resolution process in Source Catalog generation identified duplicate detections in the Tile overlaps regions and selected one for inclusion in the Source Catalog using simple geometric criteria. The selected duplicate apparitions were then required to satisfy the final source selection criteria to be included in the Catalog. In a small number of cases, one member of a group of multiple detections may have been selected by the duplicate resolution test but did not satisfy the final source selection criteria whereas the apparition that was not selected by duplicate resolution would have met the final criteria. 1,923 objects are known to have been omitted from the Catalog because of this confusion.
The positions of objects in the WISE Preliminary Release Source Catalog may be systematically offset from their try position by up to 1" in the declination direction.
Approximately 20% of sources fainter than W1~14.5 mag exhibit this bias, while an increasingly smaller fraction of the sources are biased up to at least W1=13.0 mag. The declination bias was caused by an error in the pipeline source extraction software that failed to apply a 0.5 Atlas Image pixel declination offset to some sources when performing simultaneous PSF fitting of multiple detections in close proximity (i.e. passive deblending). The sign of the declination bias depends on the scan direction and celestial hemisphere in which a source is location. The error does not affect right ascension measurements. See VI.5 for illustrations of the effects of this error and discussion of its impact.
The quoted position declination uncertainties in the Catalog, sigdec, have been inflated to reflect the presence of the bias. See the following section for a description.
Reliable positions may be extracted by measuring source positions on the Atlas Images. However, positions derived using flux-weighted centroid or peak finding and the WCS information in the Atlas Image headers is also biased relative to the Source Catalog by up to 0.5" in W1-W3 and up to 1.4" in W4. See II.3.g for a procedure to correct the Atlas Image derived source positions and image WCS.
Because there was insufficient time to correct the declination bias prior to the Preliminary Release, the Catalog declination uncertainties, sigdec, were inflated to reflect the presence of the bias. The uncertainties were adjusted by adding 0.5" in quadrature to the statistical declination measurement uncertainty (see VI.5.b). This yields a maximum absolute deviation for the distribution of uncertainties equal to the what would be produced if the declination error distribution was Gaussian.
Users who conduct studies based on the Preliminary Release Catalog positions should note that the distribution of true position residuals with respect to the 2MASS reference frame is not Gaussian, and will not scale as they would for a Gaussian (see VI.5.b).
Correction for differential aberration was not made during the first-pass processing. This can contributes a small (~0.1") increase in the observed positions residuals with respect to the 2MASS reference frame that correlates with position on the sky because it is related to the WISE relative orbital velocity. Correction for differential aberration will be included in second-pass processing.
For point-like objects, the profile-fit photometry measurements (w?mpro) provide the most accurate measurements for unresolved (to WISE) objects. The curve-of-growth corrected "standard-aperture" photometry can on average provide slightly better accuracy for sources with approximately SNR>30 because they not subject to PSF knowledge uncertainties like the profile-fit measurements. However, aperture measurements are more susceptible to contamination from aberrant and masked pixels within the measurement aperture.
The profile-fit and standard aperture photometry will systematically underestimate the brightness of objects that are extended with respect to the WISE PSF. See below for further discussion of extended objects.
If the associated measurement uncertainty in band is "null", then the
quoted magnitude is computed from a 2σ brightness upper limit
Care must be taken to not mis-interpret upper limits as detections.
As discussed in VI.4.c, WISE source
brightnesses are systematically underestimated in very high
source density regions. This occurs because background levels in
those regions are overestimated by contamination from surrounding stars.
Improved background estimation algorithms have been employed for
WISE second-pass processing.
The Preliminary Release Source Catalog contains aperture photometry measurements made on the Atlas Images for apertures ranging in size from 5.5" to 24.75" (W1-W3) and 11.0" to 49.5" (W4). These measurements have not been curve-of-growth corrected like the standard-aperture magnitudes. The large-aperture photometry is useful for estimating the brightness of small, extended sources too large to be properly measured using the profile-fit and standard-aperture photometry.
Users should be aware of features and limitations of the aperture measurements.
Sources that are extracted near regions with very low or no depth-of-coverage sometimes exhibit erroneous flux measurements because the background estimate has been compromised by the low coverage. In these cases, the background estimates can be aberrantly low, resulting in an apparent bright source where one does not exist. Many of these sources are suppressed by the minimum depth of coverage requirement for Catalog source selection (VI.3.b), but sources affected by this on one band may be pulled into the Catalog if they have better coverage in other bands.
Refer to the w?m and w?cov parameters in the Catalog to identify bands that may affected by low coverage.
WISE profile-fitting photometry attempts to deblend a detection into two components if the single-PSF fit chi-squared goodness-of-fit value is >1.8. If the resulting two-component fit reduces the chi-squared from the initial one, and the two-component chi-squared is <3 (from an initial value >3), the two sources are reported. For first-pass processing, at most one component can be added. Actively deblended sources in the Preliminary Release Source Catalog can be identified by having the parameter na=1.
Note that active deblending was not performed in the profile-fitting measurements for the Single-exposures.
WISE Source Catalog entries contain several different measurements of photometric uncertainties.
The profile-fit photometry measurement uncertainties, w?sigmpro are systematically underestimated in high
source density regions because the contribution of confusion noise
was overestimated. Consequently, the profile-fitting chi-squared values,
w?rchi2, are systematically underestimated
in these regions which can have adversely affect decisions to
deblend confused sources in crowded fields.
The profile-fit photometry flux SNR values in the WISE Source Catalog, w?snr, do not usually exceed ~60 even for extremely bright sources. This occurs because the profile-fit measurement includes a PSF uncertainty component that dominates the measurement precision at high SNR levels. As a result, profile-fit measurement signal-to-noise ratios do not continue to increase arbitrarily with source brightness.
All Catalog source magnitudes represent in-band brightness
measurements that are calibrated by comparison
with WISE measurements of a network of standard stars of presumably known
brightness within the WISE bands (see IV.3.g).
The magnitudes are not monochromatic values.
The WISE system photometric response was stable to <<1% over the life of the cryogenic survey (IV.3.g.vii). Therefore, single photometric instrumental zero points in each band are used to calibrate photometry. For first-pass processing, these zero points were defined using the first two months of on-orbit data.
Residuals biases with respect to the mean brightness of the standard
stars is <0.01 magnitudes in W1, W2 and W4 in the Source Catalog.
For W3, a there is a mean bias of approximately 0.022 magnitudes in
the sense that the Catalog W3 magnitudes may be on average
0.022 mags fainter than their quoted values (e.g. add 0.022 mag
to the Catalog W3 mags to get a better calibrated measurement).
The broad WISE bandpasses may necessitate significant color corrections when deriving monochromatic flux densities from the in-band magnitudes depending on the spectral energy distribution of the particular object. The nominal system flux zero points are defined for a fν∝ν-2 spectrum through the WISE bandpasses. Color corrections for other spectral slopes are given in IV.3.g.vi and Jarrett et al. (2011 in press).
The reliability of Source Catalog is estimated to by >99.9% for sources brighter than W1<15.6, W2<14.3, W3<10.2, W4<6.5 mag in unconfused regions of the sky with 12 or more frame coverages (VI.7). The fractional reliability decreases for fainter objects and in regions where there is less coverage. Therefore, for a catalog the size of the WISE Source Catalog, there will be thousands of unreliable entries that do not correspond to an actual object on the sky.
The source selection criteria used to construct the Source Catalog were designed designed to construct a reliable Catalog from the superset of extractions in the Working Database. Exploit the WISE survey strategy of obtaining multiple, independent exposures of each region of the sky, conservative detection limits, flagging of artifacts in processing pipeline.
Here are some tips for selecting preferentially reliable entries in the WISE Preliminary Release Source Catalog.
Source extractions that are positionally associated with the predicted locations of image artifacts produced by bright sources were identified and flagged in the Artifact Identification subsystem of the WSDS Multiframe Pipeline. Artifact locations are predicted using geometric models scaled with "parent" bright source magnitudes. These models and their parameters were based on analyses of IOC and approximately one month of survey data, so could not be fully optimized in the time necessary to begin first-pass processing. As a consequence, artifact identification and flagging in the Source Catalog is both overly conservative in some areas, and deficient in others.
With the exception of latent images, no attempt was made in first-pass processing to flag artifacts from sources detected on a given Atlas Image that were produced by bright sources outside the footprint of that Tile. This includes diffraction spikes, scattered light halos and optical ghost images. Therefore, contaminated sources and spurious detections from off-image sources may be unflagged in the Source Catalog.
To emphasize the reliability of the Preliminary Release Source Catalog, the artifact flagging model parameters of artifacts were conservatively set in the sense that sources tended to be overflagged. This results in decreased Catalog completeness around bright stars (see VI.2.c.iii, and a large percentage of all sources being flagged as contaminated in very high source density regions.
Source brighter than approximately 1.0, 0.0, -2.0 and -6.0 in W1, W2, W3 and W4, respectively, were not reliably extracted in first-pass processing (VI.4.d.iv). Therefore, artifacts produced by these objects will not be flagged in the Source Catalog. For example, the star Aldebaran is in the Preliminary Release Catalog, but it has valid photometry only in W4 (-2.90 mag). It was not properly extracted in W1, W2 and W3, and therefore artifacts and spurious detections from Aldebaran's artifacts in those bands will be in the Source Catalog without appropriate artifact flagging.
Source brighter then these limits were not reliably extracted in first-pass processing (VI.4.d.iv). If they are brighter in all bands, they may not be appear in the Source Catalog. Such objects may be in the Source Catalog if they were properly extracted in a fainter band. However, photometry will likely will be missing or badly inaccurate in the very bright bands.
Saturation begins to affect sources brighter than approximately 8.0, 6.7, 3.8 and -0.4 in W1, W2, W3 and W4. Profile-fit photometric measurements are extracted for saturated sources by fitting the PSF to the non-saturated pixels in their wings. Sources brighter than the saturation limit exhibit varying degrees of photometric bias, as described in VI.4.c.i. The most severe bias is seen in W2 where source brightness may be overestimated by nearly one magnitude approximately two magnitudes brighter than the saturation limit.
As discussed above, missing detections of bright stars can lead to the failure to flag spurious and contaminated to detections that may wind up in the Catalog.
WISE does not have a separate extended source catalog. The WISE Source Catalog contains entries for all sources detected on the Atlas Images, both point-like and small resolved objects.
The Profile-fit and standard aperture photometry in the Source Catalog are optimized for point-source characterization. They both systematically underestimate the brightness of extended sources (IV.5.c.vi). Refer to the ext_flg to identify Source Catalog entries that may be extended.
Large aperture photometry is provided for Catalog entries, and those measurement may provide better estimates for extended objects. For WISE sources that are associated with objects in the 2MASS Extended Source Catalog (XSC), measurements in elliptical apertures scaled from the object size and shape in the 2MASS XSC are provided in the WISE Catalog that better characterize the object flux.
The source detection and extraction steps in the WISE pipelines may split large, extended objects into multiple pieces. Any and all pieces that that satisfy the source selection criteria will appear in the Catalog. See IV.5.c.vi for examples that illustrated extended source splitting. Refer to the xscprox column to find WISE Catalog entries that may be sections of large galaxies identified by the 2MASS XSC. However, WISE is sensitive to large Galactic diffuse emission structures, and these objects will also be split into multiple components that will not be tracked by the xscprox column.
The WISE Source Catalog entries include photometry for the closest entry from the 2MASS Point Source Catalog that falls within 3" of the WISE source positions (IV.7). These are associations not identifications. Although the position accuracy of the two Catalogs is very good, there is a non-zero probability of chance associations between physically unrelated objects, as well as missed associations between the Catalogs. This is particularly true for objects with high proper motions, and those in regions with large foreground extinction. You should always confirm associations by reviewing carefully the entries from both Catalogs.
Last update: 2011 May 4