NORTH6CM - 6cm Radio Catalog
The NORTH6CM database is a catalog of 53,522 4.85 GHz sources generated by
Becker, R. H., White, R. L., Edwards, A. L. 1991, ApJS 75, 1. It covers between
0 degrees and 75 degrees declination using observations taken with the NRAO
Greenbank 300-ft telescope by Condon, Broderick and Seielstad (1989). The flux
limit of the catalog is dependent on declination and ranges from approximately
40 mJy at 0 degrees to 20 mJy at 60 degrees. The source positions given in the
catalog have a 95% confidence radius of approximately 50 arcsec. Spectral
indices have been calculated for 29,051 sources which have counterparts in the
Texas 365 MHz Northern Sky survey.
Bennett, C. L., Lawrence, C. R., Burke, B. F., Hewitt, J. N., and
Mahaney, J. 1986, ApJS 61, 1.
Condon, J. J., Broderick, J. J. and Seielstad, G. A. 1989, AJ 97, 1064.
Langston, G. I., Heflin, M. B., Conner, S. R., Lehar, J., Carrilli, C.
L., and Burke, B. F. 1990, ApJS 72, 621.
White, R. L., Becker, R. H., and Helfand, D. J. 1991, ApJ 371, 148.
This is a catalog of 53,522 4.85 GHz radio sources based on the northern sky
survey of Condon, Broderick, and Seielstad (1989), hereafter referred to as
CBS. The NRAO 91 m telescope in Greenbank, West Virginia was used by CBS to
survey the sky between declinations of 0 degrees--75 degrees at 4.85 GHz. The
observations were made in October, 1987. The resulting data were gridded into
285 1,024 by 1,024 FITS images with 40 arcsec pixel size. The RMS noise in the
images varied from 5--8 mJy. The images had an angular resolution of
approximately 3.5 arcmin. Sources with peak fluxes brighter than 70 Jy were
truncated in the CBS images, so they usually have been omitted from this
catalog. The extraction of a source catalog from the images was limited to
compact sources smaller than approximately 5 arcmin in diameter.
The goal was to identify sources with peak intensities greater than 5 times the
image RMS intensity level. Near declination +70 degrees the threshold is 20
mJy, while at 0 degrees it is typically 40 mJy. In fact, the RMS noise level
varies on scales of 5 degrees or less. For instance, there is a ring of high
noise at 64 degrees +/- 1 degrees declination which is approximately 30% higher
than that just to the immediate north or south. Aside from these global
effects, the completeness of the catalog suffers from local effects. The
sensitivity to weak sources decreases in the vicinity of strong sources (>
approximately 10 Jy) due to the sidelobe pattern of the 300-ft telescope. This
is a serious problem along the entire galactic plane (abs(b) < 5 degrees) as
well as near a number of isolated sources which are listed in Becker et al.
For each radio source in the catalog a position and a 4.85 GHz flux density are
given. The errors are dominated by systematic errors in the data. An estimate
of the accuracy of the catalog was made through comparisons to other data sets
as well as through checks on internal consistency. This is most readily
accomplished for the source positions using data collected at other
frequencies. In particular, a comparison was made to the Texas 80 cm Survey
(Douglas, priv. comm.) which covers nearly the entire 6 cm survey area (the
Texas survey reaches Dec approximately 71.5 degrees). Using a matching
criterion of 160 arcsec 29,051 matches were found between the two catalogs with
an expectation of 300 false matches.
The separation between matched pairs is a measure of the combined positional
errors in both catalogs. The Texas Survey has much better angular resolution
in principle (approximately 5 arcsec) but has positional ambiguity due to
interferometric sidelobes. To reduce this effect, we can limit ourselves to
the 7,531 matched Texas source which are point sources with high quality flags.
For this subset, 67% of the matches have separations of 25 arcsec or less while
95% are separated by less than 51 arcsec. The positional accuracy also depends
on the 4.85 GHz flux density of the source in question. For sources with a
4.85 GHz flux density greater than 100 mJy, the 67% and 95% confidence error
circles decrease to 18 and 38 arcsec respectively. These values are consistent
with the positional accuracy for the survey estimated by CBS. To the extent
that Texas sources contribute some of uncertainty in position, the above values
should be taken as upper limits to the positional uncertainty in this catalog.
Determining the accuracy of the flux densities in the catalog is more
problematic. A comparison between this catalog and the MIT--Greenbank 5 GHz
Surveys (Langston et al. 1990; Bennett et al. 1986) indicates a general
agreement in flux scales for brighter sources (>150 mJy) but some systematic
differences for weak or extended sources.
Declination Threshold (mJy)
0 degrees 40
10 degrees 30
20 degrees 25
30 degrees 25
40 degrees 22
50 degrees 22
60 degrees 20
70 degrees 20
The galactic latitude of the source.
The B-V color.
BROWSE classification flag.
The Declination of the source.
Ext_flag is "*" if the source is extended, else it is blank.
The 4.85 GHz flux (mJy) density of each source in mJy (the peak flux density
for point sources and the integrated flux density for extended sources).
The galactic longitude of the source.
The 6cm name, constructed from RA and Dec in the format HHMM+DDSS, with A or B
added at the end if needed to make the name unique.
NH Galactic Absorption from 21 cm (Note: All values are zero.)
The Right Ascension of the source.
The redshift of the source.
The spectral index between 80 cm and 6 cm for all sources with a counterpart in
the Texas Survey. Apart from the uncertainty in the flux measurements, the
spectral indices may also be affected by the nonsimultaneity of the two
measurements and by the tendency of the Texas interferometric survey to
underestimate the fluxes of sources larger than approximately 15 arc sec.
Spectral indices are flagged using the wid_flag parameter with an asterisk for
the 910 sources with 6 cm/80 cm position differences greater than 100 arc sec ;
such matches have a reliability less than 80%.
This is blank if the source is not detected at 0.365 GHz.
The U-B color.
The V magnitude of the source.
This is "*" if the separation between 4.85 and 0.365 GHz positions is greater
than 100 arc sec; otherwise, it is blank.
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