Foreground: S-PASS 2.3 GHz

S-PASS 2.3 GHz Polarization Survey

Coordinate System:
Projection Type:
8.9 arcmin
HEALPix, ring, res 10 (Nside=1024)
Original Data Source:
Download Links:

From Carretti et al. (2019):

The S-Band Polarization All Sky Survey (S-PASS) is a survey of polarized radio emission over the southern sky at Dec < -1 degrees taken with the Parkes radio telescope at 2.3 GHz. The main aim was to observe at a frequency high enough to avoid strong depolarization at intermediate Galactic latitudes (still present at 1.4 GHz) to study Galactic magnetism, but low enough to retain ample Signal-to-Noise (S/N) ratio at high latitudes for extragalactic and cosmological science. We developed a new scanning strategy based on long azimuth scans, and a corresponding map-making procedure to make recovery of the overall mean signal of Stokes Q and U possible, a long-standing problem with polarization observations. We describe the scanning strategy, map-making procedure, and validation tests. The overall mean signal is recovered with a precision better than 0.5%. The maps have a mean sensitivity of 0.81 mK on beam-size scales and show clear polarized signals, typically to within a few degrees of the Galactic plane, with ample S/N everywhere (the typical signal in low emission regions is 13 mK, and 98.6% of the pixels have S/N > 3). The largest depolarization areas are in the inner Galaxy, associated with the Sagittarius Arm. We have also computed a Rotation Measure map combining S-PASS with archival data from the WMAP and Planck experiments. A Stokes I map has been generated, with a sensitivity limited to the confusion level of 9 mK.

Data Release 1 (DR1) maps are available for the three Stokes parameters I, Q, U, with the data for the entire frequency range binned in one channel. A second data release DR2 will be about the spectropolarimetric data cube and will be released at a later stage.

The main specifications of the survey are:

  • Galactic coordinates
  • Aitoff projection or HEALPix maps
  • pixel nside for HEALPix maps: 1024
  • pixel size in AITOFF projection: 3.44 arcmin (see FITS file header for exact value)
  • effective central frequency = 2303 MHz
  • BW = 168 MHz (actual frequency ranges: 2176-2216 + 2272-2400 MHz)
  • FWHM = 8.9 arcmin
  • unit: Kelvin (full-beam Brightness Temperature)
  • K-to-Jy conversion factor = 1.19 Jy/K
  • Q & U maps are zero-offset calibrated by design of the scanning strategy and map-making used for S-PASS
  • Stokes I has been zero-offset calibrated with absolute flux observations available in literature.
  • Stokes Q & U sensitivity: sensitivity map is provided (in K, use same conversion factor to get that in Jy/b)
  • Stokes I sensitivity: confusion limit limited to 9 mK = 11 mJy/beam
  • Stokes I zero-offset calibration error: 70 mK = 83 mJy/beam

Conditions for using the S-PASS data:

You are welcome to use the S-PASS data, but the S-PASS team requests that:

  • If you use these data in your work you cite the paper describing the survey: Carretti E, Haverkorn M, Staveley-Smith L, Bernardi G, Gaensler B M, Kesteven M, Poppi S, Brown S, Crocker R, Purcell C R, Schnitzeler D, Sun X, 2019, MNRAS, accepted for publication, arXiv:1903.09420
  • In the acknowledgements section of your work you add the following: This work has made use of S-band Polarisation All Sky Survey (S-PASS) data.

Important Warning:

S-PASS follows the IAU convention on Polarisation Angles (PA), as is used for most astrophysical data.

That means PA is 0 northward (in the coordinate system used) and increases eastward.

This is the IAU standard.

However, there are a few data sets and software packages that use a different convention, with PA increasing westward. Examples are WMAP and Planck data, and the HEALPix software package. Passing from one system to the other means simply changing the sign of Stokes U (or the sign of the PA). For more information, see the LAMBDA Polarization Convention page.

To reflect this the S-PASS HEALPix fits file headers contain the keyword POLCCONV = 'IAU'.

The user should bear this in mind when using S-PASS (and most astrophysical data sets) in combination with WMAP and Planck data (and possibly other CMB data sets) or with the HEALPix package.

More details can be found in:

Carretti E, Haverkorn M, Staveley-Smith L, Bernardi G, Gaensler B M, Kesteven M, Poppi S, Brown S, Crocker R, Purcell C R, Schnitzeler D, Sun X, 2019, MNRAS, accepted for publication, arXiv:1903.09420


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