C1241: Going wide and deep to unravel the secrets of the obscured Universe

C1241 is a new ATCA project conducting the deepest wide-area survey ever attempted at 20 cm. The radio observations will be critical to understanding the nature and evolution of the "obscured" galaxy population which will be well-sampled by the SWIRE surveys (Lonsdale et al: www.ipac.caltech.edu/SWIRE/survey ). Furthermore the radio data, combined with data from many other facilities, will also lead to important insights into the properties of star-forming galaxies and AGNs in the early Universe. This ambitious project will see the ATCA having a significant profile within the international "legacy" programs within the Great Observatories Origins Deep Survey (GOODs).

There are two major limitations to previous deep-field research - (i) they cover wavebands which are insensitive to dust-obscured systems and (ii) they have extents of << 1 square-degree so the results are biased by small sample sizes and cosmic variance. A combination of the new wide-area surveys, with the addition of wide-band submillimetre and far-infrared coverage, will address both and extend our knowledge of the deep-field Universe.

The ability to detect and study the physical processes in dust-obscured galaxies is now of major importance. There is increasing evidence that one or more populations of obscured galaxies contribute significantly to the total star-formation density of the Universe. Only the most luminous sources, the Ultra-luminous IRAS galaxies (ULIRGs) are well studied, yet there is good reason to believe that deep surveys in the submillimetre and far-infrared will detect many more obscured, highly "active" galaxies at a range of luminosities. These galaxies may be the source of the extragalactic background at these wavelengths and may also be linked with strong nuclear activity (i.e. AGN). This link may arise because of (a) the huge amounts of available accretion fuel and also (b) hints that dusty galaxies are strongly clustered, so that the increased likelihood of interactions and mergers would trigger AGN outbursts or flaring activity.

The Spitzer Space Telescope, formerly the Space InfraRed Telescope Facility (SIRTF) is now fully operational and observing with its IRAC camera (3.6 - 8 microns) and the MIPS camera (24 - 160 microns). Over the next two years, the Spitzer Wide InfraRed Extragalactic (SWIRE) surveys will image large regions of the sky, covering a total area of around 65 square-degrees. At longer wavelengths, the survey with the new ALMA-pathfinder facility, APEX, at 850 microns will cover at least one 3-square-degree region to a depth of 2 mJy.

To study the far-infrared and submillimetre sources they must be localized so that their counterparts can be easily identified. This is not directly possible from far-infrared or submillimetre data - dusty galaxies will be detected at high surface densities by Spitzer and APEX, yet their instrumental beam-sizes are tens-of-arcseconds. The most effective way to pin-point the source is to use the tight (although physically poorly understood) relationship between the far-infrared and radio continuum.

Figure 1: The region around the Chandra Deep Field south (CDFs) showing:

1. Green circle: The 3-square-degree ATCA 20-cm mosaic region
2. White circle: The Hubble ACS pointings of the CDFs (www.stsci.edu/ftp/science/goods/ )
3. Large rectangle: The approximate border of the SWIRE Spitzer/SIRTF field (www.ipac.caltech.edu /SWIRE/survey/)
4. The regions indicated by the red lines are the KPNO mosaics with g'~25.7, r'~25, i'~24
5. The array of yellow squares: Deep optical imaging with g'~27 r'~26, i'~25, U~26 and K_s~20.7
6. The pink squares are the EIS fields (www.eso.org/science/eis/ )

Over the two years, ATCA project C1241 will conduct deep, wide-area surveys of the SWIRE and APEX regions in the CDFs and ELAIS-S1 regions. Each area mapped by the ATCA at 20 cm will be 3-square-degrees and the rms sensitivity will approach 10 microJy. The project will be requesting a total of 130, 12-hour observing slots. By the end of the project we expect to catalogue of order 15,000 radio sources across these two fields, yielding not only the means to cross-identify sources at all other wavebands, but also important radio data on the astrophysics of these galaxies.

Brian Boyle, Carole Jackson and Ray Norris
(Brian.Boyle@csiro.au, Carole.Jackson@csiro.au, Ray.Norris@csiro.au)