TargetsThe project plan includes targets in the Milky Way and a list of galaxies and galaxy groups. Nearby galaxies, galaxy clusters and radio galaxies will be observed in close cooperation with the LOFAR “Surveys” Key Science Project. Pulsars will be observed in cooperation with the “Transients” Key Science Project.
Milky WayOur location within the thin disk of the Milky Way allows studies of Galactic objects, like supernova remnants and HII regions, as well as the structure and composition of the interstellar medium in much more detail than in any other galaxy. Synchrotron emission at low frequencies originates from low-energy electrons, which escaped from their origin in supernova remnants in the Galactic disk into the weak halo magnetic field a long time ago. Details of this propagation and evolution process will be investigated with LOFAR. At low frequencies a small amount of Faraday rotation already causes a significant change of the
polarisation angle with frequency. LOFAR will make the investigation of the distribution of small clumps of thermal electrons and their relation to turbulent weak magnetic fields possible, which are of high interest to understand the polarisation properties of the interstellar medium in general. The increase of thermal absorption and of depolarisation by Faraday rotation at low frequencies will be used to model the distribution of emission along the line-of-sight, leading to a three-dimensional model of the gas and magnetic fields in the Milky Way. These results are of high importance to understand nearby galaxies where the spatial resolution is much lower. Finally, the emission of the Milky Way is a foreground for all kinds of sensitive extragalactic observations and must be properly separated for a correct interpretation of these data.
PulsarsLOFAR will detect all pulsars within 2 kiloparcs (about 6500 light-years) from the sun and discover about 1000 new nearby pulsars at high latitudes. Most of these are expected to emit strongly linearly polarised signals at low frequencies. This allows us to measure their Faraday rotation measures which will give a unprecedented picture of the structure of the magnetic field near to the sun. The strong polarisation also makes them ideal calibration sources for polarisation observations of much weaker sources.
Nearby galaxiesWith the angular resolution and sensitivity of LOFAR, it will be possible to obtain low-frequency maps providing completely new information on the relativistic electrons in nearby galaxies. The cosmic-ray spectrum derived from the radio synchrotron spectrum allows us to study and to understand the origin and propagation of cosmic rays, the energy loss processes and how the propagation is affected by the magnetic fields. Deep LOFAR observations of a sample of spiral and dwarf galaxies are planned to observe diffuse polarised emission and its Faraday rotation from the outer disks and halos. LOFAR’s sensitivity should allow us to detect much fainter emission than with present-day telescopes (see Figure 2). An even more sensitive technique to detect regular magnetic fields in galaxies is to observe a grid of Faraday rotation measurements from polarised background sources behind galaxies. This method is independent of the presence of cosmic rays in the galaxy and may allow us to detect regular fields at distances from the disk larger than the detection limit of radio synchrotron emission. The project aims to clarify the origin of galactic magnetic fields. Proposed models are the dynamo which can generate large-scale patterns of regular fields, or the galactic wind where magnetic fields from the disk are blown out and amplified by gas motions. The winds of dwarf galaxies are thought to be especially efficient to enrich the intergalactic space with gas and magnetic fields.
The intergalactic medium is probably filled with relativistic particles plasma and magnetic fields - just as frequently observed through the radio synchrotron emission from clusters of galaxies. The most promising mechanism responsible for the “enrichment” of groups are galaxies where bursts of star formation occurred. Such galaxies are frequently encountered in groups of galaxies, in particular in compact ones, where starbursts in two or more galaxies occur with short time intervals. This will lead to colliding galactic winds and to the amplification of magnetic field in the intra-group medium, as well as to shock heating of the thermal gas. With LOFAR, faint, diffuse synchrotron emission should be detected, possibly connecting the galaxies. Huge extensions which are rarely observed today (e.g. Figure 3) will be much more frequently seen in LOFAR’s radio maps.
Giant radio galaxiesGiant radio galaxies are believed to be formed from relativistic jets of matter and energy, emanating from the central regions of active galactic nuclei. Synchrotron radiation, often polarised, from the radio jets and lobes of these objects will be studied with LOFAR. The low-energy electrons responsible for the low-frequency emission can propagate large distances from their origins in the central core or lobe hotspots, and large radio cocoons are expected to surround many objects. Using high angular resolution observations with the full international LOFAR array should help us to learn more about the low-energy electron population in these objects, and hopefully to better understand the acceleration mechanisms which produce the relativistic electrons responsible for synchrotron emission. The high degrees of polarisation of giant radio galaxies makes them ideal polarisation calibrators for the observation of weaker sources.