But you need one. Either after the s, or apostrophe plus another s. This according to grammarbook.com, punctuation, apostrophes. Sorry I can’t post the link. If there’s a way to do it on a smart phone, I’m not bright enough to figure it out.
@Jeruba I’d be inclined to do that too but this was a reference to the gallery in the Salon where his pictures would have hung. I guess I might have written, “The gallery where the pictures by Degas would have hung.”
I’d go with @dappled_leaves, suggestion, go around. Perhaps saying “the works of Degas”, unless, of course, you’re referring to the actual Degas family.
You could also say, “The gallery where the Degas paintings are hung.” That makes the paintings plural rather than suggesting that there are multiple artists named Degas.
If referring to the family, you could use The Mr. and Mrs. Degas, or The Degas family.