old kernels cannot be removed in Synaptic?

I'm running MATE 16.04 on an ASUS X550LA, when I run:

$ ls /boot | grep vmlinuz | cut -d'-' -f2,3
4.13.0-38
4.13.0-38
4.13.0-39
4.13.0-39

(I have ran sudo apt-get autoremove before) Yet when I run Synaptic package manager with "linux-image-4*" in the search I see a large amounts of what I believe to be old kernels.

This is only a part of the read out, and almost all except the current kernel show a white square, and properties of any selected show "not installed". If I select any of these and right click I am offered "Mark for installation" or "Properties"

What are these? Can they be safely removed? How do I remove them?