Comment by jll29

Reminds me that a friend of mine wrote a Ph.D. thesis on "um" a couple of years ago:

Nicholson, Hannele B. M. (2007) DISFLUENCY IN DIALOGUE: ATTENTION, STRUCTURE AND FUNCTION PhD. thesis, University of Edinburgh, Edinburgh, Scotland, UK, available online: https://era.ed.ac.uk/bitstream/handle/1842/1763/Nicholson%20...

But "um" may not be quite the same as "aha" for English native speakers (and Japanese native speakers may use both "um" and "ahm" as disagrement).