  Although it is well accepted that the spin-$\frac{1}{2}$ triangular lattice
Heisenberg antiferromagnet(TLHAF) has a long range ordered ground state, a
thorough understanding of its spin dynamics is still missing. While the linear
spin wave theory(LSWT) predicts three branches of magnon mode in the magnetic
Brillouin zone(MBZ), the 1/S expansion at the next order is found to break down
in a large portion of the MBZ centered around the M point, leaving the fate of
the magnon modes there undecided. Recent neutron scattering measurement on
Ba$_3$CoSb$_2$O$_9$, an ideal realization of the spin-$\frac{1}{2}$ TLHAF,
provides a surprising answer to this issue. Two, rather than three branches of
magnon mode are observed around the M point, whose dispersion are strongly
renormalized with respect to the LSWT prediction and exhibit pronounced
roton-like minimum at the M point. This is accompanied by a strong spin
fluctuation continuum at higher energy, inside which two strong and broad
spectral peaks of unknown origin are observed. In this work, we propose a
simple picture for these spectral anomalies by invoking the resonating valence
bond(RVB) physics in the description of the ground state of the system. We find
that the roton-like minimum in the magnon dispersion can be explained by the
coupling between the collective spin fluctuation and the continuum of Dirac
spinon excitation moving in a $\pi$-flux background. We also propose that the
two broad peaks in the continuum can be understood respectively as the Landau
damped third magnon mode and the Landau damped longitudinal mode. Such a
picture can be verified by studying the polarization character of the various
spectral features.
