Magnetic excitations from the singlet dimerized state in Na 2 Co 2 ( C 2 O 4 ) 3 ( H 2 O ) 2

${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}({\mathrm{H}}_{2}\mathrm{O}{)}_{2}$ consists of two-leg ladders of ${\mathrm{Co}}^{2+}$ ions. It was reported that this is a candidate compound to show a magnetic state in the vicinity of the gapped and the N\'eel ordered state, where the quantum phase transition occurs [Z. Honda et al., Phys. Rev. Lett. 95, 087204 (2005)]. Neutron scattering experiments were performed to study the magnetic ground state of this compound. It is found that the system is well described by an almost isolated dimer model, in which dimers are formed between spins along the rung direction and the interaction along the rung direction is much larger than that along the leg direction. The intradimer coupling constant is determined to be $5.2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Other interactions were found to be very small.