Merge Aperture and Photos Libraries?

Aperture can merge libraries, but only iphoto Libraries and Aperture libraries, see: https://support.apple.com/en-us/HT201340

Do your Aperture library and your Photos Library contain different images? If your current Photos library has been created from the Aperture Library and you switched to Photos, the Aperture library may be redundant. Are you still using Aperture?

If the photos in the Aperture library are different from the photos in your current Photos Library, you can create a Photos Library from the Aperture Library and then merge the two libraries in Photos using iCloud Photo Library or by exporting the photos from the migrated Aperture library and importing them to Photos.Use "File > Export > Export unmodified originals"- Only, if you export and reimport the originals, will Photos have a chance to detect duplicates.

See the different options to merge Photos Libraries here: Notes on Merging Photos Libraries

and I can safely delete the Aperture library, or are they still contained within the Aperture library and referenced in the Photos library?

All photos from the Aperture library have been copied to the new Photos Library. If you did not delete them there, you can delete the Aperture Library. But check, if your new Photos Library is readable.

The Aperture Library and the Photos Library are sharing the photos by hard links. The files are stored on top of each other. But you can delete either the original file in Aperture or in Photos and the image will still be readable from the other application. The blocks on the hard disk will only be freed, if you delete the photo from both libraries. So deleting the Aperture library will not free much storage, just the space taken up by the previews and thumbnails, but not the space taken up by the original files. See: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

I would keep a copy of the library n an external rive, before you delete it from your system drive.

Thanks very much Leonie, I think what is confusing me is the Aperture library is showing as 75GB, and the Photos library is showing as 180GB.

There may be several reason for the Aperture library being smaller:

• Your Aperture library may not be including previews of the edited versions. Or not have been including faces. All these items need extra storage and you can disable them in Aperture
• And you may have been importing a lot of new photos to your Photos library since the migration to Photos.