What part of speech is "thank you"?

Can anybody please tell me the subject, verb, and object of this sentence:

Thank you all for conducting a landmark experiment.

I would also like to please know what part of speech thank you belongs to when it’s used the way it is in the sentence given above.

As has already been pointed out, the original phrase is "I thank you." subject-verb-object

I know from anecdotes told me years ago by an elderly relative that "I thank you." was very commonly used around the 1900s particularly by shopkeepers (in Britain).

It's still used often in certain circumstances but you don't hear it in shops any more.

This graph and the associated links may be of interest.

Google ngram: I thank you

These days, "Thank you" is a formula but it still holds the place of "I thank you"