General Description

The VLA Low-Frequency Sky Survey (VLSS) is a 74 MHz (4-meter wavelength) continuum survey covering the entire sky north of -30° declination. Utilizing the Very Large Array (VLA), the survey provides images with a resolution of 80" and with and average rms noise of 0.1 Jy/beam. Complete details are in "The VLA Low-Frequency Sky Survey" (Astron. J., vol. 134, p. 1245).

We recently re-reduced all of the survey data to create new sky maps and a new catalog. The VLSS Redux (VLSSr), corrects substantial radial flux errors from an incorrect primary beam correction in the VLSS. The new maps are at a resolution of 75" and a limiting peak source brightness of about 500 mJy/beam.

Redux Improvements

The VLSSr represents a major improvement to the original VLSS over most of the sky. It includes six previously unpublished fields, and improved dynamic range in 95% of all fields. The clean bias has been halved and is now 0.66 times the local RMS. The largest angular size imaged has been roughly doubled to 36', and the number of cataloged sources is increased by 35% to 95,000.

Sources measured in adjacent fields at different beam positions were used to derive an improved primary beam model. This corrects substantial radially dependent flux errors present in the VLSS.

The VLSSr maps and catalog use the Roger, Costain and Bridle (1973 AJ 78 130) flux scale, based on the source models presented in Scaife and Heald (2012 MNRAS 423 30).

The principal VLSS data products are:

  • A set of 358 continuum images (Stokes I) of size 14°×14° which are distributed on a grid designed such that adjacent images overlap by at least 2°.
  • A catalog of discrete sources in these images created by fitting elliptical Gaussians to all sources detected at the 5-sigma level or higher.

Survey Sky Coverage

Final sky coverage of the catalog and images is complete above -10 degrees declination, and extends down to -30 degrees declination with a scalloped edge that extends to -36 degrees at most RAs. The total observed area is 9.4 sr, and the mean rms is 0.1 Jy/beam.


Accessing the Data

Images

The primary data product is the grid of 17°×17° images. These 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, Macintosh, and Unix/X-windows systems. All images which are fully or partially completed can be obtained here or through a simplified form interface. You may also view (as either gray-scale images or contour plots) and/or copy selected subimages using our VLSS postage stamp server.

Catalog

We have extracted a source catalog from each image by fitting elliptical Gaussians to all significant peaks. The individual catalogs have been merged into a master catalog of sources from all images. One may search the full catalog database using our VLSS catalog browser. Also, the entire catalog can be obtained as a single text file. The latest version is VLSSRcatalog12Aug27.gz.

Spectral Data

We have compiled spectral data for the brightest VLSS sources to create the VLSS Bright Source Spectral Catalog (VBSSC). The VBSSC provides spectral data for the 388 sources with peak intensities above 15 Jy/beam at 74 MHz by combining the VLSS measurements with existing data from the literature and other catalogs and referencing all measurements to the same flux density scale. These data are available from our online search engine and flux-density calculator, which is described in detail in our ApJS paper.

Other Major Radio Surveys

NVSS: The 1400 MHz NRAO VLA Sky Survey (NVSS) covers the entire sky north of -40° declination (10.3 steradians) at a resolution of 45" and a limiting peak source brightness of about 2.5 mJy/beam. This survey contains publicly available images and catalog information on over 1.8 million radio sources.

FIRST: The VLA high-resolution (5" 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 mJy/beam) compact sources.

SUMSS: The Sydney University Molonglo Sky Survey (SUMSS) covers the whole sky south of declination -30 degrees with |b| > 10 degrees at a frequency of 843 MHz with similar resolution and sensitivity as the NVSS.

WENSS: The Westerbork Northern Sky Survey (WENSS) covering the 3.14 steradians north of +30° declination at 326 MHz with 54"×54"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.

TGSS: The TIFR GMRT Sky Survey is an ongoing survey to map the entire sky north of -30 degrees at a frequency of 150 MHz. The resolution is 20", and the target sensitivity is 7-9 mJy/beam.

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