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Astron. Astrophys. 357, 443-450 (2000)


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Polarimetric imaging of the polar ring galaxy NGC 660 - evidence for dust outside the stellar disk

P.B. Alton 1, D.P. Stockdale 2, S.M. Scarrott 2 and R.D. Wolstencroft 3

1 University of Wales, Department of Physics & Astronomy, P.O. Box 913, Cardiff CF2 3YB, UK
2 University of Durham, Physics Department, South Road, Durham DH1 3LE, UK
3 Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

Received 2 March 2000 / Accepted 20 March 2000

Abstract

Optical imaging polarimetry has been carried out for the polar ring, starburst galaxy NGC 660. This galaxy has a highly inclined, severely tidally-disturbed disk which is surrounded by a gas-rich, polar ring. We detect scattered light from a large part of the halo and this is attributable to dust grains residing up to [FORMULA] kpc from the stellar disk. There is evidence from emission-line imaging carried out in the past, that NGC 660 is host to an energetic outflow of hot gas along the minor axis (a `superwind'). Our results indicate that dust grains are entrained in this same outflow. Polarization due to scattering, however, is also present at positions away from the minor axis suggesting that grains may also be displaced from the stellar disk by tidal forces exerted during galactic collisions.

Where the polar ring occludes the stellar disk we observe polarization due to magnetically aligned, dichroic grains. By comparing the recorded polarization with the associated optical extinction we infer that the magnetic field in the ring has a lower (but still comparable) strength to the magnetic field in the Milky Way. We also derive a dust-to-gas ratio for the ring and this is about a factor of 2-3 lower than in the solar neighbourhood (but close to the value measured in some nearby spirals). If the ring comprises the remnants of the `interloper' which collided with NGC 660, we expect that the ruptured galaxy was a massive, metal-rich spiral.

Key words: ISM: dust, extinction – ISM: magnetic fields – galaxies: interactions – galaxies: individual: NGC 660 – galaxies: intergalactic medium – polarization

Send offprint requests to: P.B. Alton (paul.alton@astro.cf.ac.uk)

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

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