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Description
The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarization. This observation covered the frequencies between 115MHz and 175MHz. We produced an image of total emission of M51 at the mean frequency of 151MHz with 20-arcsec resolution and 0.3mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300MHz so far. The integrated spectrum of total radio emission is well described by a power law, while flat spectral indices in the central region indicates thermal absorption. We observe the disk to extend out to 16kpc and a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are larger at 151MHz, and the break is smoother at 151MHz than those observed at 1.4GHz. The arm--interarm contrast is lower at 151MHz than at 1400MHz, indicating propagation of cosmic ray electrons (CRE) from spiral arms into interarm regions.
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