Yes, ancestry.com is authentic.  It's a very good website.  Too bad they have to charge you to look for anything, BUT every time I sign on, they say I have a free trial membership.  I can't get everything I want, but it gives me a place to start.  They can't charge you unless they have your credit card number.  So if you sign on for the first time, they may give you a free trial membership and just make sure and contact them before the free trial is up.  I don't know what they do if you don't, but it is an authentic website.  They joined up with rootsweb.com.  Rootsweb.com is entirely free.  I use that most of the time.