Mid Infrared Polarisation of Ultraluminous Infrared Galaxies ⋆

The mid infrared polarisation properties of four Ultraluminous Infrared Galaxies (ULIRGs) have been investigated by broad band filter observations with the ISOCAM instrument on board the Infrared Space Observatory (ISO). The wavelength region from 5 to 18μm was selected where the emission from the putative torus peaks. We report detection of polarisation for all ULIRGs studied. The fractional polarisation ranges from ∼ 3% up to 8%. The highest polarisation is recorded in Mrk 231 which has a clear AGN signature, whereas the lowest is for Arp 220, which is generally thought to be powered predominantly by star formation. We discuss the various mechanisms that could give rise to the polarisation and conclude that the most likely interpretation is that it is due to magnetically aligned elongated dust grains. This is the same mechanism believed to be operating in a number of galactic sources. The position angle of polarisation could give the projected magnetic field direction and therefore constrain models for the formation of the tori.