  A search for new particles that decay into top quark pairs (ttbar) is
performed with the ATLAS experiment at the LHC using an integrated luminosity
of 4.7 fb^-1 of proton-proton (pp) collision data collected at a center-of-mass
energy sqrt(s)=7 TeV. In the ttbar --> WbWb decay, the lepton plus jets final
state is used, where one W boson decays leptonically and the other
hadronically. The ttbar system is reconstructed using both small-radius and
large-radius jets, the latter being supplemented by a jet substructure
analysis. A search for local excesses in the number of data events compared to
the Standard Model expectation in the ttbar invariant mass spectrum is
performed. No evidence for a ttbar resonance is found and 95% credibility-level
limits on the production rate are determined for massive states predicted in
two benchmark models. The upper limits on the cross section times branching
ratio of a narrow Z' resonance range from 5.1 pb for a boson mass of 0.5 TeV to
0.03 pb for a mass of 3 TeV. A narrow leptophobic topcolor Z' resonance with a
mass below 1.74 TeV is excluded. Limits are also derived for a broad
color-octet resonance with Gamma/m = 15.3%. A Kaluza-Klein excitation of the
gluon in a Randall-Sundrum model is excluded for masses below 2.07 TeV.
