M51 observed at 151 MHz with LOFAR

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Clik here to view.© David Mulcahy (MPIfR)

Nearby spiral galaxies have hardly been studied in the very low radio frequency range (less than 300 MHz) due to many technical challenges. Up to now, the only observations of galaxies at these frequencies were of poor resolution and no details could be made out. This is now changing with the start of operations of the brand new radio telescope, the Low Frequency Array (LOFAR).

Low-frequency radio synchrotron emission of nearby galaxies is very important as it is produced by aged and low-energy electrons that are less affected by energy losses and therefore can propagate further away from their site of origin.

A team of astronomers from several countries in Europe, including Germany, the Netherlands and the UK, have been able to observe Messier 51, aptly named the 'Whirlpool Galaxy', with LOFAR at a frequency of 151 MHz. It is the most sensitive image of any galaxy at frequencies below 300 MHz so far. With this sensitivity, the disk of M51 can be seen to extend further than ever seen before in the radio regime.

Interestingly, the disk of M51 does not extend as far out as expected due to steepening of the radial profile of the radio emission beyond the star forming regions.

With LOFAR's high angular resolution, the grand design spiral arms are visible and compared to higher frequencies are becoming broader due to cosmic ray diffusion. Using simple models it is seen that diffusion is the most dominant process of cosmic ray propagation in M51. While M51 was not detected in polarised emission due to very high amounts of depolarisation caused by interstellar turbulence within the galaxy, polarised emission emanating from 6 background radio galaxies was detected in the entire field.

The paper describing this result is published by Astronomy & Astrophysics and available at

http://adsabs.harvard.edu/abs/2014arXiv1407.1312M .