A&A 376, L35-L38 (2001)
DOI: 10.1051/0004-6361:20011085
Mid infrared polarisation of Ultraluminous Infrared Galaxies
R. Siebenmorgen1 and A. Efstathiou21 European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b.München, Germany
2 Astrophysics Group, Blackett Laboratory, Imperial College of Science Technology & Medicine, Prince Consort Rd., London. SW7 2BZ, UK
(Received 25 May 2001 / Accepted 30 July 2001 )
Abstract
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
18 
m 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.
Key words: polarisation -- infrared: galaxies -- galaxies: ISM -- galaxies: individual: Mrk 231, Arp 220, IRAS 15250+3609, Mrk 273
Offprint request: R. Siebenmorgen, rsiebenm@eso.org
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