Identify the underlined part of speech: What! can this be true?

Primary interjections

A primary interjection is a word or sound that can only be used as an interjection. Primary interjections do not have alternative meanings and can’t function as another part of speech (i.e., noun, verb, or adjective).

Primary interjections are typically just sounds without a clear etymology. As such, while they sometimes have standard spellings, a single interjection may be written in different ways (e.g., “um-hum” or “mm-hmm”).

Examples: 

Ugh! That’s disgusting.

Um-hum. I think that could work.

We won the game. Yippee!