oids on disk not in pg_class

We have a fairly large (1 TB) database we put on all SSDs because of a
very high insert and update rate (). As our business has grown, we've
been running into space constraints, so we went looking for files we
might be able to delete.

We found a large number (662 out of 1465 total ) and size (219 GB) of
files in the data directory whose name does not correspond to an oid in
the pg_class system catalog table. That amount of space would address
our current space constraint problems. Some of these tables are recent
(from today), while others are quite old (large volume in August and
May, with some smaller ones as far back as February.

What kinds of data are written to disk without being registered in
pg_class? How can we determine which (if any) are safe to delete?

Thanks.

--
Guy Rouillier