You need to do more research in the US to find their Hannoverian origin. Remember that Hannover was a country at that time, before it was made a province of Prussia. So they could be from anywhere in old Hannover, not just in Hannover the city.
I suggest you look for their passenger ship records. If you are lucky, they emigrated via Hamburg, and their embarkation list is still available. The easiest way to start, though not without its own problems, is the book index "Germans to America". See the immigration/emigration section of the German genealogy website
for a list of the books and holding libraries.
There is also a list of suggested sources for finding German places of origin in the German genealogy FAQ, available here:
http://www.genealogy.net/faqs/sgg.html
Hope this helps.
=Jim