I think it's spelled Ha'tak or Hak'tak. I watch everything in closed captioning and I've seen it both ways. I think Ra didn't think we were a big enough threat and he already had all the humans he needed, but he might also have left because we killed the Anubis of the time. Or the other goa'ulds didn't know Ra went to earth and him and Ra were killed. After a while the goa'ulds assumed he was dead and a new one stepped forward. If you read one of my threads, I said I think the iris is close enough so it won't desinagrate.
----See iris origin.