Where do atheists get their morality from?

[Vincenzo Vinny G.]

I'm arguing from a purely philosophical position, one where objective moral values don't exist. This is the default position of atheism from which our moral norms are derived. The problem is, our atheistic position cannot RATIONALLY reject the possibility of genocide, rape, murder and the likes being morally good. There is always the possibility. And more importantly, it's a metaphysical possibility, not just a logical one.

This is worrying for the rational atheists among us, and this is an area I am enormously interested in researching. My problem is, I need a better grasp of the fundamentals of ethics and moral studies.