Searches for ttbar Resonances and Fourth Generation Vector-like Quarks with the ATLAS Detector

Being the heaviest fermion, various extensions of the Standard Model predict new Physics to couple first to the top quark. The talk presents results from searches for new resonances decaying to a top-antitop pair. Theories of Physics beyond the Standard Model predict the existence of new types of quarks. We report on several search channels such as vector-like quarks decaying to a Higgs boson and a top quark or to a gauge boson and a b quark. These searches use the data sample recorded in 2012 at sqrt(s)=8 TeV centre-of-mass energy by the ATLAS experiment at the LHC and include novel boosted top quark reconstruction techniques.