Last accessed timestamp of a Netezza table?
You could setup a history database (using nzhistcreatedb) which acts as a detailed access log and then consult the views and tables that it provides (e.g. $v_hist_table_access_stats).

If you haven't already got this in place then I'm not aware of any way to find out who accessed a table unless you trawl through the logs in /nz/kit/log/postgres/.