An important goal that has driven the development of LOFAR since its inception is to explore the low-frequency radio sky by means of a series of unique surveys. Low-frequency radio telescopes are ideally suited for carrying out large-sky surveys, because of their large instantaneous ﬁelds of view and the all-sky nature of their calibration. Four topics have been identiﬁed as drivers for the proposed surveys. Three of these are fundamental areas of astrophysics for which LOFAR is likely to make substantial contributions. They are:
Figure 2. Observations of the rich galaxy cluster Abell 3667 (Johnston-Hollitt 2003). The X-ray emission (false colour image from the ROSAT PSPC satellite) is a tracer of the hot cluster gas. The radio emission (contours from the ATCA radio telescope) traces shocks and magnetic field structure in the cluster gas. The radio arcs in the outer region of the cluster are presumed to be due to turbulence and shocks caused by the merging of 2 sub-clusters to form Abell 3667.
Figure 3. A simulation on the basis of the models of Hoeft and Brüggen (2007) of a LOFAR observations of the difuse synchrotron emission from magnetised and shocked regions in a massive (1015 solar mass) galaxy cluster. The contours indicate the spatial distribution of the X-ray emission from the hot (107 K) gas.
Because LOFAR is the ﬁrst radio synoptic telescope that will essentially open up a new observational spectral window, our fourth topic is:
4. Exploration of new parameter space for serendipitous discovery.
Figure 4. Simulations of combined LOFAR and Herschel observations of distant galaxies on the basis of semi-analytical galaxy formation models as developed by the Virgo Consortium. The part of the simulations that is shown is a 120 * 20 arcmin projected slice centred on a progenitor of a nearby massive cluster at z=1.8 with a mass of 1015 solar masses. Green are the proto-cluster galaxies detectable by LOFAR and Herschel, orange are field galaxies detectable both by Herschel and LOFAR, blue are the LOFAR only detections and the grey dots trace the dark matter particle distribution.
The survey key project will be optimised for obtaining large samples of the various categories of extragalactic radio emitting objects listed above. The plan is to survey the entire accessible sky at the "key frequencies" 15, 30, 60, 120 MHz and 1000 sqr degrees at 200 MHz. The surveys will be carried out over several epochs, enabling variable sources to be recognized.
Besides their impact on these main drivers, LOFAR surveys will contribute substantially to understanding a broad range of additional astrophysical topics. These topics include:
1. Physics of radio sources, their end-stages and their environments using giant radio sources as probes,
2. Physics of active galaxy nuclear regions using peaked-spectrum radio sources as probes,
3. Magnetic ﬁelds and the interstellar medium in nearby galaxies,
4. Large scale structure of the Universe and its evolution, using clustering of radio sources, baryonic oscilations and gravitational lensing as probes,
5. Physics of galactic sources, including (i) supernova remnants, (ii) HII regions, and (iii) exo-planets.
More information: http://www.astron.nl/general/lofar/surveys-ksp/surveys-ksp.