Those are called ligatures, when two letters are tied together in a special way.
The older custom of doing so in typeset Modern English text for certain combinations of A+E or O+E no longer holds, so you should today just leave them separated except when citing an older passage verbatim.
In this answer that now resides on one of our sister sites, I previously wrote:
If push comes to shove, a reasonable general rule is:
- Use explicit lexical ligatures only for the things that make sense in the language you’re writing in, such as Æ, æ in Icelandic or Old English and Œ, œ in French.
- Leave the selection of typographic ligatures to the software, which should happen automatically for those situations where it would not look right without them.
Our word ligature comes from the Latin first declension noun ligātūra, which was in turn derived from the Latin verb ligāre meaning to bind. It’s a tie.
As for why something is “not recognized as a word” when written with a ligature, that’s a defect of whatever spellchecking software you’re being subjected to.