First results from the Compact Array 18 GHz pilot survey

Elaine Sadler (University of Sydney) for the 20-GHz survey team; Roberto Ricci (SISSA, Italy); Ron Ekers, Mike Kesteven, Lister Staveley-Smith, Ravi Subrahmanyan, Warwick Wilson (ATNF); Mark Walker (ATNF/University of Sydney); Carole Jackson (ANU); Gianfranco De Zotti (Padova Observatory, Italy)

In September 2002, as a pilot study for an all-sky radio imaging survey at millimetre wavelengths, we observed 1,216 square degrees of the southern sky at 18 GHz using a novel wideband (4 GHz bandwidth) analogue correlator on one baseline of the Compact Array. Several imaging surveys of the radio sky have already been carried out at frequencies between 0.3 and 5 GHz, where the source population is dominated by powerful radio galaxies and fainter starburst galaxies, but at frequencies above 5 GHz only small areas of sky have been studied in detail. This is mainly because large radio telescopes typically have small fields of view (one to two arcminutes) at high frequencies, making it extremely time consuming to observe large areas of sky. Measuring the high-frequency properties of extragalactic radio sources is crucial for interpreting the high-sensitivity and highresolution maps of the cosmic microwave background that are now being produced by satellite missions like WMAP and is also important for studies of active galaxies and their cosmic evolution.

The survey is made possible by using the new Compact Array 12-mm receivers in combination with a 4 GHz bandwidth prototype correlator recently developed at ATNF (the standard Compact Array correlator has a bandwidth of 128 MHz). The increased bandwidth means that the continuum sensitivity is high even for short integration times, allowing large areas of sky to be observed in a fast-scanning mode. On 13-17 September 2002, we scanned the region of sky between declinations of -60 and -70 degrees using two antennas of the array as a two-element interferometer with the wide-band correlator. We covered roughly 15 square degrees per hour to a detection limit of 60 milliJansky. The Compact Array was used in a split array mode, so that regular synthesis imaging could be carried out at the same time with the antennas which were not being used for fast scanning.

Follow-up radio imaging of the sources detected in the survey was carried out in October 2002, giving more accurate positions and flux densities. Almost half the 226 detected sources lay within five degrees of the Galactic plane, and can be identified with Galactic HII regions, supernovae and planetary nebulae. The remainder are extragalactic sources, made up of candidate quasars (60%), radio galaxies (20%) and faint optical objects or blank fields (20%).

The 18 GHz survey will continue in 2003.

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Figure 1 A histogram showing the detected source density from the 18 GHz Pilot Survey in five-degree bins in Galactic latitude. The peak near latitude zero corresponds to the population of Galactic disk sources such as HII regions. The smaller excess at latitudes of -30 to -40 degrees is due to sources associated with the Large Magellanic Cloud.
Figure 2 While many of the sources detected in the Pilot Survey are associated with distant quasars, some, like the radio galaxy shown here (an elliptical galaxy at z=0.0963) are relatively nearby. Here the red contours show lowfrequency (843 MHz) radio emission from the SUMSS survey. The blue contours show the 18 GHz emission imaged in the Pilot Survey, overlaid on an optical image from the Digitized Sky Survey. The high-frequency radio emission is concentrated at the hotspots (roughly one million light years from the parent galaxy) where relativistic jets of plasma from the galaxy's nucleus terminate and dump their energy.
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