pg_psort files

I have a fairly simple database (two tables and a few indecies). In
the data/base/xxxx directory I find the following files:

-rw-------  1 postgres  postgres    7734146 Dec 14 06:36 pg_psort.13268.0
-rw-------  1 postgres  postgres    7707487 Dec 14 06:36 pg_psort.13268.1
-rw-------  1 postgres  postgres    7776602 Dec 14 06:36 pg_psort.13268.2
-rw-------  1 postgres  postgres  936026112 Dec 14 07:20 pg_psort.13268.3
-rw-------  1 postgres  postgres    8208384 Dec 14 06:36 pg_psort.13268.4
-rw-------  1 postgres  postgres   15699116 Dec 14 06:36 pg_psort.13268.5
-rw-------  1 postgres  postgres   19002244 Dec 14 06:36 pg_psort.13268.6
-rw-------  1 postgres  postgres    2097152 Dec  9 06:33 pg_psort.25350.0
-rw-------  1 postgres  postgres    2146304 Dec  9 06:33 pg_psort.25350.1
-rw-------  1 postgres  postgres    2277376 Dec  9 06:33 pg_psort.25350.2
-rw-------  1 postgres  postgres    1597440 Dec  9 06:33 pg_psort.25350.3
-rw-------  1 postgres  postgres    1064960 Dec  9 06:33 pg_psort.25350.4
-rw-------  1 postgres  postgres    1327104 Dec  9 06:33 pg_psort.25350.5
-rw-------  1 postgres  postgres          0 Dec  9 06:32 pg_psort.25350.6

I have another database that is almost identical on a different
machine. There are no pg_psort.xxxx files there. I am wondering:

1. What are these files?
2. Do I need them?
3. If I don't need them is it safe to remove them?

--
==================================================================
Steven Saner                            SouthWind Internet Access, Inc.
ssaner@southwind.net                    Systems/Network Administrator
http://www2.southwind.net/~ssaner       http://www.southwind.net
                                        263-7963 Wichita  (800)525-7963