So that's where the supernova remnants have been hiding.
https://www.cnet.com/science/space/supernova-ghosts-discovered-in-most-detailed-radio-image-of-milky-way/?utm_source=dlvr.it&utm_medium=blogger#ftag=CAD590a51e
https://www.cnet.com/science/space/supernova-ghosts-discovered-in-most-detailed-radio-image-of-milky-way/?utm_source=dlvr.it&utm_medium=blogger#ftag=CAD590a51e