  We report on the creation of ultracold gases of bosonic Feshbach molecules of
NaCs. The molecules are associated from overlapping gases of Na and Cs using a
Feshbach resonance at 864.12(5) G. We characterize the Feshbach resonance using
bound state spectroscopy, in conjunction with a coupled-channel calculation. By
varying the temperature and atom numbers of the initial atomic mixtures, we
demonstrate the association of NaCs gases over a wide dynamic range of molecule
numbers and temperatures, reaching 70 nK for our coldest systems and a
phase-space density near 0.1. This is an important stepping-stone for the
creation of degenerate gases of strongly dipolar NaCs molecules in their
absolute ground state.
