raiser below ground

I am replacing a bathtub drain and overflow pipes with a cast iron two piece shower drain onto a concrete floor. Pipe is 1 1/2 inch and the cast iron drain is 1 1/2" . Unfortunately the brass bathtub slip joint was over 30 y.old and broke off during removal, so now I have it sticking 1/4" above the concrete floor. It is cemented into the concrete floor, and another 1/4" below it begins the P-trap which is galvanized steel pipe. Why don't I see any connection between the slip joint and the galv. pipe? Do I need to somehow chisel out the brass broken off pipe before I do anything? Most importantly how do I connect to a pipe buried 1/2" below ground? Any help would be appreciated! I've waited 2 weeks for the 1.5" cast iron drain and now am eager to finish or should I call a professional ?

Thank you, guys!!
Best.
Mario