The stars with heavier elements are found in old galaxies, because they're either red giants themselves or are created from the matterial from red giants. As it takes quite a long time to form a red giant, you can safely estimate those galaxies to be at least 8 billion years old.

That's all I know about stars with heavier elements. I don't have any clue why they should be in the outer rim. It seems to be that if disturbed matter is not swallowed, then those stars can be anywhere in that disturbed matter, be it edges or center.