IAU Symposium Abstract S186-030P

Radio continuum observation of the supermassive spiral galaxy NGC 1961: Interaction with the intergalactic medium or a recent merger?

U. Lisenfeld1, P. Alexander2, G. Pooley2
1 Univ. Granada, Dept. de Fisica Teorica y del Cosmos, Spain < ute@ugr.es >
2 Mullard Radio Astronomy Observatory, Cambridge, UK

We present new radio continuum images of the supermassive spiral galaxy NGC 1961 at 4 frequencies. Apart from its large mass (> 1012 Msun) this galaxy possesses a peculiar optical morphology and an asymetric HI distribution with a large wing at one side. Our observations allow us to seperate the thermal and the nonthermal radio emission and to determine the nonthermal spectral index distribution. This spectral index distribution in the galactic disk is unusual: At the maxima of the radio emission the synchroton spectrum is very steep, indicating aged cosmic ray electrons. Away from the maxima the radio spectrum is much flatter. We discuss various possiblities to explain this peculiar behaviour and conclude that the most likely cause are fluctuations of the star formation rate in the past. The spectral index distribution furthermore suggests the presence of interstellar shocks where cosmic rays are accelerated. This additional acceleration could explain the excess of radio emission with respect to the far-infrared emission of NGC 1961. We suggest that a violent event in the past, most likely a collision with an intergalactic gas cloud or a merger, has caused the various unusual features of the galaxy.