[Paper Review] GASS: The Parkes Galactic All-Sky Survey. II. Stray-Radiation Correction and Second Data Release

P. M. W. Kalberla, N. M. McClure‐Griffiths+9 more|arXiv (Cornell University)|Jul 5, 2010

Galaxies: Formation, Evolution, Phenomena25 references3 citations

TL;DR

This paper presents the second data release of the Parkes Galactic All-Sky Survey (GASS), correcting instrumental effects—particularly stray-radiation and radio frequency interference—using the Leiden/Argentine/Bonn (LAB) all-sky 21-cm survey as a reference. After corrections, GASS achieves the highest angular resolution and sensitivity for Galactic H i emission in the southern sky, with excellent agreement to the LAB survey and publicly accessible FITS cubes via a web interface.

ABSTRACT

The Parkes Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (HI) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release was published by McClure-Griffiths et al. (2009). We remove instrumental effects that affect the GASS and present the second data release. We calculate the stray-radiation by convolving the all-sky response of the Parkes antenna with the brightness temperature distribution from the Leiden/Argentine/Bonn (LAB) all sky 21-cm line survey, with major contributions from the 30-m dish of the Instituto Argentino de Radioastronomia (IAR) in the southern sky. Remaining instrumental baselines are corrected using the LAB data for a first guess of emission-free baseline regions. Radio frequency interference is removed by median filtering. After applying these corrections to the GASS we find an excellent agreement with the Leiden/Argentine/Bonn (LAB) survey. The GASS is the highest spatial resolution, most sensitive, and is currently the most accurate HI survey of the Galactic HI emission in the southern sky. We provide a web interface for generation and download of FITS cubes.

Motivation & Objective

• To correct instrumental effects—especially stray-radiation and radio frequency interference—in the Parkes Galactic All-Sky Survey (GASS) data.
• To improve the accuracy and sensitivity of the GASS survey for Galactic atomic hydrogen (H i) emission in the southern sky.
• To produce a second data release with calibrated, cleaned data products suitable for scientific analysis.
• To provide a web-based interface for on-the-fly generation and download of FITS cubes with customizable resolution and velocity ranges.
• To enable comparison with the LAB survey and allow users to assess residual contamination from RFI or stray-radiation.

Proposed method

• Stray-radiation is calculated by convolving the Parkes antenna's all-sky response with the brightness temperature distribution from the LAB all-sky 21-cm survey.
• Residual instrumental baselines are corrected using the LAB survey as a first-guess emission-free baseline region.
• Radio frequency interference (RFI) is mitigated through median filtering of the visibility data.
• The corrected data are validated by comparing with the LAB survey, showing excellent agreement in H i emission structure and intensity.
• A web interface is implemented to generate and download FITS cubes with user-defined Galactic or equatorial coordinates, velocity ranges, and smoothing kernels (FWHM between 16′ and 25′).
• Users can also access uncleaned data (baseline- and stray-radiation-corrected but RFI-uncorrected) and the stray-radiation correction maps for diagnostic purposes.

Experimental results

Research questions

• RQ1 How accurately can stray-radiation effects be modeled and removed from GASS data using the LAB survey as a reference?
• RQ2 To what extent does RFI contamination affect the GASS data, and how effectively can it be mitigated?
• RQ3 What is the final sensitivity and angular resolution of the corrected GASS data compared to existing H i surveys?
• RQ4 How well does the corrected GASS data agree with the LAB survey in terms of H i emission structure and intensity?
• RQ5 What level of residual instrumental effects remain after post-processing, and how can users detect or correct for them?

Key findings

• After stray-radiation and RFI corrections, the GASS data show excellent agreement with the Leiden/Argentine/Bonn (LAB) all-sky 21-cm survey, validating the accuracy of the correction process.
• The GASS survey achieves an effective angular resolution of 14.4′ FWHM and a velocity resolution of 1.0 km s⁻¹, making it the highest angular resolution and most sensitive H i survey of the Galactic southern sky to date.
• The root-mean-square (rms) noise in the cleaned data cubes ranges from 20 to 60 mK at high Galactic latitudes, with the lowest noise levels achieved at high Galactic latitudes.
• The second data release provides access to 2.8 × 10⁷ individual spectra with 5-second integration time, processed via on-the-fly mapping.
• The web interface allows on-the-fly generation of FITS cubes with customizable smoothing (FWHM 16′ to 25′), and users can retrieve both cleaned and uncleaned data products for diagnostic use.
• Despite thorough corrections, some residual RFI effects remain, with isolated channels showing contamination up to 40 mK, and users are advised to cross-check critical data with the LAB database.