The Australia Telescope 20 GHz Survey: The Source Catalogue (0911.0002v1)

Abstract: We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20G is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the whole sky south of declination 0 deg. The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources, and includes 5890 sources above a 20 GHz flux-density limit of 40 mJy. All AT20G sources have total intensity and polarisation measured at 20 GHz, and most sources south of declination -15 deg also have near-simultaneous flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected in polarised total intensity at one or more of the three frequencies. We detect a small but significant population of non-thermal sources that are either undetected or have only weak detections in low-frequency catalogues. We introduce the term Ultra-Inverted Spectrum (UIS) to describe these radio sources, which have a spectral index alpha(5, 20) > +0.7 and which constitute roughly 1.2 per cent of the AT20G sample. The 20 GHz flux densities measured for the strongest AT20G sources are in excellent agreement with the WMAP 5-year source catalogue of Wright et al. (2009), and we find that the WMAP source catalogue is close to complete for sources stronger than 1.5 Jy at 23 GHz.

• The paper presents the main contribution of compiling a catalogue of 5890 high-frequency radio sources detected with ATCA over a four-year period.
• It employs a two-phase observation methodology using rapid scanning and targeted follow-up, achieving 91% completeness for sources above 100 mJy.
• The study identifies Ultra-Inverted Spectrum sources and outlines limitations in resolving extended structures, guiding future CMB and radio research.

Analysis of the Australia Telescope 20 GHz Survey: The Source Catalogue

The paper "The Australia Telescope 20 GHz Survey: The Source Catalogue" presents the final catalog of radio sources detected by the Australia Telescope Compact Array (ATCA) over a four-year period, from 2004 to 2008. It offers a detailed profile of the radio sources observed at 20 GHz across the southern sky. This catalogue is profoundly larger than previous high-frequency initiatives, manifesting an assertive step in the radio astronomy field by documenting 5890 sources with flux densities exceeding 40 mJy.

The paper outlines the two-phase observation strategy employed in the survey. The initial phase involved an extensive blind survey facilitated through the application of ATCA’s rapid scanning techniques. This scanning phase was crucially supported by a custom analogue correlator to capture a wide spectrum bandwidth. The subsequent phase involved targeted follow-up observations to confirm the detections and acquire more precise measurements of intensity and polarization. This bipartite methodology enhanced the catalog's reliability, achieving a near-perfect reliability rate.

The detailed analyses present in the paper reflect the catalog's completeness. South of declination -15°, completeness is impressive at 91% above 100 mJy and 79% above 50 mJy. These percentages dip slightly above declination -15° due to observational limitations, reportedly from adverse weather conditions affecting certain right ascension bands. The completeness and reliability assessments also underscore strategic comparison with the WMAP 5-year source catalogue, highlighting that WMAP is nearly complete for sources stronger than 1.5 Jy at 23 GHz, corroborating this survey's rigorous standards.

A pivotal discovery in this work is the identification of sources with exceptionally high spectral indices, coined as Ultra-Inverted Spectrum (UIS) sources. Defined by a spectral index $\alpha(5,20) > +0.7$, these sources potentially unfold new understanding within radio astronomy, especially given their rarity and distinct spectral characteristics. These findings position the AT20G survey not only as a tool to fill gaps left by lower-frequency radio surveys but as a foundation to investigate these atypical radio phenomena. They further substantiate the argument against straightforward extrapolation from low-frequency data to predict high-frequency source characteristics, illustrating the necessity of discrete high-frequency observations.

The paper also discusses issues relating to significant radio sources absent in the AT20G data, fundamentally due to intrinsic survey limitations such as unresolved extended structures, which the selected baseline lengths and array configurations could not effectively capture. This was highlighted by the absence of Fornax A—a well-known radio galaxy—demonstrating the survey's resolution constraints in capturing expansive radio lobes.

From a scientific vantage point, this comprehensive source catalogue offers critical insights for future cosmic microwave background (CMB) studies, particularly in regard to foreground source subtraction vital for improving anisotropy measurements. Furthermore, resolving the high-frequency radio source distribution enables refinements in cosmological models and background studies.

This publication stands as a technical documentation offering substantial insights and extensive data for professional astronomers and astrophysicists. Its implications extend into practical applications in observational strategies and theoretical frameworks for understanding the radio universe at high frequencies. Future enhancements building upon this dataset could feature deeper survey strategies, especially targeting source varieties like UIS, potentially unveiling underlying astrophysical mechanisms. Furthermore, the paper implicitly encourages inter-observatory collaborations to calibrate and harmonize radio flux scales globally, emphasizing the necessity for integrated workflows across high-frequency radio surveys.