NRAO VLA Sky Survey
About the NRAO VLA Sky Survey
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).
Access the Data
Contents of the Data
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.
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.
The NASA/IPAC Extragalactic Database NED also provides NVSS subimages centered on all database objects north of -40 deg declination.
The NVSS mirror site at the MRAO, Cambridge should give faster response for European users.
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.