And which modern day countries are those ?
Not our economic system ?
So a country with the same legal system as the USA should have roughly the same per capita wealth ?
What "type" of society do we have, and many many "types" are there ?
What is an Indo-European area ?
"The Celts were a collection of tribes with origins in central Europe that shared a similar language, religious beliefs, traditions and culture. It’s believed that the Celtic culture started to evolve as early as 1200 B.C. The Celts spread throughout western Europe—including Britain, Ireland, France and Spain—via migration...."
Who Were Celts - HISTORY
Where in Asia or Africa can we find evidence of their migration ? (should be hard if they were from Indo-Europe as you claim)
If they're pre-Roman (as they were as 1200 BC is well before the dawn of the Roman Empire), how would the Rise of the Roman empire in the later part of 1st century BC affect their migration?