The NRAO VLA Sky Survey
- J. J. Condon, W. D. Cotton, E. W. Greisen, and Q. F. Yin
- National Radio Astronomy Observatory, 520 Edgemont Road,
Charlottesville, VA 22903
- R. A. Perley and G. B. Taylor
- National Radio Astronomy Observatory,
P.O. Box 0, Socorro, NM 87801
- J. J. Broderick
- Physics Department, Virginia Polytechnic Institute and State
University, Blacksburg, VA 24061
The NRAO VLA Sky Survey (NVSS) is a 1.4 GHz continuum survey covering
the entire sky north of -40 deg declination. A detailed description
appears in the 1998 May issue of The Astronomical Journal (Condon,
J. J., Cotton, W. D., Greisen, E. W., Yin, Q. F., Perley, R. A.,
Taylor, G. B., & Broderick, J. J. 1998, AJ, 115, 1693).
The principal NVSS data products are:
- A set of 2326 continuum image ``cubes,'' each covering 4 deg X 4 deg
with three planes containing the Stokes I, Q, and U images. These images
were made with a relatively large restoring beam (45 arcsec FWHM) to
yield the high surface-brightness sensitivity needed for completeness
and photometric accuracy. Their rms brightness fluctuations are
about 0.45 mJy/beam = 0.14 K (Stokes I) and 0.29 mJy/beam = 0.09 K
(Stokes Q and U). The rms uncertainties in right ascension and
declination vary from < 1 arcsec for relatively strong (S > 15 mJy) point
sources to 7 arcsec for the faintest (S = 2.3 mJy) detectable
sources. The completeness limit is about 2.5 mJy.
- A catalog of discrete sources on these images (over 1.8 million
sources in the entire survey).
- Processed (u,v) data sets. Every large image was constructed
from more than 100 smaller "snapshot" images. All of the edited and
calibrated single-source (u,v) data sets used to make the
snapshot images contributing to each large image have been combined into
a single multisource (u,v) file for users who want to
investigate the data underlying the images.
The NVSS has been made as a service to the astronomical community,
and the principal data products were released by anonymous
FTP as soon as they were produced and verified.
Users should read the postscript paper
(about 4.7 megabytes) or the gzip compressed
paper (about 1.2 megabytes) for an overview of the
NVSS. If you use the NVSS, please reference it as:
Condon, J. J., Cotton, W. D., Greisen, E. W.,
Yin, Q. F., Perley, R. A., Taylor, G. B., & Broderick, J. J. 1998,
AJ, 115, 1693.
The sky positions of the large NVSS images are shown in the image grid
plot. These images are in FITS format and can be read by
the major astronomical software packages (AIPS, IRAF, etc.) as well
as by a stand-alone FITS
viewer for Windows 3.1, Windows 95, Macintosh, and
Unix/X-windows systems. The full 4 deg X 4 deg FITS image cubes are
available via anonymous
FTP. You may also view (as either gray-scale images or
contour plots) and/or copy selected subimages using our postage stamp server.
We extracted a source catalog from each image by fitting elliptical
Gaussians to all significant peaks. The individual catalogs have been
merged into the master catalog of sources from all images. See the catalog
description for details about the cataloged source
parameters. An NVSS catalog browser
is available, but please exercise caution (don't ask for
a catalog of the whole sky!). The user program NVSSlist
can display selected portions of the catalog. Both the catalog
browser and NVSSlist correct the raw catalog for known biases and
computes errors associated with the source model parameters (position,
flux density, etc.) using equations derived in Condon, J. J. 1997,
PASP, 109, 166. To interpret the catalog results in complex or
crowded fields, users should look at contour plots (available on the
postage stamp server)
showing the actual brightness distributions from which sources were
extracted.
Image flux densities at specified positions in NVSS images may be
obtained here.
Users of the AIPS task IMAGR often need to image extra fields covering
nearby confusing sources. A RUN file
generator produces the required list of field offsets for
NVSS sources whose flux densities, attenuated by the primary beam, exceed
a chosen cutoff.
To avoid ambiguity, please refer to NVSS sources using the standard IAU
format NVSS EHHMMSS+DDMMSS, with E = J for equinox J2000 or E = B for
equinox B1950 coordinates, HHMMSS the hours, minutes, and truncated (not
rounded) seconds of right ascension, the declination sign (+ or -), and
DDMMSS the degrees, minutes, and seconds (truncated, not rounded) of
declination. Thus the NVSS source at J2000 RA = 00 00 00.24, DEC = -20 04
49.2 (= B1950 RA = 23 57 26.34, DEC = -20 21 31.5) should be called NVSS
J000000-200449 or NVSS B235726-202131.
A mirror
of the NVSS site is maintained by the University of Cambridge.
The NASA/IPAC
Extragalactic Database
NED also provides NVSS subimages centered on all database
objects north of -40 deg declination.
A German
version of this site was provided
by A. Romanova
Several hundred thousand identifications of NVSS sources with
optical objects in the APM or USNO-A catalogs are listed in the
QORG catalog (Flesch & Hardcastle 2004, A&A, 427, 387).
Other large-scale radio surveys which may be of interest include:
- The ongoing VLA Low-frequency Sky Survey ( VLSS) will
ultimately cover the sky north of -30 deg declination at 74 MHz with
80 arcsec resolution and an rms noise of about 100 mJy.
- Source catalogs from the Cambridge
8C survey
covering the polar cap above +60 deg declination at 38 MHz
with 4.5 arcmin X 4.5 arcmin * cosec (dec) resolution,
the
6C survey
covering most of the extragalactic sky above +30 deg declination at 151 MHz
with 4.2 arcmin X 4.2 arcmin * cosec (dec) resolution, and the
7C survey
covering most of the extragalactic sky above +20 deg declination at 151 MHz
with 70 arcsec X 70 arcsec * cosec (dec) resolution.
- The Westerbork Northern Sky Survey (WENSS)
covering the 3.14 sr north of +30 deg declination at 326 MHz with 54
arcsec X 54 arcsec * cosec (dec) resolution in total intensity and
linear polarization. Two catalogs give parameters for about 230,000
sources stronger than 18 mJy. Data from the WENSS CDROM have been
copied to a disk in Charlottesville, where they are available via
anonymous FTP.
- The Sydney University/Molonglo Sky Survey (
SUMSS) 843 MHz survey will eventually
cover the sky south of -30 deg declination with 43 arcsec X
43 arcsec * cosec (dec) resolution and about 1 mJy/beam rms noise.
- The VLA high-resolution (5 arcsec FWHM) 1400 MHz survey covering
the north Galactic cap. The FIRST
survey (for Faint Images of the Radio Sky at Twenty-cm) yields very
accurate (<1 arcsec rms) radio positions of faint (>1 mJy/beam)
compact sources.
- The Green Bank
GB6 survey covering the 6.1-sr declination
band between 0 deg and +75 deg at 4850 MHz with 3 arcmin resolution.
The GB6 catalog contains about 75,000 sources stronger than 18 mJy.
- The corresponding 4850 MHz survey in the southern hemisphere
is the Parkes-MIT-NRAO (
PMN) survey made with the Parkes 64 m
telescope. Both source catalogs and FITS-format sky images
are available.
For more information about the NVSS, contact jcondon@nrao.edu
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