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VLSS: The VLA Low-frequency Sky Survey

(Formerly known as 4MASS)

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).

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.

The VLSS has been conducted as a service to the astronomical community, and the principal data products were released to the public as soon as they were produced and verified.

Current Survey Status

The second major VLSS data release, representing roughly 95% of the planned survey area, was made available in May, 2006. In addition to the increase in sky coverage, this new data release also contains more accurate data from the previously covered regions. For more detailed information about this data including a map of the current sky coverage please see our survey status page. Observations for most of the remaining sections of the survey were taken in October, 2007, and the new images and updated catalog will be released as soon as the data are reduced and verified.

Accessing the data

Images:

The primary data product is the grid of 14°×14° 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 VLSScatalog07Jun26.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" rms) radio positions of faint (>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.

Related Links

Low Frequency Data Reduction Tutorial

The Low Frequency Array (LOFAR)

The Long Wavelength Array (LWA)

NRL Radio Astronomy Group

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Last Updated on Fri Feb 29 11:37:20 EST 2008