The Australia Telescope 20 GHz (AT20G) Survey: analysis of the extragalactic source sample

The Australia Telescope 20 GHz (AT20G) survey is a blind survey of the whole Southern sky at 20 GHz with follow-up observations at 4.8, 8.6 and 20 GHz carried out with the Australia Telescope Compact Array (ATCA) from 2004 to 2008. In this paper we present an analysis of radio spectral properties in total intensity and polarization, sizes, optical identifications and redshifts of the sample of the 5808 extragalactic sources in the survey catalogue of confirmed sources over 6.1 sr in the Southern sky (i.e. the whole Southern sky excluding the strip at Galactic latitude |b| < ). The sample has a flux density limit of 40 mJy. Completeness has been measured as a function of scan region and flux density. Averaging over the whole survey area the follow-up survey is 78 per cent complete above 50 mJy and 93 per cent complete above 100 mJy. 3332 sources with declination δ − 0.5 (S∝να). The fraction of flat-spectrum sources decreases from 81 per cent for S20 GHz > 500 mJy to 60 per cent for S20 GHz < 100 mJy. There is also a clear spectral steepening at higher frequencies with the median α decreasing from −0.16 between 4.8 and 8.6 GHz to −0.28 between 8.6 and 20 GHz. Simultaneous observations in polarization are available for all the sources at all the frequencies. 768 sources have a good-quality detection of polarized flux density at 20 GHz; 467 of them were also detected in polarization at 4.8 and/or at 8.6 GHz so that it has been possible to compare the spectral behaviour in total intensity and polarization. We have found that the polarized fraction increases slightly with frequency and decreases with flux density. The spectral indices in total intensity and in polarization are, on average, close to each other, but we also found several sources for which the spectral shape of the polarized emission is substantially different from the spectral shape in total intensity. The correlation between the spectral indices in total intensity and in polarization is weaker for flat-spectrum sources. Cross-matches and comparisons have been made with other catalogues at lower radio frequencies, and in the optical, X-ray and γ-ray bands. Redshift estimates are available for 825 sources.

[1]  Jean-Luc Starck,et al.  THE FERMI-LAT HIGH-LATITUDE SURVEY: SOURCE COUNT DISTRIBUTIONS AND THE ORIGIN OF THE EXTRAGALACTIC DIFFUSE BACKGROUND , 2010, 1003.0895.

[2]  R. Ekers,et al.  HIGH-FREQUENCY RADIO PROPERTIES OF SOURCES IN THE FERMI-LAT 1 YEAR POINT SOURCE CATALOG , 2010, 1003.4580.

[3]  Robert P. Johnson,et al.  THE FIRST CATALOG OF ACTIVE GALACTIC NUCLEI DETECTED BY THE FERMI LARGE AREA TELESCOPE , 2010, 1002.0150.

[4]  A. Bonaldi,et al.  A model for the cosmological evolution of low-frequency radio sources , 2010, 1001.1069.

[5]  Adrian T. Lee,et al.  EXTRAGALACTIC MILLIMETER-WAVE SOURCES IN SOUTH POLE TELESCOPE SURVEY DATA: SOURCE COUNTS, CATALOG, AND STATISTICS FOR AN 87 SQUARE-DEGREE FIELD , 2009, 0912.2338.

[6]  Astrophysics,et al.  High‐frequency radio polarization measurements of WMAP point sources , 2009, 0912.0621.

[7]  Roberto Ricci,et al.  The Australia Telescope 20 GHz Survey: the source catalogue , 2009, 0911.0002.

[8]  Marcella Massardi,et al.  Radio and millimeter continuum surveys and their astrophysical implications , 2009, 0908.1896.

[9]  Amber D. Miller,et al.  RADIO SOURCES FROM A 31 GHz SKY SURVEY WITH THE SUNYAEV–ZEL'DOVICH ARRAY , 2009, 0912.2335.

[10]  K. Grainge,et al.  9C continued: a radio-source survey at 15 GHz , 2009, 0908.0066.

[11]  T. Murphy,et al.  Wide‐field imaging and polarimetry for the biggest and brightest in the 20‐GHz southern sky , 2009, 0907.2084.

[12]  G. Zotti,et al.  Blind and non‐blind source detection in WMAP 5‐yr maps , 2008, 0810.2338.

[13]  P. S. Bunclark,et al.  Astronomical Data Analysis Software and Systems , 2008 .

[14]  R. Ekers,et al.  The extragalactic radio-source population at 95 GHz , 2007, 0709.3563.

[15]  Roberto Ricci,et al.  The Australia Telescope 20-GHz (AT20G) Survey: the Bright Source Sample , 2007, 0709.3485.

[16]  T. Murphy,et al.  The second epoch Molonglo Galactic Plane Survey: compact source catalogue , 2007, 0708.3092.

[17]  R. F. Reinfrank,et al.  The properties of extragalactic radio sources selected at 20 GHz , 2006, astro-ph/0603437.

[18]  D. Mesa,et al.  Predictions for high-frequency radio surveys of extragalactic sources , 2004, astro-ph/0410709.

[19]  L. Silva,et al.  Extragalactic source contributions to arcminute-scale Cosmic Microwave Background anisotropies , 2004, astro-ph/0410605.

[20]  R. Ekers,et al.  First results from the Australia Telescope Compact Array 18‐GHz pilot survey , 2004 .

[21]  G. Zotti,et al.  High-frequency polarization properties of southern Kühr sources , 2003, astro-ph/0312163.

[22]  Michael E. Jones,et al.  9C: a survey of radio sources at 15 GHz with the Ryle Telescope , 2003, astro-ph/0304275.

[23]  J. Curran,et al.  SUMSS: a wide-field radio imaging survey of the southern sky – II. The source catalogue , 2003, astro-ph/0303188.

[24]  S. O. Physics,et al.  The SuperCOSMOS Sky Survey – I. Introduction and description , 2001, astro-ph/0108286.

[25]  E. Greisen,et al.  The NRAO VLA Sky Survey , 1996 .

[26]  Chris Biemesderfer,et al.  Astronomical Data Analysis Software and Systems X , 2001 .

[27]  J. J. Condon.  Cosmological evolution of radio sources , 1984 .

[28]  A. Bridle,et al.  Variability of extragalactic sources at 2.7 GHz. III - The nature of the variations in different source classes , 1977 .

[29]  J. Wall.  The Parkes 2700 Mhz Survey: Counts of the Sources and their Distribution on the Sky , 1977 .

[30]  D. Jauncey,et al.  Radio astronomy and cosmology , 1977 .

[31]  K. Kellermann.  The Spectra of Non-Thermal Radio Sources. , 1964 .