LOFAR 150 MHz image of the field centered on nearby galaxy M 51. The size of the synthesized beam is 18″ × 15″. Roughly 3000 radio continuum sources were detected in this image within 2.5° of the center of M 51 (dotted-dashed black circle). We searched for polarization in all sources brighter than 100 mJy (201 sources; small blue squares). The locations of the sources that were found to be polarized in this work and or in other studies are also indicated. The entire field is included in the 1.4 GHz polarization catalog of Taylor et al. (2009). The regions examined in other studies are shown as the dotted black square (LOFAR 150 MHz, Mulcahy et al. 2014), dashed green circle (GMRT 610 MHz, Farnes et al. 2013, approximate), dashed magenta square (WSRT 1.4 and 1.6 GHz, Heald et al. 2009), dashed red square (VLA 1–2 GHz, Mao et al. 2015). The FITS file of the LOFAR total intensity image of the field are available at the CDS.