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NORTH20CM: Help

NORTH20CM - 20cm Radio Catalog


Overview

The 1.4-GHz Northern Sky Catalog - Version, 4 December 1991

This is the 20-cm Northern Sky Catalog of White, R. L. and Becker, R. H. (1992, Ap.J.Supp., in press) containing 30,239 sources detected from the Condon Greenbank images taken at 1.4 GHz over the declination range of -5 degrees to 82 degrees with a flux density limit of 100 mJy. This 20 cm catalog also contains the results of a cross-correlation with catalogs at 6 and 80 cm covering the northern sky between Dec=0 degrees and 70 degrees to give the spectral indices at three frequencies for about 20,000 sources.


Catalog Bibcode

1992ApJS...79..331W

References

Becker, R. H., White, R. L., Edwards, A. L. 1991, ApJS 75, 1.

Condon, J. J., & Broderick, J. J. 1985, AJ 90, 2540.

Condon, J. J., & Broderick, J. J. 1986, AJ 91, 1051.

Condon, J. J., Broderick, J. J., & Seielstad, G. A. 1989, AJ 97, 1064.

Condon, J. J., Condon, M. A., & Hazard, C. 1982, AJ 87, 739.

Douglas, J. N., Bash, F. N., Torrence, G. W., & Wolfe, C. 1980, The Texas Survey: Preliminary +18 Degree Strip (Univ. Texas Pub. Astr. No. 17).


Description

White and Becker have examined the 3 largest radio surveys currently available: the Texas 0.365 GHz Northern Sky Survey (Douglas et al. 1980), the Green Bank 1.4 GHz Northern Sky Survey (Condon & Broderick 1985, 1986), and the Green Bank 4.85 GHz Northern Sky Survey (Condon, Broderick, & Seielstad 1989). The latter, was used to generate the 6CMNORTH database of 53,000 sources (Becker, White, & Edwards 1991). The 20CMNORTH database of 30,000 sources is based on the 1.4 GHz survey and was generated in a manner similar to that described for 6CMNORTH.

Condon & Broderick (1985, 1986) used the Green Bank 300-ft telescope to survey the sky at 1.4 GHz between -5 degrees and 82 degrees declination. The survey was released in the form of 144 images. The resolution of the maps is approximately 700 arc sec, while a combination of noise and confusion results in RMS fluctuations in the maps of approximately 25--30 mJy. The resulting images suffer seriously from confusion. The threshold for identifying a source set at 100 mJy. Sources with flux densities at or near the threshold that do not have a counterpart at some other frequency should be viewed with caution.

The catalog is far from complete. The images were highly confused near the Galactic plane. Furthermore, there were many instances where two or more sources blended together making it impossible to fit the individual sources. In this catalog, any source that appeared extended is flagged because it is likely that such sources are blends of two or more sources. There are parameters for 30,239 sources including 3,552 extended sources. The fitted source positions of all sources have an accuracy of 160 arcsec (90% confidence) based on comparisons with positions in the 4.85 GHz catalog (6CMNORTH). This result is weighted towards weak sources. The positions of stronger sources will be considerably better (Condon & Broderick 1986). A match between sources in the two catalogs (defined as a separation of <= 300 arcsec) found 4.85 GHz counterparts for 80% of the 1.4 GHz sources. Approximately 1.4% of the matches are likely to be chance coincidences. Furthermore, 11% of the 1.4 GHz sources lie outside of the 4.85 GHz area (either north of +76 degrees or south of 0 degrees declination), so 90% of the 1.4 GHz sources that could possibly have matched 4.85 GHz sources do match. This strongly supports the reality of the bulk of the 1.4 GHz sources.

When a 1.4 GHz source matches two (or in a very few cases three) different 4.85 GHz sources, the alternate matches are given with the 4.85 GHz name of the alternate source(s) indented. Multiple 0.365 GHz matches are given only for sources without 4.85 GHz counterparts.


Quality

Each catalog has its own advantages and disadvantages. The Texas survey has superior positional accuracy (neglecting lobe shifts) but resolves out extended sources. The 1.4 GHz survey suffers from serious confusion, which both limits the completeness of the catalog and prevents the determination of accurate flux densities for weak sources in crowded fields. The 4.85 GHz catalog is probably the most complete of the three, but has a positional accuracy substantially worse than that of the Texas survey.

The matches among the catalogs are indicative of the reliability of the catalogs. Ninety-five percent of the unresolved 1.4 GHz sources between 0 degrees and 70 degrees are also detected at either 4.85 GHz or 0.365 GHz. The failure to find matches for some sources may reflect the limitations of the three catalogs. For example, 3,300 sources with detections at 1.4 and 4.85 GHz do not have Texas counterparts. A power-law extrapolation of the flux densities of these sources to 0.365 GHz indicates that 1,000 are brighter than the 250 mJy flux density limit of the Texas survey. Although this might reflect on the completeness of the Texas survey, it is more likely that these sources are too extended for the Texas survey to detect.

There are 48 unresolved 1.4 GHz sources at high Galactic latitudes b > 15 degrees) with 0 degrees < Dec < 70 degrees brighter than 250 mJy that do not have apparent counterparts at either 4.85 or 0.365 GHz. However, loosening the matching criterion to 400 arcsec, 60% of these sources do have matches to 4.85 GHz sources. This may be the result of the blending of several 1.4 GHz sources into a single catalog entry with a worse than average positional error. Therefore the absence of a counterpart for a source in this 1.4 GHz catalog can be misleading and should not be over-interpreted, especially for extended sources, which are likely to have less accurate positions.


Parameters

BII
The galactic latitude of the source.

Confusion_Flag
A `?` indicates that a 20cm source matched more than one 6cm or 80cm source, so confusion is likely. In that case the additional matches are listed as well.

Dec
The Declination of the source.

Ext_20_Flag
"*" if source is extended at 20cm, else blank.

Ext_6_Flag
"*" if source is extended at 6cm, else blank.

Flux_20_cm
The 1.4 GHz flux (mJy)

Flux_6_cm
The 4.85 GHz flux (mJy) from the 6CMNORTH database (Becker et al 1991)

Flux_80_cm
The 0.365 GHz flux (mJy) from Texas survey, Douglas et al., private comm.

LII
The galactic longitude of the source.

Name
The `20cm` Name, constructed from RA, Dec HHMM+DDMM with A or B added at end if needed to make name unique.

When a 1.4 GHz source matches two (or in a very few cases three) different 4.85 GHz sources, the alternate matches are given with the 4.85 GHz name of the alternate source(s) indented. Multiple 0.365 GHz matches are given only for sources without 4.85 GHz counterparts.

NH
Galactic absorption from 21 cm (NH).

RA
The Right Ascension of the source.

Sihi
Spectral index between 6 and 20 cm; blank if not detected at 6 cm. F((nu)) = C(nu)**alpha.

Silo
Spectral index between 20 and 80 cm; blank if not detected at 80cm

Wid_Flag
The 4.85 an 0.365 separation > 100 sec.


Contact Person

Questions regarding the NORTH20CM database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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