  A search for new particles decaying into a pair of top quarks is performed
using proton-proton collision data recorded with the ATLAS detector at the
Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = $13 TeV
corresponding to an integrated luminosity of 36.1 fb$^{-1}$. Events consistent
with top-quark pair production and the fully hadronic decay mode of the top
quarks are selected by requiring multiple high transverse momentum jets
including those containing $b$-hadrons. Two analysis techniques, exploiting
dedicated top-quark pair reconstruction in different kinematic regimes, are
used to optimize the search sensitivity to new hypothetical particles over a
wide mass range. The invariant mass distribution of the two reconstructed
top-quark candidates is examined for resonant production of new particles with
various spins and decay widths. No significant deviation from the Standard
Model prediction is observed and limits are set on the production cross-section
times branching fraction for new hypothetical $Z'$ bosons, dark-matter
mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with
the predicted production cross-sections, the $Z'$ boson in the
topcolor-assisted-technicolor model is excluded for masses up to 3.1$-$3.6 TeV,
the dark-matter mediators in a simplified framework are excluded in the mass
ranges from 0.8 TeV to 0.9 TeV and from 2.0 TeV to 2.2 TeV, and the
Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay
widths of the particles.
