I have looked into my library for things Celtic and have found a book by Henri Hubert titled, The History Of The Celtic People. It was originally published in 1934 as The Rise Of The Celts and The Greatness And Decline Of The Celts. As I was flipping through the book, I found an article from the Smithsonian, May 1993, by Dora Jane Hamblin, titled Once Maligned, Celtics Are Now Touted As The First Europeans stuck in the pages.
The 1993 Smithsonian article has some splendid pictures of Celtic art. I tried to pull them up on the 'net' without success. Might you try?
I've lost the trail on the Xiongnu people so, I guess I'll read Henri's book. lol.
Let me know what you think of Huberts book. It's long out of print, but maybe available...
I checked the Smithsonian site archives but no hit on the article, which sounds interesting...