join with an array

Hi,

I'm trying the following query:
select array_agg(t1.id) from table1 t1 join table2 t2 on (t2.id = any(array_agg)) group by t1.col1;

but I get this error: ERROR:  column "array_agg" does not exist

I tried aliasing array_agg(t1.id) without success.

Thanks for any suggestions,